THE long litany of works of William Wordsworth contains, surprisingly perhaps, many trivia for children. One such trivium if one might call it that, was Address to a Child During a Boisterous Winter Evening. It starts like this:
What way does the wind come?
What way does he go?
He rides over water and over the snow,
Through wood and through
vale; and o'er rocky height,
Which the goat cannot climb
takes his sounding flight.
Now as questions go, what way does the wind come? and what way does he go? are not trivial at all. The wind has its origins, as we know, in differences in atmospheric pressure over the Earth's surface, but we would normally expect air under the influence of these pressure differences to move from high pressure to low pressure. And so it would if it were not for the spin of the Earth, which, except at the equator, causes the moving air to swerve continually to the right.
We can imagine each particle of air as being under the influence of two forces: a driving force caused by the uneven horizontal distribution of pressure, and a steering force caused by the Earth's rotation. In time, these two forces come into balance, and in these circumstances the resultant motion of the air is along the isobars. In this state of steady motion, the atmosphere is said by meteorologists to be in geostrophic balance, and the motion of the air along the isobars is called the geostrophic wind. Traditionally, it has been by measuring this geostrophic wind from a forecast chart that the forecaster estimates the likely wind at any time in the future.
The geostrophic wind is inversely proportional to the distance apart of the isobars: the closer together they are, the stronger the wind will be. But there are further complications, in that the geostrophic wind also varies with latitude. A given distance apart of the isobars gives a stronger wind at, say, 30 degrees north latitude than it does at 60 degrees north.
One practical solution for the forecaster is to use a geostrophic scale. This is a special diagram constructed specifically for the type of chart in use, and which shows, in the form of a series of curved lines, the appropriate wind at each latitude for a given distance apart of the isobars. To find the appropriate wind, the forecaster uses a pair of dividers to measure the perpendicular distance between two adjacent isobars, notes the latitude, and then reads the wind speed from the geostrophic scale.
But nowadays, of course, they simply do it with computers.