S BILLY PRETTY homeward plods his weary way in Annie Proulx's novel The Shipping News, he ponders on some of 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 wind and flow of weather, the countering backwashes and eddies of storms."
Of course, some of the effects described by Billy are, at best, apocryphal. But Coriolis does indeed tend to deflect moving objects in the northern hemisphere. The effect arises ultimately from the fact that on our rapidly rotating planet points on the equator move much more rapidly from west to east than points on the surface at more northern latitudes.
Imagine, for example, the ultra powerful Derry footballer given a free kick from his home city to a goal in Dungarvan, Co Waterford. Looking at the map, it would seem that a shot due south should score.
But with the Earth moving underneath from west to east, a south bound ball, rather than striking the ground in Co Waterford, would land at a point significantly farther west perhaps on Pairc Ui Caoimh in Co Cork. To a spectator on the ground, it would appear as if the Derry free had curved gently to the right.
A similar fate befalls any volume of air tending to move across the surface of the Earth under the influence of differences in atmospheric pressure.
Looking more closely at the two places already mentioned, we can calculate that, as the Earth rotates, Derry is moving eastwards at about 600 m.p.h., and Dungarvan at about 650 m.p.h. A volume of air flowing from the former to the latter will appear to be deflected to the right, just like the football.
But conversely, a parcel of air in transit from Waterford to Derry is moving to a latitude where the surface of the Earth is travelling more slowly: the northbound air will retain most of its own initial eastward moment, and will also appear to an observer on the ground to have swerved to the right by the time it reaches the latitude of Derry.
All of this results in the general rule that air moving over the surface of the Earth in the northern hemisphere is always deflected to the right.
It may start to flow directly from high to low pressure, but gradually changes direction until it ends up moving along the isobars with low pressure on its left hand side; an effect first explained in 1835 by Gaspard de Coriolis, from whom the phenomenon takes its name.