Wind – why does air move?

Winds get their energy from differences in air pressure. These differences cause a force, that makes air move relative to the Earth. The strongest large-scale winds on Earth are found in jet streams in the upper atmosphere.

Winds are movement of the air relative to the Earth. Winds get their energy from air pressure differences, which in turn are caused, when solar heating is distributed unevenly on the Earth. In general – the bigger vertical temperature differences, the stronger winds. Upper atmosphere jet streams, the strongest large-scale winds measured in the atmosphere, are caused on the boundaries of cold polar and warmer mid-latitude air masses.

Large-scale winds are formed, when forces caused by pressure difference, called pressure-gradient forces, work to even out pressure differences between high and low pressure areas. Wind velocities depend on how big the difference in air pressure is. Deep low pressure areas cause the strongest pressure-gradient forces. In small-scale events, like wind gusts or tornados, there are also other factors that cause winds to strengthen.

What influences wind direction?

Winds circle around low pressure centres and high pressure areas. The pressure-gradient forces alone would cause winds to blow straight from high pressure areas to low pressure centres. However, the Earth rotates, and Coriolis forces caused by the rotation turn the airflow towards the right in the Northern hemisphere and towards the left in the Southern hemisphere.

In free atmosphere, far from the friction layer that lies close to the ground, pressure-gradient forces and the Coriolis force reach a balance, and in the Northern hemisphere air flows anticlockwise around low pressure centres and clockwise around high pressure areas.

Friction turns the wind

In the friction layer close to the ground the wind is affected by frictional forces on top of the pressure-gradient forces and Coriolis force. Frictional forces come into play when the roughness of the ground makes the winds slow down. In this case pressure-gradient forces, the Coriolis force and frictional forces reach a balance. In practice you can see this as a slight turning of the wind; usually twenty degrees at most. The wind turns a bit towards the centre of the low pressure area or away from the high pressure area.

In the Southern hemisphere Coriolis force acts in the opposite direction relative to the Northern hemisphere. In the Southern hemisphere pressure-gradient forces and the Coriolis force reach a similar balance as in the Northern hemisphere, but now the wind directions are turned around – winds blow clockwise around low pressure centres and anticlockwise around high pressure areas.

Article last updated 6/16/2021, 2:22:00 PM