John Campbell, age 10, of Victoria, B.C., for his question:
What are isobars?
A weather expert can look at a picture of isobars and tell you whether you should take a raincoat or a beach umbrella on tomorrow’s picnic. For of all the items on a weather map, the isobars have the most information to give, They look very complicated, but it is not hard to understand them and when you can read the isobars on a weather map, you too can be a weather prophet.
Isobars tell the weight or pressure of the air over a wide area. This is vital information, because air pressure has a lot to do with the changing weather. A dense, heavy mass of air tends to flow outward into regions of lighter air. A mass of light air tends to form a sort of pocket in the atmosphere and the surrounding, denser air blows in to fill it.
Our weather passes over the land in a series of calm and stormy air masses. A high pressure area produces gentle breezes flowing out from a central mass of heavy air. A low pressure area tends to produce stormy winds blowing in towards a pocket of lighter, low pressure air. The spinning earth twists these winds and makes them blow in spirals. Winds of a high pressure area spiral clockwise and winds of a low pressure area spiral counterclockwise.
These patterns of highs and lows govern, much of our weather and isobars give a picture of the air pressure which causes them. On a map, the isobars look like the wavy outlines of a series of islands, one inside another. The center of the series is the region of high or low pressure which influences the weather over a wide area.
Each isobar is put together from information coming in from widely scattered weather stations. Each station reports air pressure along with other standard weather items. The air pressure is given in units called millibars and one by one the farflung reports are recorded on a big weather map, Then a line is drawn through all of the places reporting air pressure of,, say, 1,020 millibars. This line is an isobar.
Another isobar is then drawn through all the points with air pressure of, say, 1,010 millibars. This isobar is a line of lower air pressure. If it is inside the first isobar, then the weather expert suspects he is dealing with a low pressure system, with its promises of blustery storms. If the second isobar is outside the first, then the approaching weather pattern may be a high pressure air mass with promises of fair weather.
Air pressure, of course, is recorded on a barometer. This instrument gives the weight of the column of air directly above it. An everyday barometer gives the air pressure in a scale of inches and standard air pressure is 29.92 inches. The barometer of a weather expert is usually marked in millibars to give a more exact measure. On this barometer, standard air pressure is 1013.2 millibars,