Fog: Its Processes and Impacts to Aviation and Aviation Forecasting

Introduction

Click image to view animation.

A ten year study of aviation accidents by the US National Transportation Safety Board (NTSB) indicated that low ceiling and visibility contributed to over 20% of the accidents. Of those accidents, 68% were specifically due to fog and low ceilings (NTSB Weather Related Accidents).

Module Objectives

This section will address issues surrounding the direct and indirect impacts of restricted ceilings and visibilities on aviation operations. After completing this lesson, the learner will:

• Identify the various users of ceilings and visibility forecasts and explain how forecasts of these conditions impact (both positively and negatively) aviation operations within each user group
• Describe the impacts of reduced visibility and ceilings on air traffic management, commercial and general aviation operations
  • Explain why forecasters need to know the characteristics of local airfields that can impact local operations
• Explain the characteristics, processes, and low-level factors that influence radiation and advection fog/stratus events
• Explain the characteristics and processes that influence terrain-induced and pre- and post- frontal fog events.

Aviation Impacts

The impacts of widespread or locally dense fog or low ceilings may not seem as dramatic as other weather hazards such as tornadoes, hurricanes, or severe thunderstorms. Fog and low ceiling events, however, occur more frequently than these other weather hazards, and their impacts to aviation operations are substantial.

These impacts, combined with the fact that ceilings and visibility represent some of the most difficult meteorological variables for numerical models to predict, make it all the more important that forecasters have a complete understanding of the processes involved in low ceiling and fog events and apply the latest techniques and approaches to the forecast problem. Issuing forecasts that are overly pessimistic can result in pilots losing confidence in the forecasts and choosing to ignore them. Since different users have different needs and limitations, your best approach is to forecast as accurately as possible and to communicate your thoughts as clearly as possible (see COMET module Writing Effective TAFs).

Specific Customer Impacts

Commercial airlines

Fog or reduced ceilings often cause delays, diversions, or cancellations. Nearly 74% of all delays at airports are caused by weather, with the majority of such delays caused by low ceilings and visibility. A 1996 study conducted by Massachusetts Institute of Technology (MIT)/Lincoln Labs on marine stratus at San Francisco International Airport (SFO) found that the airline and passenger cost of delays due to ceiling and visibility approached $100 million dollars per year(Clark, Wilson, 1996).

Airline operations and dispatchers

Every airline has its own operations center in which they monitor their flights. Airline operations function similar to air traffic managers (see below) but their focus is on their airline. Thus, they make extensive use of aviation forecast and other weather products from multiple sources.

Airline dispatchers are a part of the operations center. They plan the fuel loads and determine flight routes based on individual aircraft capabilities, international and national regulations, and weather, both enroute and at the destination.

National air traffic services may require a designated alternate destination on the dispatch release if either the destination airport's observed or the forecasted ceiling and visibility are less than 600 meters (2000 ft) and/or 4800 m (3 mi) within one hour either side of expected arrival time. Once it is determined that an alternate is necessary, the dispatcher must select an airport that meets the alternate minima. These minima are usually 120 m (400 ft) and 1600 m (1 mi) or 180 m (600 ft) and 3200 m (2 m). Smaller airports may require more stringent minima of 240 m (800 ft) and 3200 m (2 mi) or even 300 m (1000 ft) and 4800 m (3 mi).

If an alternate is required, regulations specify that additional fuel be carried to fly to and attempt to land at the destination airport and then fly to and land at the most distant alternate airport.

Upon arrival at either airport, there must be at least 45 minutes of fuel reserve remaining. If adverse weather conditions are occurring or forecast to occur, the dispatcher will first calculate a normal total fuel load and then add about 15% more fuel.

The requirement of alternates and the costs involved in carrying additional fuel could result in millions of dollars per day for all affected flights, not including the untold hours of passenger inconvenience.

General Aviation Pilots

Consider this sobering statistic: Of those General Aviation (GA) pilots involved in low ceiling or low visibility accidents due to fog, more than half were fatal. Clearly, this is a user community that needs the best forecast possible.

General Aviation describes a broad spectrum of operations. For our purposes we will define GA as "other than air carrier, air taxi, or military". Most of the flying within this GA category is not-for-hire and conducted in single- and two-engine aircraft. GA pilots obtain weather briefings through government-sponsored programs or commercial vendors of weather products. Experience levels vary from the novice private pilot to veteran airline transport pilots, and they make weather decisions based on their individual training and experience.

About half of all GA pilots can only fly VFR. Consequently, clouds can be a significant hazard for this group, as well as for a number of IFR-rated pilots who have limited flying experience in these conditions.

VFR-rated pilots flying into IFR conditions often encounter problems, which sometimes result in fatal accidents. Pilots who are not acquainted with flying without visual references tend to believe their own sensation of motion rather than their instruments. This is a particular problem for the general aviation community.

Air Traffic Managers/Controllers

Air traffic managers face a continual challenge of anticipating available operating capacity so that the demand of incoming planes can be metered to match the availability of arrival slots. The cost in both dollars and human inconvenience is very high and will only get worse as the air traffic volume increases. This is a particular problem at the major coastal airports (Cape Town, Hong Kong, London, Los Angeles, New York, Melbourne, Rio de Janeiro, etc.).

Know Your Airfield

The major factor for an airport is that no two aircraft can be on the same runway, even at opposite ends, at the same time. Sequencing aircraft for arrival and departure is based on being able to get one aircraft off the runway before another touches down or begins a takeoff roll. The lower the ceiling and visibility conditions, the greater the separation required between aircraft to achieve the one-aircraft-per-runway rule.

Landing and Takeoff Separation

Every forecaster providing TAF support to local airfields should have a working knowledge of the airfields they are supporting so that their products are sensitive to local airfield minima and restrictions. You should be aware of the factors that dictate the air traffic requirements for the specific airports you are supporting. These factors involve critical altitudes, which are based on:

• Available runways
• Runway configuration
• Types of approaches
• Required approach/landing minima
• Alternate minima
• Traffic density
• Noise restrictions

All of these will play a role in how ceiling and visibility restrictions will impact the airfield's operations. In this module we will consider runway configuration, operational thresholds, approach and landing minima.

