Foehn effect: Temperatures soar to 16C in Wales

On Wednesday February 14th, temperatures in Wales soared to 16C, aided by something called the Foehn effect.

The Foehn effect is simply a change in conditions from wet and cold conditions windward (upwind) of a mountain, to dry and warm conditions leeward (downwind).

In play across Wales, we had brisk southerly winds blowing in. This moist air mass encountered the mountainous region of the Cambrians, forcing the air upwards.

This air then cooled and condensed to form clouds and precipitation upwind. As it rains, the air loses moisture, and warms as it descends windward.

Given this side of the mountain is also drier, it is also sunnier and so receives more solar radiation allowing temperatures to rise further.

In this instance, whilst they reached 15.8C in Gogerddan, Aberystwyth and 15.7C in Trawsgoed, they were just 7C or 8C nearby.