How does fog happen in the cold temps of Winter?

Answer 1

This is a piece I wrote about fog for a local radio station:

A forecast of "frosts and fogs" is common in winter but all weather presenters dread it. There is not one of them who has not said "fosts and frogs" at least once. They need not worry in our area for a few months. After some of the warm sticky nights we have had, a frosty morning would be welcome but we will have to wait. Foggy mornings are possible still and dew is often around in the mornings. Although fog, frost and dew are very different, they are all formed by the same process.

The Sun rises each morning and heats the Earth. This heating continues until the Sun sets. We have to wait until the next day for further heating. While the Earth is absorbing energy from the Sun, it is also radiating energy back out to space. Over a year, the amount of energy lost equals the amount of energy received so the temperature of the planet remains stable. On a daily basis there are wide fluctuations in temperature.

The air in the troposphere, the lowest layer of the atmosphere and the layer where all the weather is, is warmed and cooled by contact with the surface of the Earth. The surface warms up with energy from the Sun . This warms the air above it which, in turn, warms the air above that and so on. This is why temperature decreases with height through the troposphere. At night there is no input of energy so the Earth cools down, radiating the heat into space. The cooler surface cools the air in contact with it which, in turn, cools the air above that. Sometimes on clear nights with little or no wind the temperature can fall quite rapidly. This cools the air near the ground rapidly too such that the air close to the ground is cooler than the air above it. We expect the temperature to decrease with altitude. This reversal of the process over a short distance is called a temperature inversion. They are very common in winter.

As the sun gets lower in the sky, there is a point during the afternoon where the outgoing terrestrial radiation exceeds the incoming solar radiation. This is around 3-4pm. At that point the surface stops warming and begins to cool. This is the time of the maximum temperature for the day. The Earth continues to cool through the night until the Sun rises. Two or three minutes after sunrise, the incoming solar radiation exceeds the outgoing terrestrial radiation and the surface starts to warm up. This point, shortly after dawn, is the time of the minimum temperature.

The air we breathe is a mixture of gases. One of these gases is water vapour. The amount of water vapour in the air is negligible in the desert but could be as much as 3% of the air on a tropical coast. Water vapour is an invisible gas. The water molecules tend to clump together to form water droplets and water droplets split apart to form water vapour. This process goes on all the time. When the rate of evaporation exceeds the rate of condensation, we have clear air. When the rate of condensation exceeds the rate of evaporation, the condensed water droplets form cloud or fog or dew. The temperature of the air affects the rate of evaporation. The warmer it is, the more evaporation takes place. The cooler it is, the more condensation takes place.

For any given amount of water vapour in the air, there is a temperature at which the rate of evaporation equals the rate of condensation. This temperature is called the Dew Point. As air rises, it cools. As soon as it cools below the dew point, water droplets condense and accumulate to form cloud. The dew point temperature lies at a relatively constant level so clouds have flat bottoms.

At night, the surface of the Earth cools which cools the air above it as we have seen. With no wind, the air close to the surface will cool rapidly but the air above that will cool more slowly. When there is a layer of air close to the surface that cools below the dew point, the water vapour condenses and the water droplets coat the surface to form dew. Objects that cool faster than the earth, such as your car, will get covered in dew before the surrounding area. We have seen that the coldest temperatures are at dawn. This is the time most likely for the formation of dew although it can occur earlier.

When the air is cooler and drier to start with, the dew point temperature is sometimes below zero degrees. If the surface cools to this temperature, the water vapour does not condense to form water droplets but sublimates to form ice. Sublimation is the process of changing from a gas to a solid without going through the liquid phase. The ice forms on the grass, trees and your windscreen and we call it frost.

Dew and frosts require very still air and clear skies. The clear skies allow the radiation to go straight out to space. If the air is moving very slightly, the very cold air close to the ground gets mixed with the warmer air above it. This could lead to air falling below the dew point temperature through a greater depth. When this happens, the water vapour condenses into water droplets that are suspended in the air. We can see it and we call it fog. Fogs are most common near lakes and rivers where there is plenty of moisture in the air and on clear mornings after rain. A fog that forms in this way is called a radiation fog. Sometimes a fog can form and get blown somewhere else by a light wind. This is called an advection fog.

If the wind is too strong in the morning, the cold air near the surface is mixed with a greater amount of warm air and the temperature doesn't drop below the dew point so there are no frosts and fogs on windy days. Finally, because clear skies and light or no wind are necessary for the formation of dew, fog and frost, any of the three is a good indicator that a fine day is to follow.

Answer 2

Heat rising from the earths surface combined with the moisture in the air and the cool temperatures of the air, makes fog.

Answer 3

Advection fog can result when a moist body of surface air is moved by air currents to an area with a cooler surface causing condensation.

Answer 4

The coldest parts of winter or the coldest parts of California? If you mean California, I'd guess the Lake Tahoe area though nights in the desert get pretty cold, too.

Answer 5

the ground still holds heat if it had been warm lately. so that heat rises and make fog

Answer 6

Warm moist air sinking to the ground.