If the forecaster does not thoroughly understand the potential operational impact of his/her forecast, the product is not likely to be well received by the intended user and has the potential of resulting in unintended operational impacts.

Runway Configuration

It is particularly important that you know your airport, its layout, and the surrounding terrain when you're forecasting fog. The same geophysical features that influenced the location of the airport (such as large water bodies, complex terrain, etc.) also influence fog. A study of the terrain around an airport will often reveal features that consistently promote fog.

Many airports have at least one runway configured parallel to the prevailing wind in order to maximize headwinds, minimize tailwinds, and diminish the threat of crosswinds. The four basic configurations are:

1. Single runway: This configuration is optimally positioned for prevailing winds and other determining factors.
2. Parallel runways: Operations per hour will vary depending on the total number of runways and the mix of aircraft.
3. Open-V runways: Two runways that diverge from different directions but do NOT intersect form a shape that looks like an "open-V." This configuration is useful when there is little to no wind, as it allows both runways to be used at the same time.
4. Intersecting runways: This type of configuration is used when there are relatively strong prevailing winds from more than one direction during the year.

Operational Thresholds

As a provider of critical support products to the aviation community, it is important for you to recognize that Low Instrument Flight Rules (LIFR) conditions are one among many that could cause large delays in the national air traffic system.

Every airport has different minima that impact its operations. These minima are referred to as operational thresholds. Minima are influenced by:

• tower height
• runway configuration
• airport usage (aircraft type)
• equipment (Instrument Landing System ILS, manned tower, etc.)

This includes restrictions that extend beyond the requisite Marginal Visual Flight Rules (MVFR), Instrument Flight Rules (IFR), and LIFR minima.

Instrument Landing System (ILS)

Some airports are equipped with a precision instrument landing system that normally consists of the following electronic components and visual aids:

• localizer (horizontal)
• glideslope (vertical)
• outer marker
• middle marker
• approach lights

Such systems enable approaching aircraft to land at a particular airport in poor visibility by following an instrument approach procedure. There are three categories of procedures determined by the height at which the pilot must decide to land or not (known as decision height).

• Category I (CAT I)
• Category II (CAT II)
• Category III (CAT III)

Runway Tower Visibility

The height of an air traffic control tower (ATCT) is determined by the need for air traffic controllers to see all ends of the runways and the surrounding area. Runways at today's airports may be over two miles from the ATCT. New towers are being constructed in excess of 90 m (300 feet) in order for the controller to maintain visual contact.

For example, operations at Atlanta, US (when the tower height was 64 m (210 ft)) were impacted less than 50% of the time when the cloud ceiling was at 60 m (200 ft). With the new tower height of 120 m (400 ft), operations were impacted 87 percent of the time with a 60 m (200 ft) cloud ceiling (West & Scov, 2006). How tall is the tower at your airport?

Runway Visual Range

Runway Visual Range (RVR) is an instrumentally derived value that represents the horizontal distance a pilot will see down the runway from the approach end. RVR, in contrast to prevailing or runway visibility, is based on what a pilot in a moving aircraft should see looking down the runway. This may or may not be available at your airport.

Landing

Click to open audio playback window.

Rick Curtis, Chief Meteorologist, Southwest Airlines
discusses decision height

Really, that’s not very restrictive to us with the exception of one thing…and that is that when the plane is descending, it has to be able to see the runway lights at the decision height. If they’re not able to see the runway lights at the decision height, then they have to go around and do a missed approach.

The following table summarizes some general approach criteria that are useful to keep in mind. You might want to print and post it near your workstation.

The following exercises deal with the effects of ceiling and visibility forecasts within the TAF and their effect on airport operations. However, it is important to recognize that adverse weather conditions other than ceiling and visibility restrictions will also impact flight operations. Conditions such as tailwinds, crosswinds, and precipitation can also greatly influence flight restrictions and result in delays. For the purposes of keeping this module focused on ceiling and visibility issues, we will not deal with these other conditions.

Use the Critical minima table above to answer the following questions on TAFs and landing and approach flight movements.

Summary

Low ceilings and/or reduced visibility, elements that produce instrument meteorological conditions (IMC), are safety hazards for all types of aviation. The safety and economic impacts that result from these elements make it all the more important that forecasters have a complete understanding of the airport and its users. Remember, your primary customers are:

• Commercial Airlines, Operations, and Dispatchers
• General Aviation Pilots
• Air Traffic Managers
• Airport Managers

In order to generate customer-friendly aviation weather products, aeronautical forecasters should be aware of:

• the runway configuration and operational thresholds at each airfield
• how small differences in ceiling and visibility forecasts can translate to large differences in flight movement
• the financial impacts to air carriers due to stoppages, delays and/or diversions

Forecasters can become thoroughly familiar with their users and user requirements by:

• Going to meetings, conferences, and seminars to learn more about the local aviation community's concerns
• Getting to know the customers and the type of aircraft flown at the supported airfields
• Becoming familiar with GA concerns, requirements, and restrictions
• Visiting local civil aviation facilities and meeting with local personnel

Fog Processes – Radiation fog

Now, let's direct our attention to fog and its processes. Fog is the suspension of very small, usually microscopic water droplets in the air that reduces surface-based visibility to less than one kilometer (5/8 mi) (WMO No.306, WMO No.407, and NWSI 10-813).

Some of the more common types of fog are:

• Radiation fog
• Advection fog
• Upslope/Terrain Induced fog
• Rain/Post-Frontal fog
• Blocked Flow Fog/Stratus
• Valley fog

This module is focused on the two most prominent types - radiation and advection - with the understanding that the development processes are common to all. We will use radiation fog to introduce the precondition, formation and growth, maintenance, and dissipation fog processes. The module also includes a discussion of the other four fog types.

Preconditions

The key low-level ingredients required to generate a radiation fog are moisture, rapid cooling, and calm or light winds.

Low-level anticyclones can create favorable conditions for radiation fog by suppressing surface winds and drying the air aloft through subsidence. Dry air aloft enhances radiative cooling at the surface. Radiation fog is very unlikely to form unless there is sufficient moisture in the boundary layer. Moisture may be advected into an area, or derived through daytime evaporation from surface sources such as wetlands or wet soil. Note that in some instances, pre-existing low-level moisture may be present (e.g., from evapotransportation) to create sufficient moisture for radiation fog.

