Pressure belts, the Coriolis effect and the pressure gradient

Overview

Conditions found in the equatorial low pressure belt, conditions found in the subtropical high pressure belt and conditions found in the sub-polar low pressure belt.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is atmospheric pressure and how is it measured on a weather map?

   Model answer

   Atmospheric pressure is the weight of the column of air pressing down on a unit area of surface. It is measured in millibars, also written as hectopascals, and average pressure at sea level is about 1013 millibars. Pressure is read against that average, so values below about 1013 millibars count as low and values above it as high, though forecasters usually reserve the labels for centres that depart from it clearly, below roughly 1000 millibars or above roughly 1020.

2. 2

   Why does rising air create low pressure at the surface and sinking air create high pressure?

   Model answer

   Where air rises it is removed from the surface faster than it is replaced, so the weight of the air column above falls and surface pressure is low. Air must converge inwards along the surface to feed the ascent. Where air sinks it is added to the column, so the weight above increases and surface pressure is high, and the arriving air must diverge outwards along the surface.

3. 3

   Name the global surface pressure belts and give the latitudes at which they lie.

   Model answer

   There are four in each hemisphere, arranged in bands. The equatorial low, also called the Inter-Tropical Convergence Zone, lies at about 0 degrees. The subtropical high lies at about 30 degrees North and South. The sub-polar low lies at about 60 degrees North and South. The polar high sits over each pole at about 90 degrees. They alternate low, high, low, high as you move from the equator to the pole.

4. 4

   Describe the conditions found in the equatorial low pressure belt.

   Model answer

   Intense surface heating makes air very buoyant, so it rises strongly in towering cumulonimbus clouds. Surface pressure is low, typically around 1008 to 1010 millibars, winds at the surface are light and variable, and rainfall is heavy and convectional, often exceeding 2000 millimetres a year with a downpour most afternoons. Because the surface winds are so weak and fitful the zone is traditionally called the doldrums.

5. 5

   Describe the conditions found in the subtropical high pressure belt.

   Model answer

   Air that rose at the equator and travelled polewards aloft has cooled and become dense, and it subsides here at around 30 degrees North and South. Sinking air is compressed and warmed, so its relative humidity falls, cloud evaporates and precipitation is very rare. Surface pressure is high, often 1020 to 1025 millibars, skies are clear, and this is where the world's great hot deserts are found. Surface air diverges from the belt, some flowing back towards the equator and some flowing polewards.

6. 6

   Describe the conditions found in the sub-polar low pressure belt.

   Model answer

   At about 60 degrees North and South, warm air moving polewards from the subtropical high meets cold dense air moving equatorwards from the polar high. The warm lighter air is forced to rise over the cold air along the boundary between them, which is called the polar front. Rising air gives low surface pressure, extensive frontal cloud and precipitation, and this is the zone in which mid-latitude depressions form and travel.

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