One of the characters in Annie Proulx's novel, The Shipping News, has this to say about the mysteries of the elements: "Hunters lost in the north woods unconsciously veer to the right as the Earth turns beneath their feet. In the north the dangerous storms from the west often begin with an east wind. All of these things are related to the Coriolis, the reeling gyroscopic effect of the Earth's spin that creates the wind and flow of weather, the countering backwashes and eddies of storms."
Some of these alleged effects are just, at best, apocryphal, but Coriolis does indeed tend to deflect a moving object. The phenomenon arises ultimately from the fact that the surface of our rotating planet moves fastest from west to east near the equator, and more slowly with increasing latitude. Approaching the poles, the surface completes only a very small circle in 24 hours.
The consequences of these different rates of progress for any large-scale movement over the surface of the Earth were first outlined by a Frenchman called Gaspard de Coriolis in 1835.
Coriolis was an engineer whose main interest was in large turbines and ballistics, and although he was vaguely aware of the implications of his theories for meteorology, he himself did not pay much attention to this application.
It was an American meteorologist called William Ferrel who applied the concept to the wind in 1856.
To understand the Coriolis effect, one could try to imagine a little "parcel" of air moving southwards from Sligo to Cork under the influence of a difference of pressure between the two places.
Except for a little frictional drag, the air is not fixed to the Earth in any way, and the surface of the Earth is whirling from west to east beneath it at high speed.
By the time the parcel of air reaches the point in space where Cork ought to be, Cork will have moved on: instead the little blob of air is likely to find itself over, say, Cahirciveen. To anyone observing the proceedings from the ground, the parcel of air would appear to have been deflected to the right in the course of its journey.
This example merely illustrates the Coriolis effect, which in reality is much more complex. The end result, however, is the general rule that air moving over the surface of the Earth in the northern hemisphere is continually deflected to the right.
It may start to blow across the isobars from high to low pressure, but gradually changes direction until it ends up blowing along the isobars with low pressure on the left-hand side.