Safety & Training

SCOTTISH WEATHER

Part1: An Inversion to Fog

By Alison McLure

This issue we start a series of occasional articles about the vagaries of the Scottish weather written by one of our volunteer executive committee members Alison McLure. In a former life Alison was a weather forecaster for the Met Office.

Have you ever woken up early and blearily looked out the window to see another dreich, foggy day, having eagerly packed up your hillwalking/climbing kit the night before? Many people return to their bed to wait for a better day where their navigation skills won’t be so tested. If you do that, you could be missing out on one of those gems of days in Scotland, where once you climb through the mist, the mountains are so clear, you think you could run round them in a day and where the glens are blotted out by a blanket of fog, as if there is nothing else in the world other than mountains.

There are two types of weather situation, which bring foggy days; one is when high pressure is over or close to Scotland. This brings a spell of fairly settled weather and, in the winter, often very cold conditions. The cold air falls into the glens and condenses into water droplets creating fog, especially overnight. This is sometimes called an inversion, because air is usually colder the higher you go, but in this case the rule is “inverted”. This type of fog mostly happens in November and December, where there is not much sunshine to break it up.

Above the fog there is often clear sky and a wonderful day to be had on the hills, with the possibility of brocken spectres. These are formed when the sun is behind a person on a ridge or peak and throws a shadow onto the cloud below. You can also see rings and glow around your shadow, which are formed by scattering of the light on the cloud droplets.

Another common “foggy situation” in Scotland is caused by haar (sea fog). This is where the sea is cold enough for water droplets to condense and create fog. Wind then pushes the fog inland. This tends to happen in the spring with northeast to southeast winds, mainly because the North Sea is at its coldest. Haar affects much of Eastern Scotland in these circumstances, but if you get a chance to look at satellite imagery, you can see fingers of fog reaching west through the glens. Again, there can be clear skies above and often the sun is high enough for it to clear the fog from the hills.

As always with rules in the weather, there are exceptions! Many a day I have smugly leapt out of bed proclaiming confidently that it’s an “inversion day” to find the mist following me up the hill! Despite that, it’s still worth trying, even if your day does turn into a navigation exercise.