Windy weather

(Posted: August 24, 2020)

With the gusty weather we have had over the weekend I am sure many of you experienced the blustery winds. But what is wind? And how can we identify or measure it?

Wind speeds observations over the weekend for the UoB campus site
Wind speeds observations over the weekend for the UoB campus site

How is wind formed?

Winds develop all over the world and are part of the intrinsic system which forms our weather. There are many different types of winds from gentle sea breezes to stronger, hugely damaging Derecho winds. However, essentially they are all formed from the same process.

As the sun heats up different parts of the earth and the atmosphere unequally, air will both expand and rise as it warms or contract and sink as it cools, depending on its surroundings. This causes the air to move around the globe – essentially creating wind. Many things can affect the movement and strength of the wind, from the presence of water to mountains and topographic changes.

In the UK we are predominantly dominated by the westerlies – a band of wind which is found around the 30-60 degree latitude. Its name is derived from the fact that it originates from the west and travels eastwards and it is the reason why much of the UK weather is affected by low pressure moving in from the Atlantic Ocean.

Harnessing the power of the wind
Harnessing the power of the wind

How can we forecast wind?

You will often hear meteorologists on the television telling you that the close isobars will mean strong winds and this is true. When the isobars are close it is indicating a large difference in pressure across a small area, similarly to what we experienced late last week.

Surface pressure chart for 21st August
Surface pressure chart for 21st August

However, when the isobars are far apart, we have very still and calm weather which can occur in summer and winter. Here are two examples from the recent August heatwave of 2020 to a calm, cold day we had in January earlier this year. In both of them you can the isobars are very spread out across the UK.

Surface pressure chart for 15th August
Surface pressure chart for 15th August

Surface pressure chart for 21st January
Surface pressure chart for 21st January

Wind measurements

Winds are categorised using the Beaufort Scale which ranges from still, calm winds to Hurricane force level.

For Birmingham we are fairly lucky due to our sheltered position in the centre of England. However, as seen over the weekend we can still experience strong winds as low pressure passes over.

Wind Force Description mph Effects
0 Calm <1 Smoke rises vertically
1 Light Air 1-3 Direction shown by smoke drift but not by wind vanes
2 Light Breeze 4-7 Wind felt on face; leaves rustle; wind vane moved by wind
3 Gentle Breeze 8-12 Leaves and small twigs in constant motion; light flags extended
4 Moderate Breeze 13-18 Raises dust and loose paper; small branches moved.
5 Fresh Breeze 19-24 Small trees in leaf begin to sway; crested wavelets form on inland waters.
6 Strong Breeze 25-31 Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty.
7 Near Gale 32-38 Whole trees in motion; inconvenience felt when walking against the wind.
8 Gale 39-46 Twigs break off trees; generally impedes progress.
9 Strong Gale 47-54 Slight structural damage (chimney pots and slates removed).
10 Storm 55-63 Seldom experienced inland; trees uprooted; considerable structural damage
11 Violent Storm 64-72 Very rarely experienced; accompanied by widespread damage.
12 Hurricane 73 plus Devastation

Wind is measured using an anemometer. There are several different types of these ranging from cheap cup anemometers to more expensive sonic anemometers.

Our met site displaying a cup anemometer (top left) and sonic anemometer (bottom right)
Our met site displaying a cup anemometer (top left) and sonic anemometer (bottom right)

Here at the Urban Observatory in Birmingham we have installed over a hundred sonic anemometers. Both our ClimaVUE stations and Netatmo wind anemometers record wind speed and direction with a great coverage across the city. To keep updated on wind speeds in Birmingham check out our data map.