After daytime heating ends, clear, dry conditions above the boundary layer hasten cooling at and near the surface. When skies are overcast, less than 10 percent of the radiation emitted by the earth escapes to space. Most of the radiation is absorbed and/or reflected by carbon dioxide, water vapour, and cloud droplets in overcast skies. However, clear skies allow as much as 20 to 30 percent of the radiation to escape the atmosphere.

Click image to view animation.

Earth-emitted radiation travels through the atmosphere

Note: Because winds create turbulent mixing, calm or light winds at the surface maximize radiative cooling.

As the energy escapes, the ground surface cools rapidly and induces cooling of the lowest few meters of the atmosphere, creating a shallow surface-based inversion. If there is enough water vapour in the air and enough cooling at the surface, the low-level air eventually reaches saturation.

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Formation of shallow inversion at the surface

When afternoon temperatures are cool prior to nightfall, the time required to reach saturation on a clear night is shortened.

Stable Layer Formation

As cooling continues, water vapour near the surface begins to condense onto objects as dew or deposits itself as frost. This process dries the lowest few meters of the atmosphere, while weak turbulent diffusion continues to transport moist air toward the surface.

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Turbulent diffusion and dew formation

Continued cooling in this layer causes it to become increasingly stable and resistant to the effects of weak turbulent mixing near the surface. Eventually, the near-surface turbulence ceases altogether, and with it, the formation of dew or frost at the surface. As cooling continues, excess water vapour in the saturated layer just above the surface begins to condense into fog droplets.

Note: By "turbulent mixing," here we are referring to small-scale mixing (several centimeters), not wind-induced mixing.

Click image to view animation.

Surface layer stabilization and droplet formation

Surface Heat Exchange

Different surfaces cool at different rates, depending on the surface type and thermal conductivity beneath the surface. Highly conductive surfaces, such as pavement, cool more slowly after nightfall because heat conducted upward offsets radiative cooling at the surface. Grassy surfaces have a lower conductivity than pavement, so they cool more rapidly, allowing the air in contact with it to reach saturation more quickly.

Thermal conductivity for different surfaces

The thermal conductivity of soil is also strongly dependent on its moisture content. Wet soil does not heat up as much as dry soil during the daytime because water requires more energy to heat up, and a significant portion of solar energy absorbed by wet soil contributes to evaporation. In addition, wet soil cools slower than dry soil after daytime heating ends.

Thermal conductivity differs with moisture content

Question 2

Choose the best answer.

Snow fell over an airport in your forecast area. As the conditions stabilize, which surface is more conducive to fog formation?

a) A concrete runway covered by snow

b) A freshly-plowed concrete runway

Feedback: a) is correct.

Surface snow cover is often associated with radiation fog. There are three primary reasons for this. First, snow absorbs much less solar radiation than other surfaces, and a portion of the energy that is absorbed is used for melting and/or sublimation. This limits heating on the afternoon prior to fog formation.

Surface processes in the presence of snow cover

Second, snow cover also insulates the ground at night, limiting the upward flux of heat from beneath the snow.

Insulating effects of snow cover

Third, nighttime radiative cooling occurs more quickly over snow cover than soil or vegetative surfaces.

Nighttime cooling with and without snow cover

These combined effects allow the lowest few meters of atmosphere to reach saturation more rapidly over snow-covered areas after daytime heating ends. Note, if snow is melting, it also becomes a low-level moisture source for the atmosphere.

A frozen surface warms less than an unfrozen surface during the day. This occurs because the temperature of frozen ground cannot rise above the melting point until ice at the surface is thawed. In contrast, the temperature of unfrozen ground will start to rise immediately with daytime heating. When night falls, the frozen ground starts with a much lower temperature, which hastens fog formation.

Differences in temperature between frozen and unfrozen soil

However, the presence of snow cover can also inhibit fog formation in situations where low-level moisture is shallow. Since water vapour pressure is lower around ice crystals than water droplets, rapid cooling can cause frost to grow at the expense of fog droplets. This depletes the boundary layer of the excess moisture it needs to form fog.

Click image to view animation.

Snow cover can inhibit fog formation when low-level moisture is shallow

Preconditions Questions

Formation and Growth

Radiative Cooling

During the initiation and growth phase of a radiation fog event, a fog layer forms and expands horizontally and vertically. The key processes during this phase are radiative cooling, fog layer formation, and heat flux from the surface.

During fog formation, radiative cooling progresses to the point that the air just above the ground becomes supersaturated and fog droplets form by condensation.

Radiation fog formation

Question

Choose the best answer.

Your forecast area includes three airports. One is located in an urban environment, another is in a rural area, the third is near the sea coast. If the atmosphere is conducive for fog development, at which airport would you expect fog to develop first?

a) Urban airport

b) Rural airport

c) Coastal airport

The correct answer is c).

The coastal airport is likely to develop fog first because of the presence of sea-salt particles. In some cases near the ocean or in polluted environments, droplet formation occurs prior to supersaturation since some hygroscopic nuclei, such as sea-salt particles, are active at saturation values below 100%. These environments are conducive to earlier fog formation.

Fog formation in coastal and polluted environments

Fog Layer Formation

During the initial stage of fog formation, cooling continues at and near the surface until the fog depth reaches several meters, deep enough to begin to absorb and re-emit radiation originating from the earth. This slows the rate of cooling at the surface, and the fog top becomes the level at which radiative cooling and condensation processes are most active.

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Radiative cooling as fog deepens

Depending on the surface composition, the ground may continue to conduct heat to the surface. When it becomes warmer than the air just above it, weak low-level convective currents activate, causing the base of the near-surface inversion to rise.

Note: Snow cover and frozen ground minimize heat conduction from the soil to the atmosphere.

Click image to view animation.

The base of the near-surface inversion rises as low-level convective currents form

Meanwhile, as the fog deepens, less and less radiation is able to escape from the surface and lower portion of the fog layer. The fog blankets the lower levels, restricting radiative heat loss. In the absence of significant residual heating from the surface, the lower levels of the fog can maintain a nearly constant temperature.

Click image to view animation.

Nearly constant temperature is maintained as the fog deepens

Question

Choose the best answer.

