60-second revision
- Troposphere = weather layer; ozone mainly in the stratosphere; ionosphere reflects radio waves.
- Coriolis force: zero at the equator, maximum at the poles; deflects right in the NH, left in the SH.
- Doldrums = equatorial low; Horse latitudes = subtropical highs (~30 degrees).
- Roaring Forties = southern westerlies; trade winds blow from subtropical highs to the equatorial low.
- Chinook (Rockies) and Foehn (Alps) are warm, dry descending winds; Mistral is cold (France).
- Cold currents run along west coasts in the tropics: Benguela, Canary, Humboldt (Peru), California.
- Spring tides at new and full moon; neap tides at quarter moons.
Layers of the atmosphere
| Layer | Height (approx.) | What to remember |
|---|---|---|
| Troposphere | Up to about 8 km at the poles and 18 km at the equator | All weather; temperature falls about 6.5 degrees C per km of height (normal lapse rate) |
| Stratosphere | Up to about 50 km | Contains the ozone layer; temperature rises with height; calm air suits jet aircraft |
| Mesosphere | Up to about 80 km | Coldest layer; meteors burn up here |
| Thermosphere (with ionosphere) | From about 80 km upward (the ionosphere spans roughly 80 to 400 km) | Ionosphere reflects radio waves; auroras |
| Exosphere | Outermost | Very thin air merging into space |
Pressure belts and planetary winds
Air moves from high pressure to low pressure. The Earth has seven pressure belts: the equatorial low (doldrums), two subtropical highs (horse latitudes, around 30 degrees), two subpolar lows (around 60 degrees) and two polar highs. These belts shift north and south with the apparent movement of the Sun.
| Wind | Blows from | Blows to |
|---|---|---|
| Trade winds (easterlies) | Subtropical high | Equatorial low |
| Westerlies | Subtropical high | Subpolar low |
| Polar easterlies | Polar high | Subpolar low |
Winds do not blow straight because of the Coriolis force, caused by the Earth's rotation. It deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is zero at the equator, which is why tropical cyclones do not form right on the equator.
Local winds
| Wind | Nature | Region |
|---|---|---|
| Chinook | Warm, dry; the 'snow eater' | Eastern slopes of the Rockies |
| Foehn | Warm, dry, descending | Alps |
| Sirocco | Hot, dusty, from the Sahara | Reaches Italy and southern Europe |
| Mistral | Cold, dry, blowing down to the Mediterranean | Rhone valley, France |
| Harmattan | Dry, dusty; the 'doctor' because it relieves humid heat | West Africa |
| Loo | Hot, dry, summer afternoons | Northern plains of India |
Cyclones
Tropical cyclones form over warm oceans (surface water above about 27 degrees C) away from the equator, where the Coriolis force is strong enough to set up rotation. Winds circle the calm eye anticlockwise in the Northern Hemisphere and clockwise in the Southern. Names differ by region: hurricanes (Atlantic), typhoons (western Pacific), cyclones (Indian Ocean), willy-willies (Australia, older usage). Temperate cyclones form along fronts in middle latitudes where warm and cold air masses meet.
Ocean currents
| Warm currents | Cold currents |
|---|---|
| Gulf Stream and North Atlantic Drift (Atlantic) | Labrador Current (off Canada) |
| Kuroshio (off Japan) | Oyashio (off Japan) |
| Brazil Current | Falkland Current |
| Agulhas / Mozambique (off south-east Africa) | Benguela (off south-west Africa) |
| East Australian Current | Humboldt or Peru Current (off South America) |
| — | Canary and California Currents |
Where warm and cold currents meet on shallow banks, plankton thrives and great fishing grounds form — the Grand Banks off Newfoundland and the waters off Japan are classic examples. The North Atlantic Drift keeps the ports of north-west Europe ice-free in winter.
Tides, salinity and El Nino
- Tides are caused mainly by the Moon's gravity, with help from the Sun. Spring tides (higher high tides) occur when Sun, Moon and Earth are in line — at new moon and full moon. Neap tides (lower range) occur at the first and third quarters, when Sun and Moon pull at right angles.
- Average ocean salinity is about 35 parts per thousand. Salinity is higher where evaporation is high and fresh water input is low (e.g. the Red Sea) and lower near large river mouths and melting ice.
- El Nino is the periodic warming of the central and eastern equatorial Pacific; La Nina is the opposite, cooler phase. The Southern Oscillation is the linked see-saw in air pressure, hence ENSO.
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