It begins in a flight of molecules. Bits of air spiral skyward, carrying water vapor high up to where the air cools and the vapor condenses.
In the Sahel in Africa, just below the Sahara, a farmer stands in his field, shading his eyes against the sun to look at a small collection of clouds forming in the east. He nods and goes back to hacking at the earth under his hoe. It is hard work but the earth yields to him. A few years before it would not. The Sahel had weathered a long drought, almost twenty years. Now it is ending.
Rain pours onto the brick-dry earth. The farmer does not pause in his work. The clouds pass. He turns and watches the small storm move off to the west, to the sea. The sea does not need your water, he thinks. I need it here. The sun leans on him. The ground has already absorbed the rain, but the air feels light. That is good. There will be more rain, he thinks, and returns again to his work.
The farmer is right. He does not know it, but he is working under a long trench in the atmosphere through which pass waves of low pressure. Low pressure brings winds and rain. High pressure brings sunshine and clear skies. The farmer knows nothing of highs and lows. But he knows the feel of the air.
Just off the coast, near the Cape Verde Islands, a lone fisherman eyes the flat sea, looking for his living. A shadow moves beneath the surface. He holds his breath, raises his net, sets his feet firmly against the sides of his small boat. There is a ripple. He casts the net. There is a quick flurry of movement and a small explosion of pent breath. The fish flaps desperately, pulled out of its natural element into a deadly environment of air, air full of oxygen it cannot use. In its futile thrashings, it pumps this useless matter through its gills.
The bits of air disturbed by the movement and the explosion and the struggle jostle one another, pushing for position. It is a small variation of the never-ending dance that distributes their parts of oxygen, nitrogen, argon, neon, carbon dioxide and water vapor with such efficiency that their proportions in a parcel of air at the surface of the sea are the same as that of a parcel sixty-five miles up.
But heavier. A given amount of air at sea level weighs more than the same amount above the clouds. A given amount of water, on the other hand, weighs as much at the bottom of the sea as it does just beneath the surface. This is because air can be compressed; water cannot. A person standing at at the bottom of either ocean bears the weight of all the parcels above him.
The fisherman does not stagger under his burden of air. He may not be aware that he carries it, but it is of no more concern to him than the weight of the sea under his boat. He does not know, either, that the pressure that he bears is not precisely the same as that which a fisherman in the South China Sea carries on his shoulders. Pressure at sea level varies from place to place in the world and from time to time it’s 14.696 pounds per square inch along the East Coast of the United States but it is enough of a constant to be named One Atmosphere or One G (for gravity). Scientists measure all other pressures by it, from the bottoms of mines to the tops of mountains.
Meteorologists measure pressure in inches of mercury or millibars. In 1643, Evangelista Torricelli invented the barometer, just a few years after Galileo invented the thermometer. They are still the two most basic tools of meteorology. Torricelli discovered that the weight of the atmosphere at sea level was just enough to support a column of water thirty-four feet high. He calculated that the water in the tube would fall by about one foot for every nine hundred feet of elevation.
A glass tube thirty-four feet high would be too unwieldy to haul up and down mountains. Torricelli soon found that a column of mercury about 30 inches high was the equivalent. It is this “glass” that mariners referred to in old sea logs. When the glass was dropping, it meant the atmospheric pressure was not enough to hold the mercury at the 30-inch level. Nobody knew about highs and lows then, only that a falling glass meant bad weather. Dropping below 29 inches was serious; anything below 28 inches was serious trouble. Sea level pressure on the East Coast is 29.92 inches, varying a bit with temperature.
The fisherman boats his catch, glances at the scuds of cloud to the east and decides to head home. Slants of warm rain wet him; the sun steams him dry; the clouds dissolve, reform, move west. This he can see. What he cannot see are the molecules of air he disturbed, rolling in the sun, growing warmer. The warmth excites them to greater activity. They push at one another. Small winds snatch at them, catch them up, spill them out again. Larger winds entrain them for longer runs.