Why you get cold on hill tops

There are 2 main factors which make hill tops feel much colder than the valley you walked or climbed up from: windchill and lapse rate.

Windchill is a measure of the cooling effect of the wind on exposed skin. It has a very major effect on cooling, and is a very serious factor to consider when dressing for the outdoors, especially in cool or cold conditions. Normally pleasant or tolerable temperatures can quickly become life threatening in wet and windy conditions for those who are not adequately clothed. An indication of it's cooling effect is shown below:

Wind Speed		Temperature (°C)
km/h	mph	Temperature (°C)
0	0	0	-5	-10	-15
10	6	-2	-7	-12	-17
20	12	-7	-13	-19	-25
30	18	-11	-17	-24	-31
40	24	-13	-20	-27	-34
50	30	-15	-22	-29	-36
60	36	-16	-23	-31	-38

Lapse rate refers to the amount that air temperature drops as altitude increases. Temperatures quoted in weather forecasts usually refer to air temperature at sea level, where as the actual air temperature at altitude will be:

Temperature	Meters above sea level	Air type
Less 1°C	Per 100	Dry air
Less 1°C	Per 200	Wet air
Less 1°C	Per 150	Average
Taken from 'The Mountain Skills Training Handbook', Pete Hill & Stuart Johnston

An example
The summit of Ben Nevis (a very popular destination for many visitors to Scotland) is 1344m high. If the temperature in Fort William (which is at sea level) is a pleasant 15°C, due to the lapse rate, the temperature on the summit would be a cool 6°C in still, average air. Not too much of a difference, but add to this a modest 20km/h wind and the temperature will feel more like it's -2°C, especially if the clothing you are wearing is damp from sweat (it is quite hard work getting to the top!).

More about why we get cold

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