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Desert
Desert
Regions with very low rainfall and extremely sparse vegetation.
Deserts cover about a sixth of the Earth’s land surface. They fall into three groups: hot desert climates, cold desert climates and rain- shadow deserts.
 Desert climate – Dunes are found in areas where wind-blown sand builds up.
40 30 20 10
0 –10 –20
100 80 60 40 20 0
Desert climate
Very dry climates where the amount of precipitation is so low that very little vegetation can grow. Hot desert climates are those with no average monthly temperature below 6°C. Cold desert climates are those with
at least one cold month (average temperatures below 6°C).
 Desert climate – Cairo in Egypt has no month with significant rain.
400 16 350 14 300 12 250 10 200 8 150 6 100 4 50 2
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Dew
Moisture that settles on grass and other surfaces when air cools. The temperature at which dew forms is called the dew point. (See also: Condensation.)
Dew point (dew-point temperature)
The temperature at which air becomes saturated with moisture and starts to form water droplets.
Diurnal
Daily, as something that occurs in, or lasts, one day.
Doctor, the
A regular, cool onshore wind that blows over a warm coastal area in southwestern Australia. In southern South Africa this type of wind is called the Cape Doctor.
Dog days
A term used in North America for
a period of very hot, sultry summer weather that often occurs when the Dog Star, Sirius, is visible in the night sky.
Doldrums
A zone of calms in the equatorial regions of the world’s oceans that moves slightly north or south
with the overhead Sun. It usually features daily thunderstorm rain.
Downdraught
A wind caused by rapid downwards movement of air in a thundercloud (see: Thunderstorm).
Drizzle
Light rain. Drizzle is a feature of weak depressions, especially warm fronts.
Drought
The term for an unusually long period without significant rainfall.
Some parts of the world, particularly between latitudes
1° and 20°, have a more variable
 Drought – During a drought, water levels may fall so far that rivers dry up, and water has to be sought from below the river bed.
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Rainfall in millimetres Temperature in °C
Rainfall in inches Temperature in °F