What is radiation fog

Arabia Weather - Radiation fog

How does radiation fog occur?

Radiation fog occurs at night (or occurs at the end of the night and the beginning of the day in the early morning hours) above the surface of the earth when the earth loses its heat by radiation and cools, and thus the air in contact with it cools, and in the case of the availability of water vapor and the arrival of the air temperature to or below the dew point, radioactive fog is formed .

With the condition that the wind speed is light and not static, so that the warmer air is allowed to pass over the cold surface and touch it, and thus cool down to or below the dew point.

But if the air is still (calm), then only the lower part of the warmer air close to the surface of the earth touches the cold surface of the earth, and in this case condensation occurs in a thin layer that may reach 60 cm in thickness from the surface of the earth, and here fog may form in contact with the surface of the earth in a very thin layer.

Or it may condense and form dew or frost (depending on the temperature).

Conditions for the formation of radioactive fog:

• Light winds: the wind speed is between 1-5 knots (about 8 km/h).

• High humidity up to 100%

• A clear sky where the sky is clear of clouds: (Long clear nights, so that there is enough time for the air to cool down to the point of dew).

Accordingly, radiation fog usually occurs during the winter and in late fall and early spring.

Radiation haze disappears from:

• Heating by the sun's rays , where the radioactive fog fades within several hours after sunrise, after the sun heats the air and the surface of the earth. Sometimes the fog is thick, blocking the sun's rays from reaching the surface of the earth, and thus the surface of the earth remains cold, and this condition usually occurs in the winter season and the fog can remain throughout the day.
• Increasing the wind speed: If the wind speed exceeds 8 km / h, this prevents the occurrence of radioactive fog or is a reason for its fading, because it works to mix the cold surface air with the warmer and less moisture air above it, and this prevents the air near the earth’s surface from decreasing in temperature. Its temperature reaches the dew point, and even if the case reaches the dew point and fog forms, the air will quickly dissipate the fog.
• Heating by descending air .
• Wind direction and gusts have changed from a new source.

Radiation fog is common and well-known, and it forms on the flat ground on clear and humid nights, and is usually associated with cases of high pressure.

But if the wind speed is higher than 15 knots and with the presence of air turbulence, this works to mix the air and often the fog rises to the top, forming very low stratus clouds.
Transitional fog can form very quickly during the day or night, and it is common in the winter season, when air masses move to land from the seas and oceans.