Kathy Staonyk, age 13, of Winnipeg, Manitoba, Canada, for her .question:
Is global air pressure always equal?
On a global scale, air pressure must always equal the total weight of the earth's atmosphere. So far as we know, this does not vary. On a global scale, neither does the planetary pulling force of gravity. Weight and gravity strive to create an equal balance. However, lately Andy's readers have been interested in factors that oppose the force of gravity and upset these.balanced equations.
The total weight of the atmosphere is about 5,600 million million tons. It is the force of gravity exerted from the surface to the top of the airy gases. A barometer can measure a small local sample by the pressure on a tube of mercury. It weighs a column of air one inch square that reaches from sea level to the top of the atmosphere. If the global air remained motionless, this pressure would be about 14 1/2 pounds on each square inch, or more than a ton on each square foot of the earth's surface.
Fortunately this stifling situation is impossible. Gravity cannot exert equal force on a global scale because patches of restless air constantly lose and gain weight. The gaseous air is more fluid than running water, less able to resist outside forces. Above the equator, its moving molecules use heat to spread apart in masses of light, low pressure. Above the poles, they crowd together in dense, heavy masses of high pressure. The unequal distribution of the sun's radiation creates a global patchwork of highs and lows and varies them with the seasons.
Other factors strive to stir up the patchy highs and lows and redistribute the weight of the global atmosphere. The nature of gases insists that they mix and mingle until their weight, warmth and other factors are equally shared. This sets the atmosphere in.motion. Heavy pockets of high pressure tend to blow toward light, low pressure masses, much as water flows downhill. A moving mass of high pressure tends to waft along a spell of calm, fair weather. A period of low pressure often brings a stormy spell.
The blowing air tends to change its character as it goes. Warm, moist air from above tropical seas often becomes stormy as it blows northward. When it sheds its moisture, it also loses heat and it becomes heavier. Dozens of other factors help to cause local highs and lows and stir them together in a weathery patchwork of air pressures constantly changing around the globe.
Gravity exerts its strongest force on the lowest layer of the atmosphere. About 99 per cent of the total weight is below 19 miles. Above it the calmer air grows thinner until the uppermost gases get lost in space about 1,000 miles above our heads. As we go higher, we leave part of the air's weight below us. So naturally, atmospheric pressure always decreases with altitude.