Weatherwatch with Irv Lee

Radiation Fog

Everyone in the UK is used to seeing dew on grass in the evening or morning. It can happen at any time of year, more usually when the pressure is high rather than low, and often heralds the beginning of the end of summer.

What you see in dew is radiation fog which never got airborne. To understand radiation fog, let's look at the properties of air. Air can dissolve water vapour and hold it invisibly. How much it can hold varies with temperature: the warmer the air, the more it can hold.

If the air has been warm enough to hold plenty of water but then cools, the water vapour 'comes out' of the air in some form. Air is transparent, and can't absorb heat directly from the sun. It can only take heat from the other two means of energy transmission: conduction and convection.

When the ground is warming up from direct sunlight, both conduction and convection transfer heat to the air. When the ground cools, convection is suppressed, and the lowest levels of air are cooled by conduction, or if you prefer, by contact with the ground alone. Higher pressure systems normally have less pressure gradient - the isobars are further apart, meaning lighter winds. They are also capable of producing clear skies, being almost always devoid of frontal activity.

Combine this knowledge about air with the properties of sea and land. Sea temperatures hardly vary between day and night, but change gradually through the seasons. Land quickly heats up in direct sunlight on cloudless days, absorbing heat radiated by the sun. The air at lower levels is warmed by conduction and convection, and can absorb a large amount of water vapour. Conversely, land cools quickly when the sun sets on cloudless days, through radiation of heat energy into outer space.

The air in contact with the cold ground cools by conduction. This forms a temperature inversion, and whatever wind there is in a high pressure system becomes calmer nearer the surface.

So where does the fog come from? Well, if there is no wind at all, the water vapour in the air at levels close to the ground comes out as it rapidly cools and is unable to hold it. The result is dew on the grass in the evening. This dew stays all night and only disappears when the sun warms the ground again the next day.

In winter, the dew will form frost as it comes out of the air. If there is a slight wind, say between three and seven knots, then there is enough stirring and turbulence to maintain the small water droplets in the air near the ground, and fog is formed. If the wind is over seven knots, there is enough stirring to cause the fog to lift into low stratus cloud between 1000 and 1500ft.

Coastal airfields can have different conditions when factors point to radiation fog inland. They can be clearer, as the sea helps prevent nearby land from cooling so fast, or offshore winds lead to a different local set of circumstances. However, coastal airfields should not be thought of as havens from fog - with the extra moisture in the air, denser, longer lasting fog is just as possible as no fog at all.

A typical succession of METARs for radiation fog formation:

EGXX 041750 041750Z 220/06KT CAVOK 12/08 Q1022

EGXX 041820 041820Z 200/05KT CAVOK 09/07 Q1022

EGXX 041850 041850Z 190/04KT 7000 06/06 Q1022

EGXX 041920 041920Z 190/04KT 2000 BR 04/04 Q1022 NOSIG

In aviation terms, MIST (BR) becomes FOG when visibility drops below 1000 metres.

This article first appeared in FLYER magazine's Spring 2000 edition