Thursday, 15 January 2015

Wind Chill and Lapse Rate in British Mountains

Wind Chill and Lapse Rate

Walking from my house situated at 300m a.s.l. in the Peak District today I was struck by how cold and damp it was. O.K. it’s January in the U.K. but it felt really cold.

Suitably dressed for a cold day navigating on Kinder Scout
There are two common factors that have an effect on the temperature whilst walking in British Hills. Wind Chill and the Lapse Rate.

Why do we do it?
1.            Lapse Rate
A lot has been written scientifically and in depth about Lapse Rate. There is a good paper from Bangor University. 

However it is essentially the amount that air temperature decreases as you gain altitude. Although there are variables such as whether the air is dry or saturated, generally the temperature decreases around 6.5 degrees C per 1000m climbed.

So for example if you were in Snowdonia, let’s say in Bangor at Sea Level and you were walking to Snowdon via Llanberis.

Bangor                                0m a.s.l.            10 degrees C.

Llanberis                          110m a.s.l              9.3 degrees C.

Half Way Station              500m a.s.l            6.75 degrees C

Snowdon Summit            1085m a.s.l            3 degrees C

However the lapse rate does not take into account the chilling effect of the wind.



2.            Wind Chill
"Wind-chill or windchill, (popularly wind chill factor) is the perceived decrease in air temperature felt by the body on exposed skin due to the flow of air". Wikipedia

Essentially the stronger the wind, the colder it feels! 


Wind

Temperature (Celcius)



(mph)

10
5
0
-5
-10
10

5
-1
-7
-13
-19
20

0
-6
-13
-20
-27
30

-1
-9
-16
-24
-31
40

-2
-10
-18
-26
-34
50

-3
-11
-19
-27
-35
60

-3
-11
-19
-27
-35
70

-3
-10
-18
-26
-34
80

-2
-10
-17
-25
-33
90

-1
-9
-16
-24
-31


Dressed for the cold
So on our walk up Snowdon, if the wind in Llanberis was around 10 mph the temperature at 10 degrees C  would feel more like 5 degrees C.

As we ascend Snowdon, the wind will increase with altitude, let’s say to 20 mph, so the 3 degrees C would feel more like -10 degrees C.

Today in The Peak the temperatures were around 2 degrees C.
But with the wind forecast at around 30 mph it feels more like -15 degrees C.

-15 C no wonder it felt so cold!


So the moral of this is, if you are going out to walk in the hills or mountains in winter, have the right sort of clothing to keep warm and look after yourself.