Wind Chill

Wind Chill

So you’ve heard of wind chill or 'feels like' on the BBC weather forecast, but do you know what it is?  I suppose that the name is a dead give-away, although you’d be surprised that there are a few methods of actually calculating it.

Convection
On a calm day with no wind, the actual air temperature is the temperature that you feel on your skin.  When the wind is blowing, it is responsible for transferring heat away from our bodies into the surrounding air, by a process called convection.  As wind speed, aka air velocity, increases so does the amount of heat transfer away from our bodies where skin is exposed to the wind.  Thus, for the same air temperature a person is cooled quicker if there is a wind, than without a wind.  Wind chill  is a means by which to try to describe to people the cooling effect of the wind in a language they understand, i.e. temperature, referred to as Wind Chill Temperature (or Factor).  A quoted ‘wind chilled’ temperature is not actually a temperature as such, but a means for us to relate to how cold we may feel if exposed to the wind against the skin.

In Practice

When you check the weather forecast and see the temperature and wind speed, the wind speed on the top of a hill is likely to be double or more of that in the valley.  If the temperature is cool too, there’s a danger that it’s going to feel a lot colder than you think.  For example, at 6°C with a 30 mph wind, it will feel like zero degrees on exposed skin, and at zero degrees with a 30 mph wind it will feel like -8°C.  Quite a difference to the quoted air temperature.  Also, don’t forget that the air temperature drops between 1 and 3 degrees for every 1,000 ft (300 metres) of height, so what you see on the BBC can be significantly different in practice on the mountain.

Questions For Planning

Some temperature related questions for your planning:
 

• What is the quoted temperature for the valley?
• What is the valley wind speed?
• What’s the maximum height to be gained?
• How much will the air temperature drop?
• How much stronger will the wind be on top?
• What will the wind chill factor be at the top?

 
To help with the last question, here’s the table.

Wind Chill Factors 

Please see the diagram attached. 

 Miles & Celsius

As you’ll see from the table, as the wind speed increases, it’s going to feel much colder.  This table differs from others you may find online, as it has been calculated by using wind speed in miles per hour, and temperature in degrees Celsius.  Others you’ll find are miles and Fahrenheit, or KM and Celsius, but for the unique UK audience, where we mix and match imperial and metric measurements, this table will be ideally suited for your needs. Weather forecasts in the UK do use the same calculation (as checked with the Met Office).

The Formula

The formula to calculate wind chill is quite complex (depending on your mathematical prowess), but it's included below for those who are interested.  It is the one used by the Met Office, and uses what’s called the JAG/TI method, aka the Canadian method, which is an evolution of previous calculations and now:

• uses wind speed corrected to a height (5 ft or 1.5 m) that represents the height of an average adult’s face
• is based on a human face model, i.e. exposed skin
• incorporates modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days)
• uses a walking speed of 3 mph (4.8 km h-1  or 1.3 m s-1)
• uses a consistent standard for skin tissue resistance
• assumes the worst case scenario for solar radiation (clear night sky) i.e. no heat from the sun