Last week we talked about teaching our children that they live at the bottom of an ocean of air, and that the crests and troughs in that ocean cause the high and low pressure centers we see indicated on newspaper and television weather maps.
Now let's add a couple more pieces of basic science knowledge that will help us make more sense out of a weather map and help us better understand what's happening in the atmosphere that surrounds us.Nature tries to even out those crests and troughs of air, and so air spills or flows down from a mound of high pressure into a valley of lower pressure (Every time I grudgingly sweep my carpets I'm reminded that nature and I are similar in at least one regard: We both abhor a vacuum).
The flow of that air would be straight downhill if it weren't for the fact that the Earth rotates toward the east, causing the outflowing air (in the Northern Hemisphere) to twist to the right as it rushes down and away from the center. This deflection is known as the Coriolis (kore-ee-OH-liss) force, and causes winds around a high to follow a clockwise path; winds around a low-pressure center flow counter-clockwise (In the Southern Hemisphere, the opposite is true). The steeper that mound of air is, the greater the wind speed will be around it.
Detailed weather maps show this with a series of concentric lines around each high and low. These lines connect regions of equal pressure, and so they are called isobars (EYE-so-barz), from the Greek words isos, meaning "equal" and baros, meaning "weight." Isobars that are spaced close together show great pressure differences within a short distance and warn of high winds and rough weather.
So, you should be able to tell, even from a simple weather map, the general direction of the winds in your area by looking for the local centers of high and low pressure. Having a barometer (again from the Greek baros, meaning "weight" and metron meaning "measure") in your home or classroom will allow you to see when and how the weight of the air overhead changes.
Your weather map will also show the position of weather "fronts," which form at the borders between masses of warm air and cold air. When two air masses of differing temperatures meet, they don't mix very well, and they struggle for control. Colder air is heavier than warmer air, and so is able to push the warmer air up and out of the way. You might demonstrate to your children how this looks on a small scale by putting some tap water in a glass and then slowly pouring in some chocolate syrup. The lighter, warmer air is forced to rise. It cools as it expands in the lower pressure of the upper altitudes and, because now it can't hold as much water vapor as it did when it was warmer, some of that vapor is released in the form of clouds or precipitation.
You can use your weather map, a barometer (or the pressure readings given in the newspaper or on television) and some scientific observation of your own to confirm that air pressure usually falls as a front approaches and rises after the front passes, while winds usually shift in a clockwise direction.
There is much more information about weather and weather maps to be found in the 551 section of your local library. You can also order a week's worth of detailed and instructive Daily Weather Maps by sending a check for $1.50 (payable to Department of Commerce, NOAA) to Daily Weather Maps, Climate Analysis Center, Room 808, World Weather Building, Washington, DC 20233.