In some cases, fog near the ground can evaporate. Why do you think this occurs?

a) insufficient moisture in the layer

b) heat flux from warm surfaces

c) decrease in the turbulent mixing

The correct answer is b).

Some weak heat fluxes from warmer surfaces can evaporate fog near the ground. Let's review this condition in more detail.

Click image to view animation.

Weak surface heat fluxes evaporate fog

Heat Flux from the Surface

Variations in surface composition, including soil types, vegetation, and other factors, cause local variations in humidity and the rate of radiational cooling. As a result, the initial onset and coverage of radiation fog is patchy. Upward heat flux from the ground reduces local relative humidity (RH) at the surface. This delays nocturnal radiation fog formation over highly conductive surfaces, such as pavement, and hastens fog formation over surfaces with low thermal conductivity, such as snow cover.

Fog formation over different surfaces

Formation and Growth Phase > Questions

Maintenance Phase

During the maintenance phase, an established fog layer maintains a relatively constant depth. This phase is characterized by a balance between opposing forces. These forces are fog-top radiative cooling, droplet settling, and fog-top mixing. Condensation nuclei concentrations, the presence or absence of overlying cloud layers, and surface heat conductivity are important factors influencing fog persistence.

Balance Between Opposing Forces

There is a temperature inversion in the vicinity of the fog top. The base of the inversion is typically located about 50 meters below the fog top. The top of the inversion is just above the fog top.

A skew-T diagram of the inversion near the fog top

During the maintenance phase, fog-top condensation balances evaporation and droplet settling to maintain the depth of the fog layer. Radiative cooling at fog top replenishes the supply of droplets as they settle downward, and even tries to strengthen the inversion and deepen the fog layer. At the same time, turbulent mixing attempts to weaken the inversion and erode the fog top. A radiation fog layer typically deepens during its growth phase until it reaches a height where the winds are strong enough and induce sufficient fog-top mixing to halt the growth.

Click image to view animation.

Balance among fog-top condensation, evaporation, and droplet settling

Fog-Top Radiative Cooling

The radiative heat loss is maximized when the layer immediately above the fog is relatively dry, the winds are weak, and there are no cloud layers aloft. On a clear night, the rate of fog-top radiational heat loss is much more rapid than that in the lowest few meters of the atmosphere.

Click image to view animation.

Radiative heat loss above the fog

Fog-Top Mixing

Entrainment of dry air via turbulent mixing at the fog top evaporates droplets. This process is enhanced by the presence of vertical wind speed shear at and above the top of the fog layer.

Click image to view animation.

Mixing and evaporation at the fog top

Condensation Nuclei Concentrations

Question

Choose the best answer.

Dense fog is more likely to be comprised of:

a) a large number of small droplets

b) a small number of large droplets

The correct answer is a).

When there is a high concentration of active condensation nuclei, the fog is more likely to be comprised of a large number of small droplets, rather than a small number of large droplets. Visibility is severely degraded by high concentrations of small droplets.

Fog density is enhanced by high concentrations of condensation nuclei

Click the "continue" button below to proceed.

The most active condensation nuclei, such as sea-salt, are hygroscopic, or 'water-loving'. Air pollutants can also act as condensation nuclei. Some of the densest fogs, such as the 'pea soupers' of industrial-era London, are associated with high concentrations of particulates in the air. Prior to enactment of clean-air regulations, air pollution also contributed to formation of dense fogs in the Mediterranean region.

Dense fog in an urban environment

Introduction of Overlying Cloud Layers

Question

Choose the best answer.

You correctly forecast fog at your airport during the day but it has been slow to dissipate. What could be causing the delay?

a) The increased heat flux near the surface.

b) The formation of mid- and upper-level clouds.

c) Increase in turbulent mixing.

d) The presence of large droplets in the fog.

The correct answer is b).

During the daytime, introduction of mid- and upper-level cloud layers can help to maintain the radiation fog layer. These clouds reduce the solar radiation received at the ground. This prevents warming at the surface and maintains a higher relative humidity in the lower portions of the fog layer. Options a) and c) contribute to fog dissipation while option d) contributes to fog density.

Click image to view animation.

Mid- and upper-level clouds can assist fog

The lower the level of an overlying cloud layer, the more it can reduce radiative cooling and condensate production at fog top, and allow dissipative processes such as settling to take over.

Low-level clouds reduce radiative cooling and condensation at fog top

Surface Heat Conductivity

The presence of wet ground or snow cover can prolong the maintenance phase of fog. Snow cover reduces the thermal conductivity of the surface, restricting the upward heat flux from the ground during this phase.

Snow and wet surfaces prolong the maintenance phase

Snow cover also reflects more solar radiation than other surface types, which slows diurnal warming after sunrise.

Snow cover slows diurnal warming after sunrise

Maintenance Phase > Questions

Dissipation Phase

Radiative heating both near the surface and within the fog layer combine with mechanical processes such as droplet settling and turbulent mixing at fog top to dissipate the fog. Changes in the wind and overlying cloud layers can also influence dissipation. The duration of the dissipation phase, when a fog layer rises from the ground, thins, or vanishes, can vary due to several factors.

Duration of the Dissipation Phase

During the dissipation phase of radiation fog, the depth, areal coverage, and intensity of the fog diminish. The duration of this phase can vary from less than an hour to half a day. More typically, the dissipation phase lasts for a few hours, since most fog is relatively shallow and short-lived.

Question 1

Choose the best answer.

One of the airports in your forecast area is located in a mountain valley where fog is usually slow to dissipate. Why do you think this occurs?

a) mountain valleys have abundant moisture

b) mountain valleys are protected from winds

The correct answer is b).

Events where fog takes longer than a day to dissipate occur in the more geographically protected areas, such as mountain valleys. The season also influences the length of this phase through such factors as sun angle, average wind speed, snow cover, ground moisture, and vegetation.

Click image to view animation.

Fog in mountain valleys can linger for days

Radiative Heating Near the Surface

The main source of radiant heat is the sun. During the daytime, radiation from the sun is absorbed by the ground, even when there is an intervening layer of fog. As the ground warms, it heats a thin skin of air in contact with the surface through conduction. This heat initiates weak convective mixing, which begins to warm the lowest portion of the fog layer.

Click image to view animation.

