MARINERS have some strange ideas about the origins of storms. For instance, according to David Gutterson, in Snow Falling on Cedars: "A fisherman didn't utter the words horse, pig or hog on the deck of a gill netter, for to do so was to bring bad weather down around his head, or cause a line to foul in his propeller. Turning a hatch cover upside down brought a southwest storm. Only a greenhorn would think to trim his fingernails while sitting in a seine pile, or hand a shipmate a bar of soap as opposed to dropping it into his wash basin, or cut the bottom end off a can of fruit. Bad fishing and bad weather could result from any of these things."
Meteorologists, however, look for rather different signs of imminent gales. They estimate the probable strength of the wind from isobars - the lines drawn to join points of equal barometric pressure on a weather chart.
We would normally expect air under the influence of pressure differences on the Earth's surface to move directly from high pressure to low pressure - and so it would if our earthly sphere did not rotate. As things are, however, the morning air swerves continually to the right. Indeed, 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. When the atmosphere is in a turbulent well lubricated motion, these two forces come into equilibrium, and the air can be regarded as blowing almost along the isobars - the so called geostrophic wind. It is by estimating this geostrophic wind from a forecast chart that meteorologists assesses the likely strength and direction at some future time.
The geostrophic wind is inversely proportional to the distance apart of the isobars - the closer they are together, the stronger the wind will be. But it also varies with latitude - a given separation of the lines gives a stronger wind at, say, 30 degrees north latitude than it does at 60 degrees north. To estimate the wind at any point on a chart, therefore, an inexperienced forecaster uses a pair of dividers and a "geostrophic scale", a special template from which you can read off the appropriate wind speed at each latitude for a given distance apart of the isobaric lines.
With practice, however, a forecaster quickly develops what is often called a geostrophic eye - the ability to estimate the probable wind, accurate to within a knot or two, at any given point in the area of interest merely by looking at the chart.