Fog is really just cloud on the ground cutting the visibility to below 1000 metres, and dense fog is when you cannot see any further than 200 metres.

It is, arguably, the worst weather condition that affects the motorist, although snow and icy roads can be troublesome as well. The problem with fog is that it can be very patchy and you can never be sure that there isn't a dense patch or two around the next corner.

There are three major types of fog that we get in this country, namely radiation fog, advection fog and hill or upslope fog.

Our atmosphere is made up of many gases, one of which is water vapour. It can hold a certain amount of water in the invisible gas form at any given temperature, but if the air is cooled down it becomes super saturated and has to condense some out as water droplets. These water droplets we call cloud.

Radiation fog is formed on clear, still nights. The Earth radiates its heat out to space and the ground cools down. The air in contact with the ground also cools as well, and if it cools down sufficiently so that it is holding more of this invisible water vapour gas than it can hold at the lower temperature, some of it condenses out into water droplets and forms a cloud. Incidentally if the wind is calm, then the water droplets form on the ground as dew. If the wind is too strong then clouds form but are above the ground so for the cloud to be on the ground as fog the wind should be around 5mph.

You will often find radiation fog forming in valleys because, as the air on the hillside becomes colder and denser, it flows down the hillside into the valley where it collects and more readily condenses the water vapour into cloud or fog droplets.

So to get fog the air needs to be cooled down and advection fog is formed when very mild moist air moves over a cold ground. This can often happen in early spring with mild southwesterly winds moving in across a snowy or icy ground. The lower layers of the air get cooled down rapidly to below the temperature at which fog forms.

Hill fog or upslope fog, as its name implies, is formed as mild moist air is forced to ascend a hill or mountain range. As the air moves up the windward side of the mountain it cools down, and again if the air becomes saturated then cloud is formed which, if below the top of the hills, gives fog.

So, as a driver, when we have still cold nights in winter, spring and autumn be wary of fog forming late in the night especially in the valleys. Also if the forecast is for moist southwesterly winds coming in after a cold spell watch out for fog over the hills or another favourite place is on the western sides of hills in the warm sector between the warm weather front and the cold one.

Related Links:

- Weather Basics - Fog
- Winter Weather 1 - Rain, Drizzle and Fog
- Weatherwise - The Basics - Fog