Lesson 1: What's Up? A Tour of the Atmosphere

What's Up? A Tour of the Atmosphere

We live in an ocean of air, but we tend to take it for granted. Unless you're trying to read a newspaper outside on a windy day, chances are you've rarely noticed the quintillions of molecules that are constantly flowing around your body.

When you type at your keyboard, the movement of your hands sets in motion millions of molecules of nitrogen, oxygen, and water vapor.

These are the percentages of gasses that make up our atmosphere:

Even a change of just a few hundred feet can make a difference. When your ears pop in an elevator, that's due to the change in pressure.

Try This Experiment

If you ever have the opportunity to drive up a mountain like New Hampshire's Mount Washington or Colorado's Pike's Peak, bring along an unopened bag of potato chips. Inside, the air is pushing out with a pressure of 14.7 pounds per square inch (depending on where it was originally bagged or bought, minus any leakage). By the time you reach the top, the bag should be bulging like a balloon. It may even pop because the inside pressure is now greater than the outside pressure.

How Much Does Air Weigh?

Empty air (particularly the phrase "full of hot air" in a political year) is usually synonymous with "nothing." But you'd be surprised at how much air actually weighs. The air in a small room weighs at least several pounds. The Earth's entire atmosphere weighs more than five quadrillion tons, or 10,000,000,000,000,000,000 pounds. That's a whole lot of nothing

Here's a list of the layers of Earth's atmosphere:

The troposphere is where weather happens. It extends to a height of about 10 miles at the equator, but only about 5 miles at the Poles. The difference is the result of the powerful centrifugal force generated by the Earth's rotation, making the atmosphere "bulge" at the equator. In middle latitudes such as the United States, the top of the troposphere is about seven miles high.

Nearly all clouds, weather, and water vapor exist in this bottommost layer. As wind blows across oceans and continents, it collects microscopic particles such as specks of sea salt and dust. These particles plus water vapor make up the main ingredients in a recipe for clouds and weather. Higher layers of the atmosphere don't mix much with the bottom layer. As a result, they contain almost no water vapor and therefore little or nothing that we'd think of as "weather."

Feeling the Pressure

Air, like water, is a fluid. Unlike water, however, it's invisible -- and it's easy to walk or run through. Try running neck-deep in a swimming pool and you'll notice the difference. At sea level, the atmosphere exerts a pressure of 14.7 pounds per square inch. As you go up in altitude, the pressure goes down. In a mile-high city like Denver, the pressure is only about 12.5 pounds per square inch

Waterworld

Unlike other planets in our solar system, Earth has an abundance of water. In the atmosphere, water vapor amounts to only a trace, usually less than 1 percent. But it's an awfully important trace. We wouldn't have much weather without it.

With more than 1.6 billion billion -- yes, billion billion is correct -- tons of water on the planet, there's a lot to go around. Where is it all located?

• Nearly 98 percent of Earth's water lies in oceans, lakes, and rivers

• About 2 percent is locked up in polar ice caps and glaciers

• Only a fraction of a percent of all that water is in the form of water vapor, clouds, or precipitation

Figure 1-1: Wind rises when it encounters a hill or mountain. As it rises, it cools. If it cools to its dew point, fog will appear. (Photo courtesy of NOAA/Department of Commerce; Historic NWS Collection; www.photolib.noaa.gov)

Humidity and Dew Point

Two familiar meteorological terms on TV weather reports are "relative humidity" and "dew point."

Dew point refers to the temperature at which the water vapor in the air will condense, forming a cloud or fog. The closer the dew point is to the outside temperature, the greater the relative humidity is. When your warm breath forms a "cloud" on a cold winter day, it has cooled to the dew point. For a more in-depth look at dew point, see my book Tying Down the Wind, pages eight and nine.

Most weather books explain how warm air "holds" more water vapor than cold air. (See Tying Down the Wind, page 32, to see why some meteorologists dislike that explanation.) Just because two air masses have the same relative humidity doesn't necessarily mean they contain the same amount of water vapor, or moisture. An air mass that has a temperature of 90 degrees and a relative humidity of 80 percent contains much more moisture than air with a relative humidity of 80 percent and a temperature of 32 degrees. Both air masses have the same relative humidity, but more water vapor is present in the warmer air. That's why we call it "relative humidity" rather than just "humidity."

Dew point is a more useful number because it depends on just one thing: how much moisture is in the air. A dew point of 10 degrees means the air is fairly dry, and a dew point of 75 means it's muggy. But relative humidity depends on two factors: temperature and moisture. Dave Thurlow, host of the radio program The Weather Notebook, once explained, "Relative humidity goes up when the sun goes down and goes down when the sun goes up." (Take a look at figure 3.1 on page 19 of Arthur Upgren's book Weather to see how humidity fluctuates with temperature.)

How Much Does a Cloud Weigh?

A typical cotton ball cumulus cloud weighs anywhere from dozens to hundreds of tons. That's a lot of moisture. A thunderstorm tips the scales at millions of tons, and a hurricane weighs even more.

Let's leave the troposphere for a moment (don't worry, we'll come back) and continue touring the higher layers of the atmosphere. Better bring an oxygen mask, though. Up here, the air's much too thin to breathe. A coat would be a good idea, too, because it's getting chilly.

About 7 to 10 miles high, we encounter the tropopause. This is where the troposphere ends and the stratosphere begins. The temperature here is about -80 degrees Fahrenheit. In the troposphere, the higher you go, the colder it gets. But the tropopause is an inversion, a boundary between colder air below and warmer air above. In the stratosphere, the temperature actually starts to rise. The question is, why?

The Ozone Layer

Ozone is the answer. Most ozone exists in the stratosphere between about 15 to 20 miles high. It absorbs radiation from the sun, allowing a buildup of heat. On page 161 of Weather, figure 14.2 shows how much colder the stratosphere would be if there was no ozone layer.

When it occurs at ground level, ozone is pollution. But high in the atmosphere, it shields us -- and all life on Earth -- from harmful ultraviolet rays.