Some Geophysical Background Notes Related to Climate and Weather - The Changing Arctic Environment: The Arctic Messenger - page 374

Coriolis effect:
Earth rotates towards the east, but the velocity of rotation varies according to latitude on
Earth's surface. If I stand at the equator, where Earth's circumference is 40,075 kilometres, I
will travel eastwards at 1,669 kilometres an hour. If at the same time my wife were standing
at the Arctic Circle, where the circumference is only 17,662 kilometres, she would lazily
amble eastwards at a sedate 736 kilometres an hour. If she were standing exactly at the pole,
she would not have moved at all (only rotated). Anything that moves north from the equator
that is not attached to the ground, such as a parcel of air, will conserve (retain) the eastward
speed that it acquired at the equator. Therefore, as it moves north, it will be moving ever
faster than the ground immediately beneath because it is moving closer to the axis of Earth's
rotation. The result is that air (or water) travelling northwards away from the equator will
seem to be deflected towards the right (east). This deflection to the right of the direction
of movement in the Northern Hemisphere is said to be caused by the Coriolis effect. In the
Southern Hemisphere, air moving away from the equator is again deflected to the east, but
this time, the deflection is to the left. The Coriolis effect is zero at the equator.
In reality, the faster the air flows down a pressure gradient (from an area of high pres-
sure to an area of low pressure), the harder it is for it to be pushed to the right. Eventually,
a balance is reached between the pressure gradient force and the Coriolis effect, resulting in
an airflow that is parallel to the isobars (lines on a chart showing areas of equal pressure).
Winds that are in such a state of balance are called geostrophic winds . The result is the char-
acteristic anticlockwise (cyclonic) circulation around cyclones (low-pressure systems) in the
Northern Hemisphere.
Air being displaced out of a high-pressure system (anticyclone) in the Northern Hemi-
sphere will move to the right, producing a clockwise (anticyclonic) circulation.