Meteorologists are very fond of isopleths. They draw them quite compulsively anytime and anywhere they can. The opportunities to indulge themselves are legion in their walk of life. An isopleth is a line drawn upon a chart which joins points, having assigned to them the same numerical value of any given element.
They come in many different guises, having names like isotherms (which join points of equal temperature); isohyets (lines of equal rainfall); isohels (which show the distribution of recorded sunshine); and isotachs (which join the points where the speed of the wind is uniform).
But perhaps the most familiar one to readers of this column is the isobar, a line joining points of equal pressure.
A well-drawn isobar is a thing of beauty to a weather-person. He or she sees it as a personal work of art: a thin black line sweeping its way across the chart in graceful curves and slowly, gently changing course to follow the local flow of wind.
When the pattern is complete, the isobars show clearly the areas of high and low pressure, the depressions and anticyclones that shape our daily weather. The whole ensemble is made possible by noting the pressure values reported from hundreds of weather stations over the thousands of square miles represented on the chart.
But the concept is less simple than it seems. Pressure, as we know, varies considerably in the vertical; it decreases by about one hectopascal for every 30 feet in height, a very much greater rate of change than often appears over considerable distances in the horizontal.
And since weather stations differ considerably in their altitude, the atmospheric pressure read directly from the barometer at each station must be corrected to refer to a standard level, the chosen one being Mean Sea Level.
The correction consists of "reducing" the pressure read at each station to a notional value appropriate to sea level immediately beneath it, by assuming that air, rather than rock or soil, fills the intervening space. It is accomplished by adding to the pressure reading from the barometer, an amount equal to the weight of an imaginary column of air extending from barometer down to sea level.
This is not difficult to calculate, and depends mainly on the air temperature at the time of the reading; in practice, however, the required corrections are worked out for each station for a complete range of possible temperatures, and all the observer has to do is to read the appropriate figure from a card.