Wind

Why air moves

The Sun heats the Earth unevenly. Warm air rises and leaves lower pressure behind it; cooler air sinks and builds higher pressure. Air flows from high pressure towards low pressure, and that flow is wind. The bigger the pressure difference over a short distance, the stronger the wind.

Why wind curves

Because the Earth spins, moving air is deflected: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This is the Coriolis effect. North of the equator, air circles anticlockwise around low pressure (storms) and clockwise around high pressure; south of the equator it is the other way round. High above the ground the wind blows almost along the lines of equal pressure; near the ground, friction slows it and lets it drift partly towards the low.

Reading the speed

Sailors describe wind with the Beaufort scale. Force 3 (12–19 km/h) moves leaves and small twigs. Force 6 (39–49 km/h) moves large branches and makes umbrellas hard to use. Force 8 (62–74 km/h) is a gale that breaks twigs off trees. Force 12 (118 km/h and above) is hurricane force.

What the toggles show

Speed colours (eye button)

Shades the map by wind speed so strong-wind areas, such as storms and jet-like channels between mountains, stand out even when the streaks are thin.

Pressure

Draws isobars every 4 hPa with H and L at the centres of highs and lows, from NOAA's GFS forecast, following the timeline.