Weak surface convective mixing initiates during the day

The relative humidity in this layer begins to decrease, slowing the formation of fog droplets and eventually evaporating existing droplets. As the fog thins, the warming process accelerates, allowing more solar radiation to reach the ground. With moderately strong sunshine, the base of a fog or low cloud layer can lift at a rate of up to a few hundred meters per hour.

Click image to view animation.

Weak surface convective mixing erodes the lowest layer of fog during the day

Radiative Sub-Process: Radiative Heating Within the Fog Layer

While the main effects of solar radiation occur through the convective mixing process, a secondary heating process can also contribute to fog dissipation. Carbon dioxide and water vapour within the fog layer absorb and re-emit some of the radiation from the earth. As depicted in the animation below, air warms as it absorbs the heat energy, causing its temperature to rise and relative humidity to fall.

Click image to view animation.

Carbon dioxide and water vapor absorb and re-emit solar radition

Droplet Settling

Regardless of their size, all fog droplets continually settle. The depth of a fog layer decreases when the droplet formation rate cannot keep up with the settling rate. Fog droplets vary in size, with smaller droplets settling more slowly than larger ones. An average fog droplet, which is less than 20 micrometers in diameter, will settle at the rate of 1 cm/sec.

Click image to view animation.

Droplets settling

So, fog initially 30 meters (or about 100 feet) deep should settle to the ground in about an hour if the maintenance processes are removed. This would result in a rapid improvement to unrestricted visibility. In the real atmosphere, ongoing maintenance processes diminish gradually, causing a slower improvement in visibility.

Fog-Top Turbulent Mixing

A fog layer's capping inversion is often accompanied by a layer of significant vertical wind shear. The base of the inversion is typically about 50 meters below the fog top. Turbulent mixing of warmer and drier air into the top of the fog layer can reduce relative humidity in that layer and lower the inversion. The weaker the capping inversion is, the more susceptible it is to this ongoing mixing and erosion process.

Click image to view animation.

Turbulent mixing near the fog top

Changes in the Wind

Moderate to strong low-level wind can cause fog to dissipate both at the fog top and near the surface. At the fog top, winds entrain warmer, drier air from aloft into the fog. Near the surface, winds cause mixing of the surface-warmed air with the fog above. Both promote evaporation of fog droplets and improved visibility.

Click image to view animation.

Moderate to strong low-level winds dissipate fog

Cold advection above the fog layer can also dissipate fog by weakening the fog-top inversion, which enhances mixing processes.

Note: On the other hand, advection of drier air above the fog top can enhance radiative cooling.

Click image to view animation.

Cold advection weakens the fog-top inversion

Overlying Cloud Layers at Night

At night, loss of radiant heat is most rapid when there are no clouds above an established fog layer. If a broken or overcast layer of mid-level or a thick layer of upper-level clouds is introduced, fog-top cooling decreases because less radiation is able to escape the atmosphere. This effect can slow the rate of new droplet formation and contribute to fog dissipation.

Click image to view animation.

The presence of upper-level clouds decreases fog-top cooling

Dissipation Phase > Questions

Advection Fog

The other major fog type is advection fog. Advection fog can result from various scenarios. In one scenario, advection fog forms when cold air advects over a warm water surface. As the cold air moves over the water surface, water evaporates into the air mass and increases the near-surface moisture content. This results in fog formation in the lower levels. This type of advection fog is commonly referred to as steam or evaporation fog.

In the situation where warm air advects over a colder water surface, the warmer air cools down to its dewpoint causing fog to form. Some moisture may also be introduced into the air mass in the lower levels, but the main process at work is cooling of the air mass to saturation through conduction and light turbulent mixing. This type of advection fog is commonly referred to as sea fog.

Advection fog forms primarily through boundary layer dynamic and adiabatic processes, and is dominated by synoptic-scale processes that affect the lifetime of the event. Radiative processes still play a role in its development and life cycle, but are not dominant. Formation can occur with light to moderate winds in the low levels.

Advection fog events occur most frequently in the warm season when oceanic high pressure ridges persist. Advection fog or stratus forms in response to poleward flow around the western periphery of these ridges. This is especially true if the climatologically favored flow follows long trajectories over a warmer ocean environment before passing over rapidly cooling Sea Surface Temperatures (SST). Thus, regions with strong SST gradients are high frequency areas of extensive marine fog and stratus. The graphic below shows the frequency of sky-obscuring fog over the world's oceans. Note the high frequency of fog over the northwest Atlantic and northwest Pacific in June-July-August and southwest Atlantic in December-January-February.

Preconditions

Conceptual Model of Advection Fog Preconditioning

In this graphic, the left-hand side is a cross-section through the area, and the right-hand side is a plan view. You can see the cooler air mass advecting in and replacing the milder and relatively drier air. As it advects in, the cooling of the surface and the upslope flow also force the air up the terrain feature. And finally, the large-scale subsidence aloft is conducive to the formation of fog and stratus in the region.

Elements of advection fog formation

Example of Sounding for Advection Fog

This is a typical sounding for an advection fog event. The sounding shows a saturated layer near the surface, a somewhat drier air aloft and a moderate shear across the inversion.

The moderate shear and decoupling of the boundary layer from the air above it is typical of both radiation and advection fog. You are also seeing warm advection above the boundary layer. This warm air advection is going to help not only establish, but also continue this inversion that is locking in the saturated air.

Evolution of a Sounding for Advection Fog

The next graphic shows the evolution of the sounding associated with this situation. The cool air moves in and is forced up the terrain, cooling the low levels of the atmosphere. We see this in the change of the temperature profile in the skew-T diagram between the first and second panels. As that cold air continues to move into the area (3), the vertical structure of the atmosphere continues to modify; and we see the inversion set up in the boundary layer. There are still relatively strong winds in the boundary layer. In the last panel (4), the winds have decreased, and there's a saturated region with an inversion that is trapping that moisture in the boundary layer.

Advection fog development

Conceptual Processes Involved in Advection and Radiation Fog

The conceptual graphic below shows the differences between radiation and advection fog processes. With radiation fog, the radiative processes dominate the situation. You can see cooling of the ground surface and the adjacent water surface. The water surface also acts as a moisture source, so thicker fog forms over the water area than over the land surface. Again with radiation fog, local-scale processes are more important than with advection fog.

