One of the more surprising features of the weather map is the fact that the wind blows along the isobars - along lines of constant pressure - rather than flowing, as one might reasonably expect, from places where the atmospheric pressure is high to where it is relatively low. The latter would, in fact, happen if the Earth were smooth, flat and stationary.
But the Earth rotates. The surface of the planet moves fastest at the equator, where it must complete a circle of 25,000 miles in 24 hours at a speed of just over 1,000 miles per hour.
North and south of the equator, however, the surface moves more slowly, since it describes a smaller circle in a day - and approaching the poles the circle is very small indeed.
The consequences of these different rates of progress for any large-scale movement over the surface of the planet were first outlined by a Frenchman called Gaspard de Coriolis in 1835; the resulting phenomena are therefore said to be caused by the Coriolis effect.
The Coriolis effect is a relatively simple concept mathematically but is difficult to visualise in physical terms. The end result, however, is also very simple: the general rule is 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.
Coriolis was an engineer by profession, and his main interest was in large turbines and ballistics.
Although he was vaguely aware of the implications for meteorology of his theories, he paid little attention to this application; it was an American teacher called William Ferrel who applied the concept to the wind.
Ferrel was born in 1817 in Pennsylvania. Of farming stock, he followed his profession of schoolteacher for over 20 years in Tennessee, Kentucky and Montana.
He had also, however, developed an interest in the mathematics of the weather, and it was in 1857 that, largely independently, he applied the concepts originated by Coriolis to the movement of air over the surface of the Earth.
Ferrel's theories are still with us, to the extent it has often been suggested that in the context of meteorology the phenomenon might more properly be known as the Ferrel effect.
William Ferrel went on to make several more significant contributions to meteorology. He died on a visit to Canada 110 years ago today, on September 18th, 1891.