To understand why air moves, it helps to understand air pressure, which is the amount of force that moving air exerts on an object. There are several ways of measuring atmospheric pressure, the most common being inches of mercury, which we use in the United States, and millibars, the metric equivalent.
If you could somehow isolate a 1-inch-square column of the atmosphere, from the surface all the way to the top of the troposphere, it would weigh just about 14.7 pounds. So meteorologists say that air pressure at sea level is 14.7 pounds per square inch, or psi. That translates to 29.92 inches of mercury (abbreviated as Hg, the symbol for mercury on the periodic table of elements) or 1013.25 millibars. In case you’re wondering, one millibar is equal to 0.02953 inches of mercury.
With nearly 15 pounds of pressure pushing against every square inch of your body, you’d think it would be hard to even take a breath. Fortunately, nature does its best to stay in balance, and there is just as much pressure pushing outward in each cell of your body as there is outside pushing inward. This shows you just how well we’ve adapted to living on the surface of this planet. But what if you’re not on the surface, but up higher where air pressure is less, as you find when climbing a mountain or flying in a plane? As you climb higher, the pressure in your body becomes greater than the pressure outside, and you’ll probably start to notice an uncomfortable pressure in your inner ear as those 14.7 pounds of pressure try to get out.
Atmospheric pressure is measured using a device called a barometer, which is either liquid filled (which is where the inches-of-mercury method comes from) or metal based. Although you’ll hear your local TV weatherperson use the term “inches of mercury” a lot, liquid barometers are rarely used these days; the aneroid barometer, which uses variations in the shape of a metal cell to measure air pressure, is now much more common, as are newer electronic models.
Unlike temperature, air pressure decreases the higher you go in the atmosphere. (You’d think temperature would go down with increases in altitude, and it does to a certain point. But then it goes up again before coming back down. It isn’t what you’d expect, is it? For details, see the section titled What’s the Atmosphere?) The only thing keeping all of Earth’s air from leaking out into space is gravity, which pulls air molecules toward the earth’s surface. Air at ground level is under more pressure because of the weight of all the air above it, so the higher you go, the less pressure you’ll find. At a height of 3.5 miles, air pressure is only half what it is at the surface, so at this altitude you’re above half of all the air molecules in the atmosphere.
When warm air rises, it relieves the pressure of the air beneath it and so creates an area of low pressure. But if that same rising air mass cools, then it sinks and presses down on the air below it to create an area of high pressure. Because the atmosphere is always trying to keep itself in balance, and because low-pressure systems are actually partial vacuums, air moves from high-pressure systems to areas of low pressure, producing wind.
The difference in air pressure between air masses is called a pressure gradient, and the higher the gradient, the faster the winds will blow. Because the earth rotates, those winds turn to the right in the Northern Hemisphere and to the left in the Southern, following a path first discovered in 1835 by Gaspard-Gustave de Coriolis, a French engineer and mathematician. Coriolis applied the element of rotation to Newton’s Laws of Motion, describing how a free-floating object near the earth’s surface appears to curve as the globe rotates beneath it. You can duplicate the Coriolis effect by having someone turn a globe while you try to draw a straight line on it from north to south with a piece of chalk: what you’ll end up with is a curved line.
The Coriolis effect is what imparts rotation to weather systems. It affects any moving object not attached to the earth’s surface, from space shuttles to artillery shells.
You’ve probably seen weather maps with swirled lines that sometimes look like fingerprints. These swirled lines are called isobars, and they connect locations with equal air pressure. Multiple isobars usually form a target shape, and in the middle you’ll find a capital “H” or “L”—a high- or low-pressure area. Because high-pressure areas contain air that is sinking toward the surface, they’re usually associated with fair weather; while low-pressure systems, which contain rising air, are more unstable and often mean a dose of rain, snow, or worse. Remember that air is being pushed out of high-pressure systems as it hits the surface and spreads out, while lows tend to suck in air at the surface and pile it up into clouds and storms.
Because lows turn counterclockwise and highs clockwise in the Northern Hemisphere, you can look at an isobar map and tell which way the wind is blowing. Wind blows parallel to isobars above the surface, and the closer together the lines are, the faster the wind speed. In a hurricane, isobars are so tightly packed they almost merge together.
However, down at ground level, the friction caused by air blowing over objects such as mountains, trees, dogs, and people slows down the wind and partially cancels out the Coriolis effect, allowing air to cross isobars as it flows toward low-pressure areas. The section of atmosphere below around 3,300 feet is called the friction layer for that reason.
The isobar maps you see on TV and in the newspaper are called constant height charts because they show equal areas of air pressure at a single height, such as sea level. Another type of map that meteorologists often use is called a constant pressure chart, because it connects areas with the same air pressure whether they’re found at the surface or higher in the atmosphere. On a constant pressure chart, meteorologists pick a pressure and show you at which altitude that pressure can be found in different locations.
If you were able to ride along a line on a constant pressure chart, you’d rise and fall as you curved around a low or a high, because air pressure varies by height depending on air temperature and other factors. You can think of isobars on a constant height chart as being narrow speedways around lows or highs, whereas the contour lines on a constant pressure chart are more like roller coasters.
While surface maps tell us what the weather is like outside our windows, upper-air maps can tell forecasters what kind of weather we may experience in the near future. With nothing to slow them down, winds aloft almost always blow faster than air that flows along at ground level. Winds in the upper atmosphere generally blow from west to east, creating a zonal flow where the wind follows the lines of latitude that wrap around the earth horizontally. In a zonal flow, storm systems follow the course of least resistance, making a beeline across the country as fast as the wind will carry them. Because temperatures don’t differ much within a zonal flow, they don’t usually bring severe weather.
So what happens when something comes along to disturb the air’s nice straight course; something like a large mass of cold air moving down from Canada, or a big sticky bubble of hot air floating northward from the Gulf of Mexico? Then we have a meridional flow, so-called because those systems move roughly north or south along meridians, the lines that mark off longitude. A meridional flow indicates that air masses from the north and south are mixing, and that can mean stormy weather as areas of differing temperatures battle it out for air superiority.