Janet Hodgson, age 12, of Toronto, Ont.,

What causes high and low pressure areas?

We hear of the approach of high pressure areas and low pressure areas when we listen to the daily weather reports. And we soon come to like the highs and dislike the lows. For a high pressure cell tends to bring a spell of fair weather. The steady temperature may be warm or cool, the winds are light and skies are generally clear with little or no rainfall. A low pressure cell brings a spell of unsettled weather with strong, blustery winds, cloudy skies and usually some rain or snowfall:

Air pressure is the weight of the air pressing down on the face of the globe and the restless air is constantly piling up into dense, heavy masses and spreading out into thin, light masses. Dense, heavy air tends to flow and blow towards masses of thin, lighter air much as water runs downhill. An area of high pressure is a great mass of dense air with a wind system tending to blow outwards to mingle with surrounding lighter air. An area of low pressure is a pocket of light, often rising air. Winds from the heavier air masses which surround it tend to blow into the pocket of low pressure.

An air mass takes its character from the land and sea on which it sits. A high pressure cell hatches when masses of cool, heavy air compress and descend to the surface. This happens over polar regions. It also happens over tropical seas where warm air expands and rises and in so doing cools itself, compresses and descends in a heavy mass to the surface. The major high pressure cells which bring spells of fair, steady weather to North America are hatched in the far north or over the warm seas of Florida and Southern California. The vast air mass may be a few hundred or several thousand miles wide.

A low pressure cell, maybe 20 miles wide, may form from the light, rising air under a thunderhead. Others form above the warm and protected ground on the sheltered side of tall mountains. But our major lows are formed, or so we think, by the areas of .high pressure. Two highs of different temperatures tend to force the air masses between them to pile up in a dense, turbulent mass which becomes an unsettled area of low pressure„ These lows tend to alternate with highs parading one after another along certain paths across the globe.

The wind system of a weather cell is given a twist by the spinning earth. In the northern Hemisphere, the winds of a high pressure area spiral outward in a clockwise direction and the wind system of a low spirals inward in a counterclockwise direction. South of the equator, the spirals take the opposite directions. As a rule, the highs and lows are nudged along by the prevailing winds, though some are guided by shores and mountains. When two different air masses collide, they may change course or declare war and fight it out in a stormy weather front,