"Feels Like" Explained

Ever wondered why the Dan Corbetts of this world show you a multitude of temperatures in a weather forecast and then quantify them by going on to say what it will feel like?

Since December 2007 the BBC Weather team have been using temperatures to suggest what it will feel like outside as well as showing the usual max and min temperatures.

Feels Like is based on temperature and other meteorological factors to show how the combination of these will feel to the skin. It is shown in degrees Celsius.

Why is it called "Feels Like"?

Our aim was to find a way to provide a quick indication of what it would Feel Like when you step outside, all year round.

This winter has seen the first use of Feels Like but we have been using wind chill for many years. The difference now is that it will be used in the summer during periods of extreme heat.

Although the formulas used to calculate the temperatures are different depending on whether it is summer or winter, ultimately the end result is the same - what it will Feel Like.

It's not a new phenomena either, the combination of wind with low temperatures and its impact has been a feature of forecasts since it was developed around the time of World War 2. It was produced to determine risk factors when operating outdoors in extreme weather.

The Feels Like temperature is used around the world, particularly in very cold climates such as the Arctic and Antarctic. In other less cold places in winter, it impacts on decisions about what to wear, whether or not outdoor activities will be possible and in some cases in the form of warnings by the media of the dangers that the combination of cold and wind can bring.

For example in July 2007 areas of the Australian State of Victoria were covered by snow and this was compounded by the high winds, leading to the conditions being even more extreme. In February 2007 blizzards swept into Ontario, Canada, the storm brought severe disruption and heavy snow as well as warnings for wind chill.

In the UK the typical weather scenario for referring to Feels Like temperatures is a wintertime east to northeasterly wind driven across Scandinavia or Russia on the southern side of a high pressure system. Such winds are cold because of their track across the mountains and tundra. They can often be strong too, because low pressure over Southern Europe can result in a very tight 'squeezing' of the isobars.

We have used the term wind chill during the winter in the past. For example in March 2005 the south-east and north-east of England were hit by severe winter weather. Heavy snowfall brought disruption to transport, but that was not the whole story. As the snow died out, ice became the main feature of the weather story, with wind chill being a significant fact due to the strong northerly wind.

The Met Office are supplying us with forecast data for sites around the UK, allowing us to display the forecast, both on national and regional forecasts. The JAG/TI algorithm, which measures 'face only wind chill' is a Canadian method. The Met Office use this method as it has been clinically tested, it is simple to use and based on advances in science, technology, and computer modelling.

Specifically, the new JAG/TI Wind chill Temperature index:

-Calculates wind speed at an average height of five feet (typical height of an adult human face)

- Is based on readings from the national standard height of 33 feet (typical height of an anemometer)

- Is based on a human face model

- Incorporates modern heat transfer theory (heat loss from the body to its surroundings, during cold and breezy/windy days)

- Lowers the calm wind threshold to 3 mph Uses a consistent standard for skin tissue resistance

- Assumes no impact from the sun (i.e.clear night sky).

The formula
T(wc) = 13.12 + 0.6215T - 11.37V**0.16 + 0.3965TV**0.16

(Where T(wc) is the wind chill index based on the Celsius scale, T is the air temperature in °C, and V is the air speed in km/h measured at 10 m (33 ft, standard anemometer height).

So, how are we going to be presenting it?

Feels Like formed part of our weather forecasts in the 2007/08 winter season when the winds were strong and the temperatures low. The forecast temperature icons rotate on the map and reveal the equivalent Feels Like temperature, allowing viewers the chance to see what it might feel like outside.