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D
is for …
Depression
This
term is used meteorologically to describe cyclonic weather systems
i.e. those in which pressure decreases towards the centre.
The central pressure value is usually between 950 to 1020 millibars,
and can rise or fall.
When the pressure rises, the system weakens and becomes less active
- it is said to "fill"- and when it falls further the
system "deepens" and becomes more active, producing poor
weather conditions.
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| Paul tests the BBC's depression simulator |
A typical
depression in temperate latitudes has a series of different airmasses
(see A-section) and fronts associated
with it, and can provide rather unsettled weather in the form of
rain, drizzle, showers, snow and strong winds.
The winds within the depression blow anti-clockwise around it, parallel
to the isobars.
Most of the depressions which affect us are generated over the north
Atlantic by the clash of warm air from the south and cold air from
the poles: as they meet they start up the depression’s characteristic
spinning motion.
Depressions are very mobile weather features and Britain often lies
in their path as they head eastwards, blown along on the prevailing
west-to-east circulation of the northern hemisphere.
In more tropical regions depressions can be much more severe. They
are then called hurricanes in the Atlantic, and typhoons in the
Pacific.
Doldrums
"In the doldrums" is a commonly-used expression which
has a meteorological origin!
The Doldrums is an zone approximately 5 degrees to the north and
south of the equator in which the winds fall extremely light, and
the weather is often very poor with thunderstorms, heavy rain and
squalls.
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| Popeye was distinctly underwhelmed by his posting
in the Doldrums |
In
the days of sailing ships sailors often used to find themselves
stranded in these windless regions, often with little or no food
and water.
Understandably they were depressed and frustrated by their lack
of progress, and the term "in the doldrums" is still used
with that meaning today ... often about summers in the north of
England!
Desert
A desert is a region with insufficient rainfall to support vegetation.
Most of the worlds deserts are at latitudes of less than 50 degrees.
The main causes of desert conditions are the presence of a persistent
anticyclone (area of high pressure) or ground which shelters an
area from rain-bearing winds.
The change from a vegetated area on the boundary of a desert into
an extension of the desert area is called desertification.
This can be caused by long term climatic change or by environmental
pressures such as deforistation or over-grazing of domestic animals.
This is not a common phenomenon in the north of England!
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| The
Gobi Desert |
Drizzle
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| A familiar weather picture!! |
Drizzle is a form of precipitation which is made up of very small
water drops (typically 0.2 - 0.5mm in diameter).
The drops form in low "stratus" cloud by the collisions of even smaller
droplets.
The air beneath the cloud must be humid to prevent the drops evaporating
before they reach the ground.
Drizzle often causes a marked reduction in the visibility. This is
a common phenomenon in the north of England!! |
