Global patterns of temperature, pressure, wind and precipitation

Overview

Global pattern of annual precipitation, belt of very heavy rainfall along the equator and why precipitation totals rise again through the mid-latitudes.

What you need to know

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Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Describe the global pattern of surface temperature shown by a world isotherm map.

   Model answer

   Isotherms run broadly east to west, roughly parallel to lines of latitude, with the highest values in a belt across the tropics and values falling steadily towards each pole. The pattern is not perfectly zonal: the isotherms bend polewards over warm ocean currents and over land in summer, and equatorwards over cold currents and over land in winter. The highest annual means, approaching or exceeding 30 degrees Celsius, occur over subtropical continental interiors such as the Sahara rather than at the equator itself, because equatorial cloud cover reduces insolation.

2. 2

   Why do the isotherms on a January map bend so much more sharply than those on a July map in the Northern Hemisphere?

   Model answer

   The Northern Hemisphere has large land masses, and land cools far faster than the sea. In January the continental interiors of Asia and North America become intensely cold while the oceans, warmed by currents, stay relatively mild, so isotherms swing far south over land and far north over the Atlantic and Pacific. In July the contrast is smaller because land heats quickly but the oceans lag only moderately behind, so the isotherms are straighter. In the Southern Hemisphere, which is mostly ocean, the isotherms stay close to parallel with latitude in both seasons.

3. 3

   Describe the global pattern of annual precipitation.

   Model answer

   There is a narrow equatorial belt of very heavy rainfall, commonly over 2000 millimetres a year, straddling the equator. On either side, at about 20 to 30 degrees North and South, lie the driest zones on Earth, with under 250 millimetres. Totals rise again through the mid-latitudes, giving 500 to 1500 millimetres between about 40 and 60 degrees, with the wettest parts on windward western coasts. Poleward of about 70 degrees totals fall again to under 250 millimetres. Superimposed on this, eastern sides of continents in the tropics and monsoon Asia are much wetter than the zonal pattern alone would suggest.

4. 4

   Explain the belt of very heavy rainfall along the equator.

   Model answer

   This coincides with the equatorial low pressure belt and the rising limb of the Hadley cells. Intense insolation heats the surface, air becomes buoyant and rises, and because the air is very warm it holds a great deal of water vapour. As it rises it cools to dew point, condenses and forms towering cumulonimbus, releasing latent heat that drives the ascent still further. Convergence of the north-east and south-east trades reinforces the uplift, so heavy convectional downpours occur on most afternoons throughout the year.

5. 5

   Explain the dry belts at about 20 to 30 degrees North and South.

   Model answer

   These lie beneath the descending limb of the Hadley cells, where air that rose at the equator subsides after cooling and losing its moisture aloft. Descending air is compressed and warms, so its relative humidity falls and any cloud evaporates. There is no mechanism to lift air to its dew point, so precipitation is very rare and highly unreliable. The effect is intensified on western coasts by cold offshore currents that stabilise the lowest air.

6. 6

   Explain why precipitation totals rise again through the mid-latitudes.

   Model answer

   The mid-latitudes lie under the sub-polar low pressure belt and the polar front, where warm tropical air meets cold polar air and is forced to rise. A steady procession of depressions travels along the front, steered by the Polar Front Jet Stream, each bringing frontal cloud and precipitation. Prevailing westerly winds crossing open ocean pick up abundant moisture, and where they meet high ground on western coasts relief rainfall adds a great deal more, which is why places such as western Ireland and western Scotland are so wet.

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