How Does the Weather Work?

Snow! We skiers think about snow a lot this time of year: Where it comes from, how likely it is, but who really understands the weather? Let’s see if we can shift our imagination into high gear and give this a shot.

There are two huge engines that drive the weather. One is the sun heating up the air, evaporating moisture into it, and causing it to circulate in convection currents that take masses of air up from the surface of the earth and back down again as they are heated and cooled. The other driving force is the rotation of the earth. This rotation, usually not apparent to us, is the basis of the Coriolis force, which, as we shall see, can have quite spectacular effects.

Imagine a sphere in empty space. Let’s say it’s about 25,000 miles around (the same size as our earth, in fact). And, we’ll cover it with a smooth linoleum floor (no oceans or land at this point). But we will give it a spin, so that it rotates once every twenty-four hours. And, to better see this rotation, we’ll put a big light in nearby space. Say, 93,000,000 miles away.

Now, because our sphere rotates, it has an axis with two poles. Arbitrarily, we’ll call one of these the North Pole, and the other the South Pole. Imagine standing exactly at the North Pole with your arms outstretched. Standing on this immense linoleum plain, you are slowly rotating counter clockwise. Once every twenty-four hours your right arm moves slowly forward, your left arm moves backward, and you make a complete turn.

An ice skater goes into a spin by pulling her arms and legs in after she starts a turn. You could do the same. If there were a pivot point under your feet with no friction and you pulled in your arms you would actually begin a slow spin. Instead of rotating once in 24 hours, you might now “spin” around once every two or three hours. This sort of a spin is simply the conservation of angular momentum you already possess.

Imagine a bunch of huge merry-go-rounds each a mile in diameter and scattered all around this linoleum landscape at our North Pole. Imagine looking down on all these merry-go-rounds. Each one appears perfectly still, but it nevertheless rotates once every 24 hours along with the rest of our great sphere. Now imagine thousands of people getting onto the merry-go-rounds and rushing into their centers. What happens? The merry-go-rounds (and the people), already spinning once each 24 hours now begin to spin faster (because of the angular momentum of the people). They spin counter clockwise because that’s the way our sphere is already spinning. The more the angular momentum already possessed by these systems (of merry-go-rounds and people) is concentrated toward the axis of the system, the more quickly the whole system spins.

Now, let’s go from this perfect sphere back to the familiar but chaotic world around us. When water in a bathtub moves toward the drain, it tends to spin in a counter clockwise direction. This is in the Northern Hemisphere where things are already spinning that way (in the Southern Hemisphere, it’s the opposite). The same thing happens to a mass of air. If air is warmed it expands; if cooled it contracts. Imagine a huge mass of air being cooled. It contracts. It moves inward. It was already rotating along with the rest of the earth once every 24 hours. Now this rotation is amplified. This is why northern air masses always rotate counter clockwise around a region of low pressure. Likewise, if air rises, it is replaced by air flowing in from the surrounding area near the ground. The spin picked up by air rising in a column can be most dramatic, because the column is often very thin, concentrating the spin very tightly. We call such a column a tornado.

We know that the sun heats the air more at the equator than it does at the poles. All other things being equal, this causes the warm air to rise at the equator and flow north and south at high altitude towards the poles. It then cools, settles, and returns to the equator by being pushed along the ground. Both the air and the ground at the equator are moving over a thousand miles an hour (the equator is 25,000 miles around and it rotates once every 24 hours; 25,000 divided by 24 is a little over 1000). What happens to this speed as the air moves toward the North Pole? It becomes the jet stream. The air, already moving with the earth at 1000 miles an hour begins to move over regions of the earth that are moving slower and slower, until the north pole is reached and the ground isn’t moving at all (it is simply rotating in place). Of course, a lot of this speed is lost to friction, but the effect is that the jet stream in the upper atmosphere is seen as a current of air moving from west to east often exceeding two hundred miles an hour. This can cause quite a head or tail wind for aircraft flying west or east at high altitudes.

Now, what happens when the air cools in the polar regions, sinks to the ground, and is pushed back to the equator? The reverse occurs. As the air moves away from the pole it is going slower than the ground under it. The ground is moving to the east faster than the air, so the air appears to be moving to the west. This is what causes the trade winds that move (mostly across the oceans) from east to west. The regions closer to the poles, before these winds have a chance to build up, are known to sailors as the doldrums. As the winds finally begin to match speed with the rotating earth beneath them, between about thirty degrees north and south latitude, we have what sailors called the horse latitudes. Between these two were the trade routes for sailing ships driven by the trade winds.

The Chinook and Santa Ana winds are different forms of the jet stream and trade winds, respectively. The Chinook happens when the jet stream touches down over the Rocky Mountains. The Santa Ana forms across the deserts of the southwest. The Chinook usually comes from a westerly direction, the Santa Ana from the east.

Other factors also contribute to the weather. Land warms in the sunlight more quickly than water. When dawn breaks, warm air begins to rise on shore and this pulls in the cooler air from the sea causing a sea breeze in the morning. When the sun goes down, the air cools more quickly over the land, and the air blows back out to sea later in the day and during the night. When water picks up the energy from the sun it evaporates into the air. This energy is released when the water condenses as rain or snow.

Why does it rain or snow? Because when the warm air rises, the pressure on it, higher in the atmosphere, goes down. The temperature also drops. The lower temperature and pressure cause the moisture to condense, form tiny droplets that are first seen as clouds, and then aggregate into larger drops to fall as rain, sleet, or snow, depending on the temperature and the amount of water vapor in the air. To complicate the process further, when the clouds form, they shade the sun and cause temperatures to fall quickly and unevenly. Between clouds, of course, the sun shines through. It hits and warms the ground, and causes the air in contact with the ground to rise. Great convective cells can form along with great differences of temperature and pressure. Intense low-pressure regions can give rise to the swirling air of tornadoes on the local scale, and to hurricanes on a much larger scale. Great differences of temperature in convective cells can cause drops of rain to cycle high into the atmosphere until they freeze and become large enough to fall as hailstones.

The transport mechanisms of heat and global rotation move the atmosphere all over the world. In November, a lot of the moisture that evaporates into the air from the Pacific rises, moves north, and finds itself turning into the jet stream and moving east. As it moves first north, then east, and rises higher, snow falls on the Sierras, the Cascades, the LaSals, and the Rockies. This causes skiers to appear on the mountain slopes, sales of Chapstick to increase, and thoughts of snow and winter wonderlands to occupy people’s minds.