The wind chill factor
It's feeling chilly today with fresh to strong winds and heavy showers and cold enough for snow on the mountains.
At the time of writing, the temperature in Llanberis is 8 deg;C Celsius but at Clogwyn station, a mile below the summit of Snowdon, on the Snowdon Mountain Railway the temperature is below freezing with a severe wind-chill.
As a rule of thumb, the higher you go the colder, wetter and windier it gets and when air is forced to rise, it always cools.
I took this photo today of a slight dusting of hail/sleet/snow on the wind farm above Llandinam. Image by Bill Pugh.
The rate of cooling isn't constant but on average, temperature drops by around 2 deg C per 300 m (1,000 ft) of ascent. This means that at the summit, the temperature will be much cooler than in the valley below.
At 900 m (3000 ft) the wind speed is on average double that on low ground, and the overall wind-chill effect on a wet or perspiring human body can be very large. A wind of 40 mph and an air temperature of 3 °C will give a wind-chill temperature of -10 deg C.
Snow on Snowdonia again above Llyn Padarn with the 'window' in the Llanberis Pass which means more rain for us locals. Image by Brian Wakeham.
Mountains produce a number of potentially hazardous weather phenomena including: gales, storm-force winds, persistent heavy rain or blizzards raging at the summit whilst it's calm lower down.
You might also encounter ice and snow lasting well into spring and summer, especially on north-facing slopes away from the warmth of the sun.
So, do take care if you're heading off for a mountain walk. It really pays to be prepared for all weather conditions if you're out trekking at any time of the year but especially so in winter.