Eileen Tamaki, Age 12, Of Toronto, Ont., Canada, for her question:
How do they forecast the weather?
Weather forecasting is based on reports from thousands of wind vanes and rain gauges, balloons, barometers and pictures and data sent down from weather satellites. These data, plus other local and global evidence, is processed and analyzed by meteorologists. The result is a picture of the weather on its way here from somewhere else.
Old timers looked at the sky for signs of the weather ahead and often the signs kept their promises. You, too, can try to guess the coming weather from what you see. Chances are the day will be fine and fair if gentle breezes blow from the west or if fluffy white clouds drift over a b1ue sky. Chances are the day will be rainy when the moon wears a halo or when the sky is curtained with dark clouds.
You can make better forecasts with the help of a few weather gadgets. A barometer is a glass tube holding a column of mercury that rises and falls with changing air pressure. Fair weather often follows a rising barometer. A falling barometer may promise a storm. A wind vane gives you a record of the changing breezes, their strength and directions. You can predict a storm when a strong north wind shifts west and then south.
A rain gauge gives you a record of seasonal rainfall. The weather of any locality tends to repeat itself, more or less, year by year. With a weather thermometer you can keep seasonal records of temperature. A more complicated gadget is used to measure the humidity, which is the moisture in the air. The weather at ground level is often changed by winds aloft, and weathermen send up balloons to gather news from upstairs.
If you keep faithful records of your instruments, you have a real weather station. Most countries have a weather bureau to gather data from such stations scattered far and wide. Some send in daily reports, others report every hour and more data is radioed in from planes and ships. The weather bureau coordinates all data on a large map. A picture grows, showing the antics of the atmosphere over a vast area.
Experts read the signs from far and wide and forecasting is done on the basis of the maste.r_map. They know, for example, that a pocket of high pressure promises fine weather and that a low pressure cell threatens to bring storms. They know also that highs and lows tend to travel eastward, carried by the prevailing westerly winds. These and countless other factors are used to forecast what the weathery atmosphere plans to bring us tomorrow or next week.
The world's weather is created in the lowest level of the atmosphere. Masses of warm and cool air, damp and dry air merge and mingle around the globe with no concern for the man made boundries between countries. The weather is a global affair, and weathermen make their most accurate forecasts when they have data from around the world. For this reason, the weather bureaus of many countries exchange information on a friendly basis of give and take.
Weather satellites today have the ability to measure temperatures and pressures at various atmospheric levels, temperatures of the ocean waters and more important the ability to photograph cloud cover and storm clouds and transmit photos instantly to earth stations throughout the world. Gathering data is essential in weather prediction.