Eva Crossan of Stirling asks :-

We often see sea fog form over the sea, then drift up the river before spreading over the land. Why is this?

Tony Trewinnard, of Blue Skies Weather and Climate Services Ltd, responded.

Fog is air where the water vapour is visible. Air contains water vapour all the time, but usually it is invisible. When the amount of water vapour becomes greater than the capacity of the air to hold it, it condenses out as tiny water droplets, and that is what we see as fog. The process is much the same as pouring water into a sponge - the sponge absorbs the water until it can hold no more, then the excess water drips out.

There are two ways air can get more water vapour in it. Firstly, water can be evaporated from puddles, streams, lakes, rivers or the sea. Secondly, air can be cooled. Cool air can't hold as much water vapour as warm air, and when air is cooled the water vapour condenses out - this forms the dew and frost on cold nights.

The sea is a great source of water - so air over the sea usually contains more water vapour than air over the land. That is one reason why fog can more easily form over the sea. But, the sea is also often colder than the land. So if air from the land moves over the sea, it will cool down and fog will form.

Sea fog will often "come ashore" over rivers as they are the wettest areas and more likely to evaporate moisture. They are also usually colder than the land. As the fog drifts over the land it looses moisture and warms a little, so the water droplets evaporate back into their invisible form as water vapour.