Radiation and advection fog elements

With advection fog, large-scale mass transport dominates the situation. In this example, advection occurs off the large water body and thin fog develops just onshore. A more dense fog develops along the slopes as this moist air is being forced up the terrain. The densest fog forms where we have advection onshore, over the lake surface (which increases the moisture in the air mass), and then up the terrain feature. With advection fog, again, the large-scale transport dominates the situation, but local features have a secondary effect. The local and radiative processes are secondary, but they certainly are affecting the intensity of this event.

Use this skew-T diagram to answer the following questions:

Question 1

Choose the best answer.

Do you think this is a radiation fog event or an advection fog event?

a) Advection fog event

b) Radiation fog event

The correct answer is a).

The key factor in this case that gives away the dominant process at work is the wind speed. If you have strong winds and veering winds with height (indicating that some dynamic and warm advection processes may be involved), then you probably have an advection fog event.

Dissipation

The dissipation processes for advection fog are the same as for radiation fog but its duration is dependent on the strength of the formation processes. For example, advection fog is likely to dissipate as the upslope wind dissipates.

For more details see the Dissipation PhaseDissipation Phase section in the radiation fog portion of this module.

Summary

The table below summarizes the processes that dominate radiation and advection fog events. A key feature of advection fog events is that the boundary layer can tolerate relatively high wind speeds while still maintaining the fog event. In contrast, radiation fog events are normally associated with boundary layer winds less than five knots because windier conditions would cause entrainment of drier air aloft and would mix out the saturated conditions closer to the surface.

Radiation and Advection Fog Characteristics

This table deals with the different characteristics of both radiation and advection fog.

Note: The delineation between fog types is not always straightforward because over time, fog that is formed primarily by radiative processes may be supported by dynamic or advective processes, and vice versa.

Additional Fog Types

Now that we have reviewed the two most common types of fog, let's consider the following additional types:

Terrain-Induced Fog

Upslope Flow

Another fog or low-stratus forming mechanism is associated with the development of upslope or terrain- induced flow. As moist air is forced upslope, it cools adiabatically and saturation may result.

Blocked-Flow Fog/Stratus

Cool moist “on-range” flow may be blocked by a mountain range if the flow speed (kinetic energy) is not great enough to lift air parcels upslope against the affects of negative buoyancy. The upslope ascent initiates saturation on the slopes of the mountain.

Valley Fog

Valley fog forms when air near the terrain height cools, usually by radiation at night. As the air cools it becomes denser than its surroundings and sinks towards the centre of the valley. This results in the creation of a pool of cold air at the valley floor. Fog formation occurs if the air is cold enough to reach its dewpoint temperature.

Rain/Post-Frontal Fog

When precipitation falls through dry air the liquid drops or ice crystals evaporate or sublimate directly into water vapour. The water vapour increases the moisture content of the sub-cloud layer while cooling the air. Fog can form as a direct result of precipitation falling through the boundary layer, or subsequently due to the increase in the amount of moisture within the boundary layer. Fog can form once the associated precipitating clouds clear and overnight cooling ensues.

Let's review the processes associated with all four types.

Terrain-Induced Fog

Terrain has many effects on fog and low stratus development, maintenance, and dissipation. Some terrain features enhance the fog or low stratus and some act to inhibit its occurrence. In this section, we will address the primary terrain influences, including upslope, blocked, and downslope flows, and mountain/valley circulations. In particular, we'll address terrain effects on low-level inversions, moisture, and turbulence.

Upslope and Blocked Flow

Upslope and flow blocked by terrain barriers often go hand-in-hand and can enhance fog and low stratus development. Blocked flow, also known as cold air damming (CAD) can occur on any windward slope.

Purely upslope flow without blocking (shown in the top panel) results when there is sufficient flow for the air to make it over the terrain barrier and down the leeward side. In these cases, the air may not be as stratified as in blocked cases, and the upslope flow will be stronger. However, the mechanical uplift provided by the mountain barrier can still cause lower ceilings, fog, and precipitation. Much of the impact on ceilings and visibility is found near the crest of the terrain barrier. In these situations, the clouds, fog, and precipitation can quickly dissipate once the upslope component ends.

Blocked flow is the response of the atmosphere when the flow is not sufficiently strong to go over the topography and instead is blocked by it (shown in the bottom panel). This situation occurs when the atmosphere is highly stratified and/or the flow toward the mountain barrier is relatively weak. Much of the impact on ceilings and visibility is found along the lower portions of the terrain and upstream of the terrain barrier. The low ceilings are found at the level of the minimum lifted condensation level (LCL) as required for cloud formation.

These events are often long-lasting and self-maintaining. A blocked or cold air-damming event is shown in the picture below. Note how the cloud tops are well below the terrain crests. In the process discussion that follows, we will focus on upslope flow with blocking since this situation provides the greatest and longest lasting impact on ceilings and visibilities and presents the greatest difficulty in forecasting the end of the event.

Process Discussion

Click image to view animation.

The presence of upper-level clouds decreases fog-top cooling

Flow blocking is not an instantaneous event, but one that establishes over time as the flow begins to interact with the mountain barrier. Once established, it has a characteristic structure that is slow to change. Because of its persistent, self-perpetuating nature, flow blocked by terrain can lead to long-lasting fog and stratus events.

Consider the situation of the near-surface flow encountering a long mountain barrier as depicted in the diagram to the right.

At some point, the easterly flow begins to be affected by the mountain and slows down. As it slows, the pressure gradient becomes stronger than the Coriolis force, and the wind must turn towards lower pressure. As this happens, the strength of the flow toward the mountain decreases, and it no longer has enough momentum to go over the barrier. As a result, it continues to turn more toward lower pressure. The evolution of the flow as it develops around the barrier requires that the air near the mountain develop a different structure than air further upstream.

The vertical structure is depicted in the cross section on the left, which shows the blocked region to be a wedge of air near the mountains, capped by a sloping layer of higher static stability (the blue line). The air is forced up and over the blocked region. The cooler and moist air within the blocked region simply flows along the barrier toward lower pressure.

