The varying
distribution of heat from the sun on the earth determines
areas of high and low pressure in the atmosphere. The
main mechanism is as follows: where heating is greatest,
the air, which is less dense and lighter, tends to rise
vertically and low pressure forms in the area left behind.
Warm,
less dense air rises resulting in an area of low pressure.
The air at an altitude then cools, descending back towards
the ground, forming areas of high pressure.
As the
air rises, it becomes denser and heavier as it cools
and tends to spread out until, having cooled completely,
it works its way back down, where it generates high
pressure. The whole time, the atmosphere tends to restore
the balance, meaning air moves from high-pressure areas
(where there is a greater buildup) to low-pressure areas
(where it is less concentrated), in much the same way
as a liquid tends to fill any gaps it encounters in
its path. The resulting motion is wind. The higher the
difference in pressure between the two configurations
(high and low), and the closer they are, the higher
the wind speed.
What actually happens is that wind is deflected off
its otherwise straight course from high to low pressure.
Wind actually deviates to the right, in our hemisphere,
circulating clockwise around the centres of high pressure,
and anticlockwise around low-pressure centres. This
behaviour was discovered a long time ago. The Dutch
meteorologist C. H. Buys-Ballot (1817-1890) found that
a person standing with his back to the wind would have
the low-pressure area on his left and high-pressure
area on his right. In the southern hemisphere, the opposite
occurs. The deflection of the air is a direct result
of the earth’s rotation, as French mathematician
G. G. de Coriolis (1792-1843) demonstrated in 1835.
Indeed, at any point on the earth - with the exception
of the equatorial belt - the rotation affects a moving
body more and more the closer it is to the poles. Consequently,
in a given area in the northern hemisphere, air moving
northwards, for instance, would be deflected in a north-easterly
direction.
Moving
air is deflected towards the right in the northern hemisphere.
This is due to the fact that the earth’s surface
the air is moving over is, at the same time, rotating
anticlockwise. Air particles A heading towards B will
end up at C.
In actual fact, wind does not blow exactly parallel
to the isobars because of friction between the air and
the earth’s surface. Instead, it converges slightly
in low pressures and diverges in high pressures. Wind
intersects with the isobars at approx. 30 degrees over
land and approx. 10-15 degrees over the sea, where friction
is reduced. To get an idea of the force of the wind,
we must instead allow for the “pressure gradient”,
which is the ratio between the difference in pressure
measured at two geographical points and their horizontal
distance. At first glance, we can obtain a rather rough
measurement of speed qualitatively speaking: the closer
the isobars are together, i.e. the higher the pressure
gradient, the stronger the wind. To estimate quantity,
however, a scale is used like the one in the top right-hand
corner of the map with the isobars. This scale features
the latitude, on the left, and wind speed, horizontally.
WIND
ON WEATHER MAPS
On weather maps used by forecasters in Weather Centres,
each parameter observed is represented on a geographical
map, in certain areas where the weather stations are
located, using a simple set of international weather
symbols.
Weather
map with isobars on surface at 4 hPa intervals. Top
right, the scale for estimating wind strength. Suppose
we want to determine the wind over the middle of the
Tyrrhenian Sea. The wind blows, leaving low pressure
on its left and drops slightly towards the minimum,
hence an east wind. To determine its strength, since
we are at a latitude of 40° N, we need to apply
the distance between two consecutive isobars (measured
with a compass or ruler) to the scale, starting from
the left edge on a line with the relevant latitude.
Hence, estimated wind speed will be 35-40 knots.
This gives the meteorologist an immediate idea of what
the weather is like at a given time in a given area.
Wind direction, on such maps, is represented by a shaft
pointing to the centre of a circle, which indicates
the observation centre, from the direction the wind
is blowing from. Barbs and half barbs sticking out from
the direction shaft towards the low pressure instead
indicate its speed. Each barb stands for 10 knots, a
half barb for 5 knots, and a small triangle (pennant)
for 50 knots. In accordance with the international symbol
rules, the value of the pressure is featured to the
right of the circle indicating the weather station,
and just above it. This value is indicated with a three-digit
code, the third part being the digit after the decimal
point.
Symbols
on a weather map featuring pressure and speed. Pressure
is expressed with a three-digit code, the third part
being the digit after the decimal point. It must be
read bearing in mind the type of pressure structure
(high or low) and placing a 10 in front when the pressure
is 1000 hPa, and a 9 whenever it is lower. For instance,
988 is equivalent to a pressure of 998.8 hPa and 025
equivalent to 1002.5 hPa. Wind direction is given by
the shaft pointing to the circle indicating the station;
and speed by the barbs on the shaft (one barb stands
for 10 knots, a pennant for 50 knots).
