Predicting Snow

Predict Snow - Understanding 'Thickness'

Being able to understand and forecast snow conditions in winter is a useful skill to have and can help make sure you get the most out of your winter walking experiences. The helpful advice below has been provided by Andrew Bateman from Mountain Innovations Walking Holidays and is a great introduction to understanding more about Scottish weather and forecasting whether snow will fall.

If you look at the above image you will find one of the Met Offices weather charts. It shows Mean Sea Level Pressure (continuous black isobar lines) and Thickness which is indicated by the numbered dashed lines. It's these thickness lines which are interesting when it comes to predicting snow fall. Thickness is the term used to refer to air masses of particular densities. The colder the air mass, the denser it will be and so the air pressure will decrease more rapidly as you rise up through it when compared to warmer air.

Meteorolgists have been able track the movement of cold air down from the polar regions by monitoring this phenomenon. They send up weather balloons and note the altitude when the air pressure has halved (around 500mb). The lower this occurs, the denser - and so colder - the air. From these measurements they create these thickness lines to mark the southern limit of these various cold air masses. Of the various thickness lines produced, meteorologists have come to realise that the line marked 528 is very likely to produce snow at low levels.

So, in the above example, if the 528 crosses Britain you can be pretty sure of snow at low levels and if it comes to within 200 miles of the British Coast you can expect snow on at least our northern hills. In this example, looking at the attached chart, by Friday there's going to be snow on the Cairngorms (528 line shown looping over Shetland).

The great thing is that these charts are produced 5 days in advance, so a quick look on Monday can give you plenty of time to plan for your snowy mountain trip at the weekend.

To access the Met Office charts showing these thickness lines go to the portal: www.cygmet.com/coolmenus/uk1.htm and click on any of the UKMO's 36 to 120 hour Prognosis.