For a given mountain height (h), the stronger the stratification, the further upstream that the mountain barrier effect can be felt. The actual distance (L) also depends on the incoming flow speed. The upstream distance is less for stronger flows toward the barrier, and in some situations, the upstream distance may barely extend beyond the base of the barrier slope. The COMET Web module Flow Interaction with Topography contains a more in-depth presentation on this topic.

In cases of blocked flow, the air mass under the inversion can quickly cool and saturate due to a combination of adiabatic cooling, cold advective and diabatic processes. Fog and/or low stratus can quickly develop in blocked flow and can persist for several days until the flow weakens or larger-scale downward forcing flushes out the stratified surface layer.

Once fog or low stratus has formed, it is continuously regenerated as long as flow persists across the top of the cold layer and/or until the cold layer is sufficiently warmed and moistened. In the first case, the synoptic flow will play a key role. In the second case, the boundary layer settling becomes the most important factor. Also in the second case, fog/low stratus may persist for several days in spite of synoptic variations.

Dissipation

Forecasting the conclusion of a CAD event can be as difficult as accurately forecasting its onset. Clues that cold air damming and upslope flow are decreasing include:

• Evidence of warming (or warm advection) at the surface, including equatorial winds
• Falling surface pressures in the cold air
• Cold air depth decreasing.
• A pressure gradient (or pressure changes) conducive to westerly flow that would decrease the depth of the cold air mass
• For CAD with overrunning easterly flow, look for decreased strength of this overrunning flow
• For CAD with equatorial or westerly overrunning flow, look for subsidence aloft (i.e., subsidence within the overrunning flow)
• Snow cover decreasing and/or rainfall decreasing and/or ground surface warming

Some useful fog forecast tools to use include:

• Surface pressure analyses
• Upper air charts
• Soundings
• Profiler data
• Satellite images
• Boundary layer and surface features

Valley Fog

Mountain/Valley Breeze

Mountain and valley breezes are components of a local wind pattern that routinely develops along mountain slopes. Their formation is favored under weak synoptic pressure gradients, such as large high-pressure regimes. The biggest potential forecast concerns associated with mountain-valley breezes are gusty surface winds and light turbulence, mountaintop convection during the day, and nighttime valley fog and stratus.

Mountain and valley breezes are two complementary components of a diurnal circulation. Their formation and intensity depend on the surface temperature contrast created by daytime heating and nighttime cooling, which are both enhanced by clear skies and dry ground. Slope orientation to the sun, as well as the direction of the prevailing synoptic flow, also affect the strength and evolution of mountain-valley breezes. For example, in the Northern Hemisphere, valley winds tend to be strongest on south-facing slopes and weaker or even absent on north-facing slopes.

Process Discussion

Typically, local mountain winds begin as the morning sun heats south- and eastward-facing slopes (in the Northern Hemisphere). In turn, the air above the slopes also warms and starts to rise, causing a valley breeze up the mountainside. This produces compensating subsidence over the valley and/or leads to a deep turbulent mixed layer that homogenizes the air. The initial rising motion combined with high relative humidity sometimes result in low clouds and fog that 'climb' the mountain slopes as the circulation first becomes established. Given adequate atmospheric instability and moisture, this process often breaks the inversion, breaks up the valley fog, and triggers the first convection of the day.

In the afternoon, the valley breeze reaches its maximum intensity (6 to 12 knots or stronger, especially off the surface) and can contribute to thunderstorm development.

Click image to view animation.

At night, the circulation reverses itself as a drainage current and becomes a concern for fog and stratus formation. The mountain slopes lose their heat, cooling the air in contact with them. This air becomes denser than the surrounding air, continuously sinks down the mountain slopes, and initiates weak compensating rising motions in the valley. This flow pattern is called a mountain breeze or a drainage wind.

Throughout the night, the valley continues to cool due to both drainage and radiative heat losses. These enhance a strong inversion that normally decouples the mountain breeze from the synoptic-scale winds. As the boundary layer becomes isolated from the flow, fog may form in the valley, especially when the valley contains a local moisture source.

An excellent example of valley fog persistence is seen in the visible satellite image shown here. Note the fog in many of the valley locations. Also, note how the fog has dissipated in some of the smaller valleys but persisted in several of the larger valleys. This is likely because these valleys are colder and deeper, and it is more difficult to break the valley inversion.

Valley fog persistence: Fog in the Po valley and in the valleys of the southern Alps.

Rain/Post-Frontal Fog

Pre-Warm Frontal Events

Fog and very low ceilings often form in the cold wedge of air beneath and adjacent to a warm front boundary as the warm air overruns the cold wedge. Fog and stratus are especially likely if precipitation is falling through the cold air. This allows the underlying cold air to become saturated through a combination of evaporational cooling and moisture advection into the layer. In these situations, precipitation is often observed in conjunction with very low ceilings or fog. Visibilities can be reduced to below airport and flight minima, especially if the underlying surface is very moist and cold, such as with the presence of snow cover.

Click image to view animation.

Below are the three basic types of low-pressure systems that are prone to forming pre-warm frontal fog.

One type is caused by slow moving, non-intensifying lows or open waves with a flat orientation to the isobars. The fog forms in a broad zone poleward of the warm front and near the weak low center.

The second type is caused by lows where there is a distinct surface flux of cold air toward the warm front. This frequently happens in cold air damming situations. Fog or low ceilings form in the cold wedge along and poleward of the front and can be extensive.

The final type is caused by filling lows that are still producing rainfall. Fog or low ceilings tend to form in extensive areas poleward of the occluded or warm front as gradients weaken and evaporational cooling allows saturation. These features also produce fog in and around the filling low center as winds weaken and convergence occurs.

Note: Lows that are intensifying do not usually produce much pre-warm frontal fog. In fact when a low is intensifying, any existing fog tends to dissipate due to increased pressure gradients, vertical motions, and subsequent mixing.

Post-Cold Frontal Events

In the cool season, low stratus and fog often form within the cold air behind a cold front. Post-cold frontal stratus and fog occur when a shallow dome of cold air advects into a region and is overrun by moist, warm air aloft.

This is situation is analogous to a cold front displaying anabatic characteristics (also known as an anafront), where there is upward vertical motion behind the front resulting in extensive clouds and some precipitation. Typically, a cold front displays katabatic characteristics where there are downward vertical motions behind the front, which will tend to inhibit any stratus or fog development.

