Pressure and wind: isobars, Buys Ballot and Beaufort
Read a pressure chart at sea: isobars, ridges, troughs and cols, why the wind blows, Buys Ballot's law, the surface wind and the Beaufort scale.
How the lesson starts
Air has weight. The weight of all the air above a place presses down on it, and that push is the pressure.
Forecasts and Met Office charts measure it in hectopascals, written hPa. One hectopascal is the same as one millibar, the older name you will still hear: 1 hPa = 1 mb.
The standard pressure at sea level is 1013.25 hPa, so call it about 1013. The real pressure moves above and below that figure, and those movements are what this module is about.
On board you read it on a barometer. A yacht's barometer is an aneroid one: it works without any liquid inside.
Isobars
An isobar is a line joining places with the same pressure, like a contour line on a land map. On Met Office charts they are drawn every 4 hPa.
Pressures are corrected to sea level before they are drawn, so the height of the land does not change them.
What you practise inside
- Pressure and isobars
- Why the wind blows2 exercises
- Beaufort and the sea
This is a preview. The lesson itself is interactive: you work through it step by step, and every answer is checked as you go.