Wind is the movement of air masses caused by the unequal heating of the Earth, among other things.
Cold air is denser and tends to fall, while warm air is less dense and tends to rise. The combination of these two movements contributes a rotating motion called a convection cell.
A mass of cold air that falls forms a high-pressure area at ground level. Conversely, a mass of warm air that rises forms a low-pressure area at ground level.
Air masses always move from a high-pressure area toward a low-pressure area.
Sea breezes and land breezes illustrate how a convection cell works.
During the day, the Sun's rays fall on the sand and heat up the ground. The air that touches the hot ground also becomes warmer. Warm air is less dense than cold air, so it rises.
At the same time, the air that touches the sea is cooled by the fresh water. Since cold air is denser than the warm air, it tends to fall.
The movement of these masses of cold and warm air creates a sea breeze.
At night, the Sun's rays no longer warm the sand, making it colder. The air that touches the sand is also cooled. Since cold air is denser than warm air, it falls.
Above the sea, the air gradually warms. Since warm air is less dense than cold air, it rises.
The movement of these masses of cold and warm air creates a land breeze.
A weathervane is an instrument that indicates the direction of the wind.
This is a pointer (an arrow, a rooster, etc.) mounted on a rotating axis. The pointer can rotate freely around its axis and is therefore aligned parallel to the wind. It is the thinnest part of the pointer (the tip of the arrow, the rooster’s beak, etc.) that tells us where the wind is coming from. A stationary cross indicating the points of the compass may be fixed under the pointer of a traditional weathervane.
An anemometer is an instrument that indicates wind speed.
The anemometer was invented by Italian architect Leon Battista Alberti in 1450, and consists of empty half-spherical cups facing in the same direction that rotate freely around an axis. In modern anemometers, an electronic system is used to calculate the number of revolutions made by the cups over a given period of time. The wind speed is then displayed on a small screen.
The Beaufort scale is a scale used to estimate wind strength and speed.
This scale estimates wind strength and speed, with 12 levels of force. Each of these forces is associated with an approximate wind speed and observable effects.
| Force | Approximate Wind Speed (km/h) | Observable Effects |
| 0 | 0 to 2 | Smoke from a chimney or fire rises vertically in a straight line. |
| 1 | 2 to 5 | Smoke from a chimney or fire rises in the same direction as the wind. However, the weathervane does not move. |
| 2 | 6 to 11 | You can feel the wind on his face. The weathervane turns to indicate where the wind is coming from. The leaves on the trees move slightly. |
| 3 | 12 to 19 | The leaves and branches of the trees are in constant motion. Flags unfurl. |
| 4 | 20 to 29 | Dust, leaves and small objects are lifted. |
| 5 | 30 to 39 | The shrubs move slightly. Waves form on inland waters. |
| 6 | 40 to 50 | Big tree branches shake. Umbrellas turn upside down. Telephone wires swing. |
| 7 | 51 to 61 | The trees sway. Walking against the wind is difficult. |
| 8 | 62 to 74 | Walking against the wind is an ordeal. |
| 9 | 75 to 87 | Shingles, house antennae and other structures were ripped off. |
| 10 | 88 to 101 | Trees were uprooted. Houses suffered severe damage. |
| 11 | 102 to 116 | A violent storm hits and the devastation is widespread. |
| 12 | More than 116 | A hurricane hits and devastation ensues. |
When winds exceed 116 km/h, they are known as hurricanes. There is a scale for distinguishing between the different categories of hurricane: the Saffir-Simpson scale. There are five categories, the last of which represents force 5 hurricanes (winds in excess of 250 km/h).