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 Atmosphere – This diagram shows the way in which the winds move across the Earth. Hot air rises over the equator, and cold air sinks over the poles. Hot and cold air mix in the mid- latitudes to make changeable weather.
 Atmosphere – A satellite picture of Earth from space shows Africa with clouds over the equator (centre) and clear skies over the deserts (top).
Atmospheric pressure
The weight of the air above a given point. It is also called barometric pressure and is measured in millimetres of mercury using a barometer. A place where the air pressure is greater than normal can be called a high pressure region
or anticyclone; a place where the pressure is lower than average can be called a low pressure region, a depression or cyclone.
Aurora
Also known as ‘northern lights’ and ‘southern lights’. It appears as shining folded curtains in the winter sky at high latitudes.
 Aurora – The colours of the aurora are produced when the charged particles of the solar wind are captured by the Earth’s magnetic field. They collide with oxygen and nitrogen atoms in the upper atmosphere, emitting light, of which green, white, red and blue are the most common.
Barometric pressure
Azores–Bermuda high
The subtropical high usually positioned over the North Atlantic Ocean, level with North Africa. Stronger in summer than in winter, it shifts its position between the region of the island of Bermuda, in the west and the islands of
the Azores in the east. This high influences the track of depressions across the Atlantic Ocean. (See also: Anticyclone.)
 Barometer – The aneroid barometer works by amplifying changes in the size of a partially evacuated bellows using a set of levers.
B
Barometer
An instrument that measures the pressure of the atmosphere and is used in weather forecasting. There are two kinds of barometer. The most accurate kind uses a column of mercury. It is known
as a Fortin barometer. The most common kind uses a thin-walled metal can from which the air has partly been sucked out. It is called an aneroid barometer.
Barometric pressure
(See: Atmospheric pressure.)
Sealed capsule is compressed during high pressure and expands as the pressure drops.
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