What is wind?

 

Wind is all around us but what is it and what actually causes it? Here we examine one of the simplest aspects of weather and some of the terminology that surrounds it.

Wind simply is the movement of air. And the air moves because of differences in temperature and pressure around the globe.

This difference in temperature and pressure is because at the equator (the hottest area on earth) the hot air rises before cooling and sinking as it moves towards the poles. This leads to the basic concept of atmospheric circulation… the three-cell model (see right).

As the air cools it shrinks and as it heats it expands. This is the basis of air pressure, with high pressure essentially pushing down harder on the Earth’s surface because there is more air present it is just more dense.

It is these differences of temperature and pressure that cause winds on many different scales from large-scale, such as monsoon and trade winds, to localised winds such as the Mistral in France.

How Strong?

Wind is measured in several scales depending on the use using an anemometer. Most commonly used are miles per hour or knots although metres and kilometres per second are also used. Wind is a vector force so always has a speed and direction (given in degrees). The Beaufort scale is also used to describe wind speed based on observed conditions.

Beaufort Scale Knots General Term
0 <1 Calm
1 1–3 Light air
2 4–6 Light breeze
3 7–10 Gentle breeze
4 11–16 Moderate breeze
5 17–21 Fresh breeze
6 22–27 Strong breeze
7 28–29 Moderate gale
30–33
8 34–40 Fresh gale
9 41–47 Strong gale
10 48–55 Whole gale
11 56–63 Storm
12 64+ Hurricane


Types of Wind

There are many types of wind due to the scales at which they occur. At different scales different factors are important such as the rotation of the Earth (the Coriolis force) to the landscape (e.g. mountainous) to the buildings present in an area.

To read more about famous winds around the world see here.

To read more about the terminology used in meteorology for wind see here.

What is Wind Chill?

As the winter draws in and the weather gets colder an often used phrase is wind chill but what is it?. Wind-chill is a measure of the amount of heat lost from the skin as the wind blows across it. So if it is 0°C with no wind that is what it will feel like. However if that the same temperature there was a wind of 5 knots blowing it will feel like -5°C in calm air. As the wind increases the more heat is lost from the skin when it blows across it.

This is why in winter it feels so much colder when there is a wind blowing.

Summary

Wind is caused by the movement of air around the globe. The amount the air moves is dependant upon the differences in temperature and pressure. Wind occurs on different scales and are measured using an anemometer or using the Beaufort scale by eye. Wind chill is the amount of heat lost from skin as the wind blows across it and is stronger the harder the wind blows.

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