New light on how fog forms in mountainous areas

Of the world's various weather phenomena, fog is perhaps the most mysterious, forming and dissipating near the ground with fluctuations in air temperature and humidity interacting with the terrain itself.

Because it reduces visibility, fog poses serious hazards to the traveling public. For example, fog is the second leading cause of aircraft accidents after high winds. It leads to automobile crashes and disrupts ferry operations.

Thousands of people die every year in fog related accidents throughout the world!

 Today, most forecasting uses a computer model known as Numerical Weather Prediction (NWP), which processes massive meteorological observations with computer models to output predictions for precipitation, temperature, and all sorts of other elements of the weather. However the current computer model doesn't work well for fog, and researchers hope that improvements can be made using the masses of data they gathered over seven weeks in the winter of 2022 at several sites.

Fog involves a lot of physics processes so it requires a computer model that can better represent all these processes. Because fog is clouds near the ground, it requires a high-resolution model to resolve it, so we need models at a very fine scale, which are computationally very expensive. The current models (relatively coarser in resolution) are not capable of resolving the fog processes, and we need to improve the models for better fog prediction.

Valley fog is a perfect example of how topography and atmospheric processes converge to create a distinctive weather phenomenon.

The ground is cooling overnight while denser, cooler air drops from mountain tops collecting in the valleys, in a phenomenon known as "cold air drainage." Cooled by the ground, the dropping air temperature can approach the dew point, and if there is sufficient moisture in the air, fog begins to form, becoming the most dense around sunrise when surface temperatures are lowest. Winter nights create favorable conditions for different forms of fog, such as cold-air pool fog, ephemeral mountain valley fog and radiative ice fog.

By observing how these varying kinds of fog form and dissipate, the researchers are continuing to learn about the meteorological conditions and physical processes governing the formation of fog.

 Zhaoxia Pu et al, Cold Fog Amongst Complex Terrain, Bulletin of the American Meteorological Society (2023). DOI: 10.1175/BAMS-D-22-0030.1

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How different types of fog actually forms

Radiation Fog

This type of fog forms at night under clear skies with calm winds when heat absorbed by the earth’s surface during the day is radiated into space. As the earth’s surface continues to cool, provided a deep enough layer of moist air is present near the ground, the humidity will reach 100% and fog will form. Radiation fog varies in depth from 3 feet to about 1,000 feet and usually remains stationary. This type of fog can reduce visibility to near zero at times and make driving very hazardous.

Valley fog is a type of radiation fog. When air along ridgetops and the upper slopes of mountains begins to cool after sunset, the air becomes dense and heavy and begins to drain down into the valley floors below. As the air in the valley floor continues to cool due to radiational cooling, the air becomes saturated and fog forms. Valley fog can be very dense at times. This type of fog tends to dissipate very quickly once the sun comes up and starts to evaporate the fog layer.

Advection Fog

Advection fog often looks like radiation fog and is also the result of condensation. However, the condensation in this case is caused not by a reduction in surface temperature, but rather by the horizontal movement of warm moist air over a cold surface, such as warm moist air flowing over snow. Advection fog can sometimes be distinguished from radiation fog by its horizontal motion along the ground.

Freezing Fog

Freezing fog occurs when water droplets remain in the liquid state until they come into contact with a surface upon which they can freeze. As a result, any object the freezing fog comes into contact with will become coated with ice. 

Evaporation or Mixing Fog

This type of fog forms when sufficient water vapor is added to the air by evaporation and the moist air mixes with cooler, relatively drier air. The two common types are steam fog and frontal fog. Steam fog forms when cold air moves over warm water. When the cool air mixes with the warm moist air over the water, the moist air cools until its humidity reaches 100% and fog forms. This type of fog takes on the appearance of wisps of smoke rising off the surface of the water.

The other type of evaporation fog is known as frontal fog. This type of fog forms when warm raindrops evaporate into a cooler drier layer of air near the ground. Once enough rain has evaporated into the layer of cool surface, the humidity of this air reaches 100% and fog forms. 

Hail Fog

Hail fog is an unusual type of fog that forms shortly after a heavy hailstorm.  The cold balls of ice fall into warm, very moist air near the surface.  As the hail accumulates on the ground, it cools the air just above the ground to the dew point, resulting in fog.  The fog forms when winds are light, and it usually quite patchy and shallow.

Source: US  NWS

How to avoid accidents in fog and be safe :

If possible, avoid driving during foggy conditions. Use fog lights and drive at low speed if you must. Do not change lanes and avoid overtaking. Use low beam headlights as high beams reflect off the moisture droplets in fog making visibility difficult. Or use fog lamps. Follow roadside blinkers, if available. Maintain sufficient distance between your vehicle and the vehicle ahead. Drive at a controlled speed. Turn on your car heater as fog outside will cause condensation inside . Keep your vehicle’s windscreen and windows clean. Activate wipers and defrosters. Ensure your vehicle’s light indicators are working and use them appropriately. Blow horn repeatedly, especially while changing lanes and at turns.Do not stop your vehicle in the middle of the road in case of no visibility; pull over to the side of the road. Try to listen to the sounds of other vehicles. Avoid sudden manoeuvres. Turn on your vehicle’s emergency flashers and low beam headlights on. Do not apply sudden brakes to avoid a rear vehicle hitting you. Don't drink and drive. Drive closer to the kerb. Keep track of road markings, while driving. Stay focussed on the road. Always keep an emergency kit in your car.