Coriolis effect or Coriolis force

Coriolis effect or Coriolis force The Coriolis effect, often referred to as the Coriolis acceleration or force, produces an apparent deflection of airflow relative to the Earth's surface. For horizontal airflow this deflection is to the right in the northern hemisphere and to the left in the southern hemisphere. The degree of deflection is zero at the geographical Equator, and at a maximum at either pole. The Coriolis acceleration (or force) is an important determinant in the geostrophic wind.

Understanding the Coriolis effect centres on an appreciation of the consequence of the Earth's rotation for air motion within the atmosphere: atmospheric air motion must be considered in relation to the Earth's surface. The Earth rotates on its axis (which passes through the North and South Poles) from west to east approximately once every 24 hours. Viewed from a point over the North Pole, this rotation is anticlockwise. Viewed from a point over the South Pole, the rotation is clockwise. However, points on the Earth's surface near the Equator have a speed of movement relative to space in excess of 1600 kilometres per hour, whereas, points at either pole simply rotate once per day about the axis.

These facts have two important consequences. First air moving across the Earth's surface will carry with it an ‘inherited’ velocity which can be greater or less than that of the Earth's surface at its new location. In either hemisphere air moving from lower to higher latitudes will be moving over areas where the surface speed is decreased. In the anticlockwise-rotating northern hemisphere, and viewed from the surface, there will thus be an apparent deflection to the right, because the air sample will now be moving faster relative to the surface beneath. Air moving in the reverse direction will be moving over areas where the surface speed is increased, so that, again, there will be an apparent deflection to the right. For the clockwise-rotating southern hemisphere the effect is mirrored, so that deflection is to the left.

Second, the Earth is nearly spherical, so that the orientation of its axis of rotation relative to air motion must be also considered. Exactly on the Equator horizontal airflow (i.e. that tangential to the Earth's surface) is normal (perpendicular) to the ‘sense’ of the Earth's spin, so that the effect of the rotation on airflow is zero. At the poles horizontal airflow is in exactly the same ‘sense’ as the Earth's spin, so that the effect of rotation is at a maximum. A similar, but contrasting, argument may be applied to air moving vertically relative to the Earth's surface, so that the Coriolis effect applies to all air motion in the atmosphere, and may be resolved into its vertical and horizontal components, more commonly the latter.

In the general case for horizontal motion, the Coriolis acceleration, a, is proportional to the sine of the latitude, F, and the velocity of airflow, V:a = 2 Ω V sin Φ, where Ω is the angular velocity of the Earth's rotation.

Graham Sumner