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Heat wave
A few days of unusually hot weather. Heat waves are a feature of the mid-latitudes and result from a tongue of tropical air moving polewards. Heat waves happen only in the summer.
High
A region in which the air pressure, as measured by a barometer, is higher than average. Highs are places of sinking air and clear, settled weather. Also used as
a shorthand for high pressure system or anticyclone.
High pressure
The build-up of air in some parts of the atmosphere. It is often used as an alternative to anticyclone.
Hoar frost
Frost that occurs when there is both wind and very moist air. Hoar frost can build up to be many centimetres long.
Hot climate
Where it is always hot, and average monthly temperatures do not fall below 18°C. There are equatorial climate and tropical climate types, including tropical continental climate, tropical marine climate and tropical monsoon climate.
 Hoar frost – Hoar frost has built up on thin cattle wire to make this fence look as though it is solid.
Hot desert climate
Where it is hot and permanently under the influence of trade winds blowing over the land. (See also: Desert climate.)
Hot dry climate
Dry climates of the tropics and subtropics, which may have a short period of unreliable seasonal
 Hot climate – Places with a hot climate and long dry periods are often favoured tourist destinations.
rain, often part of a summer monsoon. These regions are characterised by a sparse vegetation of grass and small trees, called scrub. (See also: Hot climate.)
Humidity
The amount of moisture in the
air. The full term is relative humidity, which means the amount of moisture in the air compared with the maximum amount that the air could hold. If the relative humidity is close to the maximum that the air can hold, little moisture evaporates from the skin and the air feels sticky and uncomfortable (humid). High temperature combined with high relative humidity can pose a serious threat to health. High humidity, wind and low temperatures make the air feel very ‘raw’.
When air holds as much water vapour as possible, the relative humidity is said to be 100%.
Humidity
Horse latitudes
The part of the Atlantic Ocean where the winds change from westerly to easterly. They exist near 30°N and 30°S and make up a
zone of plentiful sunshine and long calms. (See also: Doldrums).
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