Worksheets › Class 11 › Atmospheric Circulation and Weather Systems › Cheat sheet
Class 11 Atmospheric Circulation and Weather Systems: cheat sheet
CBSE Social Science. Air pressure and its belts, the forces that steer winds, the general circulation, local winds, air masses, fronts and cyclones.
Key ideas
- Pressure falls with height; across the earth there is an equatorial low, subtropical highs near 30°, subpolar lows near 60° and polar highs.
- Winds blow from high to low pressure, steered by the pressure gradient force, friction and the Coriolis force.
- The Coriolis force turns winds to the right in the northern hemisphere and to the left in the southern; it is zero at the equator.
- The general circulation has three cells: Hadley (tropics), Ferrel (westerlies) and polar; together they carry heat towards the poles.
- Local winds: sea breeze by day and land breeze at night, valley breeze by day and mountain wind at night, and cold katabatic winds.
- An air mass is a large body of air of even temperature and moisture; a front is the boundary between two air masses.
- Tropical cyclones need warm seas above 27 °C and the Coriolis force; they have a calm eye and weaken over land, where moisture is cut off.
Remember
| 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 |
Watch out
- Saying the Coriolis force is strongest at the equator (it is zero there).
- Mixing up the sea breeze (day, sea to land) and the land breeze (night, land to sea).
- Making winds blow clockwise round a northern-hemisphere cyclone.
Worked example
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.
Practise Atmospheric Circulation and Weather Systems
Revise with this page, then practise: every Atmospheric Circulation and Weather Systems sheet in the app is new, with an answer key and working.
Coming soon for Android and iPhone.