'Where the wind's like a whetted knife…'
by Penny Johnson
John Masefield was going down to the sea again, rather than going up a
mountain, but he obviously knew a thing or two about wind chill. Wind chill is a
phrase often mentioned in weather forecasts, and even more often on OUMS
meets on top of an icy mountain top (usually in the same breath as a few
unprintable adjectives). The wind chill information given in forecasts is useful
information for the hill walker, as it helps you to work out how cold you might
feel. However, even the 'wind chill temperature' is not the whole story, as your
ability to withstand such temperatures depends on such things as your age and
physical fitness. If you are wet you will also cool down faster, which is why
garments that wick away sweat are so useful.
The table shows the actual air temperatures, and what air at that temperature
would feel like at different wind speeds.
Force
0
1
2
3
4
5
6
7
8
9
10
11
12
* based
Description
mph
calm
<1
light air
1-3
light breeze
4-6
gentle breeze
7-10
moderate breeze 11-16
fresh breeze
17-21
strong breeze
22-27
moderate gale
28-33
fresh gale
34-40
strong gale
41-47
whole gale
48-55
storm
56-65
hurricane
>65
on the Beaufort wind scale
20
27
22
19
18
16
15
14
13
13
13
12
12
12
Air
15
25
18
14
12
10
8
7
6
5
5
4
4
4
temperature (°C)
10
5
0
-5
22
20
17
15
13
9
5
1
9
4
-1
-7
6
0
-6
-12
3
-3
-10 -16
1
-6
-13 -20
-1
-8
-15 -23
-2
-9
-17 -25
-3
-10 -18 -26
-3
-11 -19 -27
-3
-11 -19 -27
-3
-11 -19 -27
-3
-11 -19 -27
-10
13
-4
-12
-18
-23
-27
-30
-32
-34
-35
-35
-35
-35
It is interesting to note that above wind speeds of around 40 mph, any further
increase in the wind speed makes little difference to how cold it feels. Hopefully
this is only of academic interest as I, for one, have no intention of walking
anywhere in winds that strong! The main message from this article is that a
windproof layer is as important as layers of fleece when dressing for winter
walking (as most of you will already know!). For those of you who are interested
in why the speed of the wind matters at all, read on.
Air feels cool or cold when it is colder than your body temperature. If the air
surrounding you is cooler than your body, heat energy will be transferred from
you to the air around you. The colder the air, the faster the energy is transferred.
If you are standing in still air, the warmth of your body heats up the air around
you. This warm air will gradually rise and carry the warmth away from you,
sucking in cooler air which heat from your body will warm up. This only happens
relatively slowly, though, which is why calm air actually feels warmer than it
really is.
If there is a wind blowing, the wind blows this warmed layer of air away much
faster, replacing it with cooler air. The faster the wind, the faster this warmed air
is blown away, and the colder it feels. So if the air temperature is 5°C and the
wind is 20 mph, it feels the same as it would do if the air temperature was -6°C
on a still day.
Wind chill does not affect how cold something will get if it is left out long enough.
Your body will never get colder than the air temperature (in Britain, at least).
Again, this is academic as on a winter's day you would have been dead long
before your body temperature got anything like that low. A windy day only feels
colder than a still day because the wind is carrying away the air around us that
our bodies have warmed up, and so we cool down faster. If something like a Sigg
bottle full of water starts off warmer than the air temperature it will lose heat
until it reaches the same temperature of the air. It will cool down faster on a
windy day, but it will never get colder than the air temperature, no matter how
fast the wind*. If the temperature is 5°C and the wind is 20 mph, you might feel
as if it is several degrees below zero, but the water in your bottle will not freeze.
If you are interested, this website has more info. The table on it is in Fahrenheit
(trust the yanks!) but there is a calculator at the bottom where you can select
your units, and also a page of useful FAQs.
http://www.weather.gov/om/windchill/
*not necessarily true on a rainy day, but that's another story!
[And if anyone with long memories remembers seeing an article like this in the
Cairn before, your memory does not deceive you! I was asked for a repeat by the
new editor.]