Causes of wind

Wind is the horizontal motion of air in the atmosphere. It arises primarily from pressure differences, which themselves come from temperature differences.

Source: WMO Guide to Meteorological Practices; ICAO Annex 3; FAA-H-8083-25B PHAK Ch. 12.

Primary cause — temperature and pressure

Wind is generated by pressure differences resulting primarily from temperature differences:

The sun heats the Earth unevenly (equator > pole, land > water by day, etc.).
Warmer air expands, becomes lighter → rises → surface pressure drops (low).
Colder air sinks → pressure rises (high).
Air flows from high to low → wind.

→ Without temperature differences there would be no wind (at least no large-scale wind).

Horizontal pressure gradient

The horizontal pressure gradient results from the pressure difference, related to a certain distance:

Unit: hPa per distance (e.g. hPa/100 NM or hPa/100 km).
Steep gradient → strong wind.
Flat gradient → weak wind.

Wind on weather charts

In the surface weather chart, areas with high wind speeds can be recognized by narrow spacing of the isobaric lines:

Closely spaced isobars = strong pressure gradient = strong wind.
Widely spaced isobars = low gradient = weak wind.

The result of a low horizontal pressure gradient (wide spacing of isobaric lines) is weak wind, with still air conditions locally.

Conversely: areas with strong horizontal pressure gradient can be recognised in surface weather charts by several isobaric lines with narrow spacing.

Isobars and isohypses

Isobars: lines of equal pressure on a weather chart, typically found on surface weather charts.
Isobars connect places with equal QFF — i.e. equal pressure reduced to MSL.
Isohypses are lines of equal geopotential altitude of a pressure level above MSL — used on upper-air charts.

Geostrophic wind and Coriolis

On the rotating Earth, Coriolis force also acts:

Northern hemisphere: deflection to the right.
Southern hemisphere: deflection to the left.
Result: wind does NOT flow directly from high to low, but almost parallel to the isobars (geostrophic wind, see next lesson).

Factors affecting surface wind

Direction and speed of wind close to the surface depend on temperature, air pressure, centripetal acceleration and Earth's rotation.