Click image to view animation.

In certain synoptic conditions, multiple processes such as upslope flow and cold air damming may be responsible for the fog formation. The presence of multiple processes contributing to a post-cold frontal fog or stratus event often makes it hard to define what is the specific cause. In cases where the cold air is deep enough to produce precipitation, the event is similar to the pre-warm frontal situation in that warm overrunning precipitation is falling into the cold air and causing saturation of the surface-based cold layer. Differences between the two, however, include the following:

• Cold air is typically advancing, while in warm frontal situations it is often eroding or retreating
• Fog/stratus areas can be widespread and progress equatorward behind the cold front
• In the winter, precipitation is often of the mixed, freezing, or frozen variety
• The low-level air mass and surface may be pre-conditioned from pre-cold frontal precipitation
• Visibilities and ceilings often fall below minima in fog closely behind the front

Stationary Fronts

Stationary frontal boundaries can provide a focus for fog and stratus development, especially if the boundary is oriented east-west. The basic processes are similar to those described in the warm and cold frontal situations with the presence of warm advection aloft and a colder underlying air mass. However, as the graphic illustrates, a stationary boundary implies flow nearly parallel to the front, at least through the depth of the frontal layer. This means that advection is likely to be neutral or weak in this layer. Above the frontal layer, however, warm advection can occur and produce overrunning precipitation, sometimes equally north and south of the boundary. In these cases, much of the fog and stratus will be found along and on either side of the boundary.

Other considerations with stationary front fog events include:

• Convergence along the stationary front may help form fog and low stratus, even equatorward of the front. As a result, fog and stratus can occur on either side of the front for different reasons
• Little movement of the front suggests a tendency for the event to be persistent
• Fog or stratus may not break up until one of the following events occurs:
  • The front dissipates
  • The moisture source is shut off
  • The front advances or retreats due to approaching synoptic-scale forcing
  • An isothermal layer is established and fog lifts to a low cloud deck and then to a higher base overcast

Forecast Considerations

The following table provides an overview of some of the more important factors to consider when faced with potential fog or stratus associated with a frontal feature.

Frontal Fog/Stratus Events

Summary

Terrain Influences

• Mountain/Valley Breeze
  • Important Ingredients include:
    • Adjacent mountain-valley terrain features
    • Weak pressure gradients and synoptic flow (< 11 kts), particularly near the surface
    • Mostly clear skies which allow for daytime heating of mountain slopes and strong nocturnal cooling
  • Valley fog caused by a mountain breeze is favored by:
    • Clear skies
    • A nocturnal inversion
    • A local moisture source or enhancement of local moisture

• Upslope/Blocked Flow (Cold Air Damming)
  • Upslope flow—the mechanical lift provided by the mountain barrier that can cause low ceilings, fog, and precipitation
    • Ceilings and visibility are most impacted near the crest of the terrain barrier
    • Clouds, fog, and precipitation can quickly dissipate once the upslope component ends
  • Blocked flow occurs when the atmosphere is highly stratified and/or the flow toward mountain barrier is relatively weak, or both
    • Impact to low ceilings and visibility is mainly along the lower portions of the terrain and upstream of the barrier
    • The air mass under the inversion can quickly cool and saturate
    • Ceilings will be at the level of the minimum LCL
    • Events establish over time and are often long-lasting and self-maintaining as long as flow persists across the top of the cold layer and/or until the cold layer is sufficiently warmed and moistened
    • The distance that the mountain barrier effect can be felt upstream depends on the mountain height, the strength of the stratification, and the incoming flow speed
    • Clues for dissipation:
      • Evidence of warming (or warm advection) at the surface, including equatorial winds
      • Falling surface pressures in the cold air
      • Depth of cold air decreasing
      • Pressure gradient (or pressure changes) conducive to westerly flow that would decrease the depth of the cold air mass
      • For CAD with overrunning easterly flow, look for decreased strength of this overrunning flow
      • For CAD with equatorial or westerly overrunning flow, look for subsidence aloft
      • Snow cover decreasing and/or rainfall decreasing and/or ground surface warming
    • Sometimes after scouring, the boundary layer immediately sets up for a radiative fog event
    • Downslope Flow
      • Can quickly dissipate fog/stratus by adiabatic warming, warm advection, and the drying of the air mass
      • Initial mixing of two different air masses can result in a brief period of "mixing fog"
      • Look for:
        • Weakening of initial easterly upslope flow, inversion depth, and inversion strength
        • Decrease in depth of easterly upslope
        • Decrease in depth of low-level saturated layer
        • Westerly winds in low levels
      • Remember:
        • To incorporate model biases in the forecast process
        • Weak events with only low cloud cover are susceptible to erosion by insolation and mixing from above

Frontal Events

• Pre-Warm Frontal
  • Fog/stratus often forms in the cold wedge of air beneath and adjacent to the warm frontal boundary as warm air overruns the cold wedge
  • Fog/stratus is especially likely if precipitation is falling through the cold air
  • Low-pressure systems that typically form pre-warm frontal fog include:
    • Slow moving, non-intensifying lows or open waves with a flat orientation to the isobars
    • Lows where there is a distinct surface flux of cold air toward the warm front
    • Filling lows that are still producing rainfall
  • Intensifying lows do not usually produce much pre-warm frontal fog

• Post-Cold Frontal
  • Post-cold frontal fog/stratus occurs when a shallow dome of cold air advects into a region and is overrun by moist, warm air aloft
  • Analogous to a cold front displaying anabatic characteristics
  • Multiple processes may be responsible for the fog formation
  • Where cold air is deep enough to produce precipitation, the event is similar to a pre-warm frontal situation (warm overrunning precipitation falls into the cold air, causing saturation of surface-based cold layer)
  • Fog/ stratus can still form where the cold air is too shallow to create precipitation

• Stationary Fronts
  • Contain warm advection aloft with a colder underlying air mass. They also have a flow that is nearly parallel to the front, at least through the depth of the frontal layer.
  • Neutral or weak advection is likely within the frontal layer
  • Warm advection can occur and produce overrunning precipitation above the frontal layer. In such cases, much of the fog and stratus will be found along and on either side of the boundary
  • Convergence along stationary front may help form fog/stratus
  • Events tend to be persistent