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Sally Caine, age 12, of Montreal, Quebec Canada, for her question:

What does the weatherman mean by barometric pressure?

The weatherman may give the barometric pressure in inches, centimeters or millibars and he usually adds that it is rising, falling or steady. He also reports the local wind direction and nay riention its speed. Mysteriously, rising or falling baro¬metric pressures may precede either storms or clearing skies. Naturally we want to know what goes on behind the scenes.

Barometric pressure is the weight of the atmosphere pressing down on the surface of the earth. Naturally the weatherman doesn't weigh the whole thin, which is estimated to be about five quadrillion tons. He weighs a very small sample of the local atmo¬sphere. The instrument he uses is the barometer. It measures a small sample, a column of air that presses down on one square inch of the surface, all the way down from the very top of the atmosphere.

Warm air tends to be thinner and lighter than cold air. Hence, cold air tends to weigh more and exert riore pressure. The standard mercury barometer weighs a sample of local air by the pressure it exerts on a glass tube of mercury. Since the weathery air changes from day to day, so does the local barometric pressure.

Therefore, a sort of midway point is needed to estimate the barometric rising and falling, highs and lows. The standard unit is taken from a column of average air press

ing, down on one square inch at sea level. The weatherman may give this barometric pressure at 29.92 inches or as 76 centimeters. then talking with fellow meteorologists, he may report it more precisely as 760 millibars.

By itself, barometric pressure tells only the weight of a local sample of the atmo¬sphere. True, an enormous air mass may be passing slowly overhead. If the barometric pressure is a steady 30.10 or 30.20 inches and moderate winds are northwest or southwest, your local weatherman is likely to predict a few days of fair weather, with little change in temperature.

However, the picture changes when masses of warm and cool air collide. Winds tend to blow from masses of dense, high pressure air into a region of light low pressure. As these weathery conflicts pass overhead, the airy turmoil causes the barometric pressure to rise or fall    as the local air gains or loses weight.

Of course, the global weather picture changes with the seasons and varies from  place to place between the equator and the poles. But as a rule, the ups and downs of barometric pressure indicate what's ahead in this or that local situation.

Let's see what sometimes happens in southeastern Canada. Suppose the wind is southeast, the barometer 30.10 inches and falling. The weatherman may forecast stronger winds and rain within 24 hours. E'hen the wind shifts to the southwest and the barometer rises slowing above 30 inches, he may forecast clearing skies and a few days of fair weather.

 

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