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The lines shown on a weather map are isobars - they join
points of equal atmospheric pressure.
The pressure is measured by a barometer, with a correction
then being made to give the equivalent pressure at sea level.
Meteorologists measure pressure in units of millibars (mb),
though instruments sometimes give pressures in terms of inches
of mercury. The term hectopascal (hPa) is often used instead
of millibar, where 1 millibar equals 1 hectopascal. In the
British Isles the average sea-level pressure is about 1013
mb (about 30 inches of mercury), and it is rare for pressure
to rise above 1050 mb or fall below 950 mb.
Charts showing isobars are useful because they identify
features such as anticyclones and ridges (areas of high pressure)
and depressions and troughs (areas of low pressure), which
are associated with particular kinds of weather. These features
move in an essentially predictable way.
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Also, wind speeds and directions are related to the
spacing and orientation of the isobars.
| Relationship between winds and isobars |
There are three important relationships between isobars and winds.
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The closer the isobars, the stronger the wind.
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The wind blows almost parallel to the isobars.
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The direction of the wind is such that if you stand with your
back to the wind in the northern hemisphere, the pressure is
lower on the left than on the right.
These make it possible to deduce the wind flow from the isobars.
The direction given for the wind refers to the direction from
which it comes. For example, a westerly wind is blowing from the
west towards the east.
Measurements of wind strength are made at 10 metres (33 feet)
above the ground. A specified height has to be used because the
wind speed decreases towards the ground. In this country winds
are measured in knots (nautical miles per hour). However, forecast
winds are often given in miles per hour (where 1 knot is equivalent
to 1.15 m.p.h.) or in terms of the Beaufort Scale. There are rapid
variations in the wind - these are referred to as gusts. Gusts
are higher inland than over the sea or windward coasts, although
the mean wind speeds tend to be lower inland. Typically, gusts
can be 60% higher than the mean speed, although in the middle of
cities this can reach 100%. Northerly winds tend to be gustier
than southerly ones.
| Relationship between wind direction and weather |
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In general, the weather is strongly influenced by the wind
direction, so information about the wind provides an indication
of the type of weather likely to be experienced. However,
this approach is effective only if the wind is blowing from
the same direction for some time. A marked change in wind
direction usually indicates a change in the weather.
Northerly winds tend to bring relatively cold air from polar
regions to the British Isles. Similarly, southerly winds
tend to bring relatively warm air from the tropics. The characteristics
of the air are also affected by its approach to the British
Isles. Air picks up moisture if it travels across the sea,
but remains relatively dry if it comes across the land.
As cold polar air moves southwards over an increasingly
warm sea, the heating of the air by the sea causes cumulus
clouds to form. These clouds may grow sufficiently for showers
to develop and, consequently, winds from the north-west,
north or north-east usually bring cold, showery weather to
the British Isles.
Warm air from the tropics moving northwards over the sea is cooled
from below. Sometimes the cooling is sufficient for sea fog or
a thin layer of stratus to form. The cloud can become thick enough
for drizzle, especially on windward coasts and over high ground.
In general, winds from the west or south-west are associated with
overcast, wet weather.
Winds from the south and south-east mainly occur in summer
and these bring warm, dry weather. However, southerly winds
can sometimes bring hot, thundery weather.
Easterly winds in winter bring very cold air to the British
Isles. The characteristics and path of the air determine
whether it is cloudy (with perhaps rain, sleet or snow) or
fine and sunny. In summer, an easterly wind will mean it
is cool on the east coast but warm elsewhere, usually with
clear skies.
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| Depressions and anticyclones |
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In a depression (also referred to as a 'low'), air is
rising. As it rises and cools, water vapour condenses to
form clouds and perhaps precipitation. Consequently, the
weather in a depression is often cloudy, wet and windy
(with winds blowing in an anticlockwise direction around
the depression). There are usually fronts associated with
depressions.
In an anticyclone (also referred to as a 'high') the winds
tend to be light and blow in a clockwise direction. Also
the air is descending, which inhibits the formation of
cloud. The light winds and clear skies can lead to overnight
fog or frost. If an anticyclone persists over northern
Europe in winter, then much of the British Isles can be
affected by very cold east winds from Siberia. However,
in summer an anticyclone in the vicinity of the British
Isles often brings fine, warm weather.
A trough has characteristics similar to those of a depression,
and in a ridge the weather is similar to that in an anticyclone.
The changeable weather in the British Isles is caused
by a succession of depressions with their associated fronts
and anticyclones (or ridges) running across the country
from the Atlantic Ocean.
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The boundary between two different types of air mass
is called a front. In our latitudes a front usually
separates warm, moist air from the tropics and cold,
relatively dry air from polar regions. On a weather
chart, the round (warm front) or pointed (cold front)
symbols on the front point in the direction of the
front's movement. Fronts move with the wind, so they
usually travel from the west to the east. At a front,
the heavier cold air undercuts the less dense warm
air, causing the warm air to rise over the wedge of
cold air. As the air rises there is cooling and condensation,
thus leading to the formation of clouds. If the cloud
becomes sufficiently thick, rain will form. Consequently,
fronts tend to be associated with cloud and rain. In
winter, there can be sleet or snow if the temperature
near the ground is close to freezing.
It is convenient to distinguish between warm fronts,
cold fronts and occluded fronts.
A front which is moving in such a way that the warm
air is advancing to replace the cold air is called
a warm front. As the warm front approaches,
there is thickening cloud and eventually it starts
to rain. The belt of rain extends 100-200 miles ahead
of the front. Behind the front the rain usually becomes
lighter, or ceases, but it remains cloudy. As a warm
front passes, the air changes from being fairly cold
and cloudy to being warm and overcast (typical of warm
air from the tropics travelling over the sea). Also
there is a clockwise change in wind direction, and
the wind is said to 'veer'.
A cold front moves so that the cold air is
advancing to replace the warm air. This means that
as a cold front passes, the weather changes from being
mild and overcast to being cold and bright, possibly
with showers (typical of cold polar air travelling
over the sea). The passage of the front is often marked
by a narrow band of rain and a veer in the wind direction.
An occluded front can be thought of as being
a result of the warm and cold fronts meeting. Consequently,
ahead of an occlusion the weather is similar to that
ahead of a warm front, whereas behind the occlusion
it is similar to that behind a cold front.
The characteristics given for the fronts apply to
active fronts. If the front is weak, the rain associated
with it is light or non-existent, and the changes across
the front are less marked.
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NOAA-12 visible image at 09UTC on 23 April 1992
Surface analysis at 12UTC on 23 April 1992, shortly
after the satellite image depicted above
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