What causes the
winds and controls their direction?
variations of pressure across the earth cause wind.
In the northern hemisphere
the air rotates clockwise around a high pressure area and counter-clockwise
around a low. The air would like to move directly from high to low to
equalize the pressure but cannot do so because the earth is rotating, so the
air mass ends up rotating too. The fact that the air does not go directly but
moves around means that the weather associated with the air 'hangs' around
for much longer than you would expect. It can also mean that the weather
condition covers a much wider area.
Winds are described
in terms of the direction from which they come. This makes sense as the
winds tend to bring the weather and temperatures with them, so knowing
where they have come from can often give information about the weather
they might bring. For example, a north wind might well have
come from the cold polar regions and so may bring cooler weather or air
with it "When a north wind blows it doth bring snow."
Obviously storms are
associated with high winds and an anemometer
may give some indication of the increase in wind, or for that matter the
passing of a storm as the wind drops.
Measuring wind speed and direction