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Feels Like...

Richard Chapman Richard Chapman | 13:27 UK time, Thursday, 27 March 2008

Since the Easter weekend there has been a lot of discussion about the wintry weather. Viewers have been asking how unusual it was to get snow falling at Easter and we have been asking ourselves to what extent we got the weather story effectively across to the audience.

Weather map showing temperaturesPart of this story was the use of the Feels Like temperature icons. These were used across the weekend to highlight the fact that the heavy snow showers were not the full story, rather the biting northerly winds would make it feel very much colder than the thermometer might suggest.

So is the data scientific? The answer is yes. The data is supplied by the Met Office who are using a well respected formula, the JAG/TI, to produce the temperatures for the winter season.

The JAG/TI algorithm, measures 'face only wind chill' and is one used in Canada. The Met Office use this formula as it has been clinically tested, it is simple to use and based on advances in science, technology, and computer modelling.

Specifically, the JAG/TI Wind chill Temperature index:

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

Weather map showing Feels Like temperatureWhy 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.

At the end of the day we will be using it when it helps give the complete weather picture. It might be bright sunshine out of the window, and the thermometer may read 5C but if there is a breeze, then it will feel colder than 5C and this might have an impact on what people want to wear. As with our wind graphics and isobar charts we will use these graphics when relevant to the weather story of the day.

The branding of the white temperature disk and blue square outline is a recent development on the weather graphics system, introduced in 2005. It was developed from the previous wind chill graphics - a blue temperature and a yellow outline – to fit in with the current look and feel of the weather graphics.


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