How do you forecast fog?

We know that fog is essentially a very low-lying cloud, and that for clouds to form the air must be cooled to a point where its moisture condenses out into a visible cloud. The cooling normally occurs as air rises, but in the case of fog we have to wait for the air to cool down to its fog point naturally. This will be very dependent on the time of year, how much cloud cover there is, the strength of the wind, the amount of moisture in the air and the geographical location.

The main types of fog are radiation, valley, advection, upslope and evaporation. Here we look at radiation fog, and the ideal conditions for its formation are clear skies, light winds and plenty of moisture. If there is a high moisture content in the air, the temperature will not have to fall as far to reach the fog point and there will be a greater risk of fog forming. This is often why fog forms more readily around lakes, rivers and reservoirs.

Winds need to be gentle enough to keep the moisture in the air at the right concentration. Too light and it’ll form dew; too strong and the moisture will be mixed through the air and probably keep temperatures too high.

Clear skies at night allow temperatures to drop as heat escapes more readily from the Earth’s surface. With the right wind, moisture and lack of cloud, the fog point will be reached.

However, in order to forecast the fog point for the coming night we need to know the maximum temperature reached during the day. Bearing in mind we may be forecasting fog a few days ahead, the maximum temperature will also need to be forecast.

To do this we must consider; the amount of sunshine and how strong the sun is at a particular time of year, the wind speed and direction and whether they will change, how much cloud there is, whether there will be any rain, how much heat the ground has absorbed, and so on.

If any of those variables is different, even by a small amount, it will have a knock-on effect throughout the forecast. With so many factors at play, it’s easy to see that fog is one of the trickiest weather phenomena to forecast.