B is for …

Black Ice
Black ice is a greatly over-used phrase. You'll often hear traffic and travel reporters mention it on winter mornings.

Icicles hanging from a roof

People tend to refer to any frost or ice on a road surface as black ice but, meteorologically speaking, the description should only be used for one specific instance.

True black ice is formed when light rain or drizzle falls onto a road or pavement surface which is already at a temperature of below zero Celsius.

The water droplets freeze on contact with the cold surface, forming a film of ice. Because the ice, or glaze as it is sometimes called, is transparent the blacktop surface of the road is still visible, hence the name black ice.

As well as giving the phenomenon it's name, the transparency also means that it is one of the most treacherous of winter hazards for the motorist.

Backing wind
The wind is said to be backing when the direction is changing in an anticlockwise direction with reference to the compass.

For example, a change of wind direction from Northwesterly to Southwesterly would be a backing wind. The opposite of backing is veering.

Blue Sky
Why is the sky blue? Sunlight is scattered by the molecules in the atmosphere, a theory pioneered by Rayleigh.

Blue skies ahead?

He showed, by using mathematics, that short wavelength light (blues and violets) are scattered more than long wavelength (reds and yellows). Therefore, the light which reaches us after scattering through the atmosphere is high in blue and violet content, but since our eyes are less sensitive to violet, the sky appears blue.

Blizzard
This term originated in North America, where it meant a very cold, strong to gale force wind laden with snow, some of which has been raised from snow-covered ground.

All snowed in

In UK forecasts "blizzard" is defined as winds of at least force 7 (see Beaufort Scale below), along with moderate or heavy snowfall, causing drifting and reduction of visibility to 200m or less.

Criteria for a "severe blizzard" are winds of force 9 or more, with visibility reduced to nearly zero.

Beaufort Scale
First adopted by Admiral Beaufort, on board an early 19th century British man-of-war vessel, this scale estimates the force of the wind (in case you don't have a handy anemometer - see our A section!)

It relates wind speeds to disturbance of the sea, or of fairly standard land-based features, as set out in the table below. You'll often hear us refer to this scale whilst forecasting for our coastal waters.

Beaufort scale