Joe B. Long, ago 10, Bellaire., Texas, for his question:
How is, a barometer useful in weather forecasting?
A barometer measures the weight of the air pressing down upon the earth or the ocean. And the weight of the air changes with the weather. If you take readings from a barometer every few minutes, you can tell whether the air pressure is rising or falling ‑ whether the air is getting heavier or lighter. Rising air pressure is often a sign of good weather and falling air pressure may herald a storm.
Your barometer can give a hint about the weather forecast from one single place. The weatherman has co‑workers at countless weather stations all over the world. They gather information such as air pressure, wind speed and direction, temperature and rainfall, and flash it into headquarters by radio and telephone. The weatherman at headquarters can get a picture of surface weather conditions any place on earth every six hours.
He pinpoints the countless bits of weather information over a large map. And perhaps the most important information concerns the air pressure as recorded by many barometers at many different places. He figures the force of air pressure in units called bars and one bar equals one thousand millibars, The standard unit is the force of air pressure at sea level when the temperature is at 59 degrees Fahrenheit. This unit of one standard atmospheric pressure is 1013.2 millibars.
Almost always, the weather report shows that the air pressure is either more or less than one standard unit. When it is, say, 1032, the weatherman knows where to find an area of high pressure. Here the air is dense and heavy,, and winds tend to blow out from it rather like water running down a hill. If the earth were not spinning around like a top, the winds would blow straight out from a high pressure area like the spokes of a wheel.
But the spinning of the earth gives then a twist and they blow out in spirals.
The weather map‑maker notes the pressure at many places around the high pressure area. He notes all the places which have, say, a pressure of 1030 millibars. He connects these areas with a line called an isobar. He notes all the spots with pressure of, say 1025 millibars and connects them with another isobar. Soon he has a series of isobars on his map. They tell him about the shape, size and strength of the air mass. Other information tells him when we can expect this period of pleasant weather and he hands on the good news to us in a weather forecast.
When the weatherman hears of an area of low pressure he will issue a storm warning. Here the winds tend to blow inward to a region of light rising air, spiralling as the‑ go. If this region of high pressure is off the West Indies during the summer months, the weatherman calls for more and more information in a hurry. Pretty soon he may be sending the weather planes aloft every six hours to chart the course of a growing hurricane.