Air pressure and its belts, the forces that steer winds, the general circulation, local winds, air masses, fronts and cyclones.
| Mean sea-level pressure | about 1,013.2 mb |
| Fall with height (near the ground) | about 1 mb for every 10 m: pressure = sea-level − height ÷ 10 |
| Pressure gradient | pressure difference ÷ distance (e.g. mb per 100 km); wind blows from the higher to the lower |
| Winds round a low | anticlockwise in the north, clockwise in the south (the reverse round a high) |
| Names of tropical cyclones | cyclones (Indian Ocean), hurricanes (Atlantic), typhoons (Western Pacific), willy-willies (Western Australia) |
| Air mass codes | mT maritime tropical, cT continental tropical, mP maritime polar, cP continental polar, cA continental arctic |
| Travel time | days = distance ÷ distance per day |
A tropical cyclone over the Bay of Bengal moves about 450 km a day and is 2,250 km from the coast. (a) In how many days will it make landfall? (b) How far will it travel in 132 hours? (c) Why does it weaken over land?
Working: 2,250 ÷ 450; 450 × 5.5
Answer: (a) 5 days (b) 2,475 km (c) Its supply of moisture, its source of energy, is cut off.