Inversions

The tropopause is a planet-wide inversion, acting as a "roof" to weather. It stops the vertical growth of thunderstorms. In Los Angeles, a much smaller-scale inversion also creates a "roof," trapping haze and pollution and blocking the view of the mountains

How High Is the Sky?

How high is the sky anyway? On our field trip through the atmosphere, we've discovered that the air becomes too thin to breathe after only a few miles. At the top of Mount Everest, two-thirds of the atmosphere's mass is below us. By the time we've climbed midway through the stratosphere, already 99 percent of all molecules in the atmosphere are far below us. Hardly any air is left, but we still have three more layers of the atmosphere to visit!

Mesosphere

The mesosphere is next, starting at an altitude of about 50 miles. It's somewhat arbitrarily designated as the beginning of "space." As far as breathing is concerned, you wouldn't be able to tell the difference between the mesosphere and a vacuum. This is the coldest layer of Earth's atmosphere, bottoming out at about --150 degrees Fahrenheit.

Thermosphere

The thermosphere is utterly inhospitable. Temperatures here vary from 1000 degrees to over 3600 degrees Fahrenheit, depending on solar activity. The space shuttle orbits in the thermosphere, with a maximum ceiling of about 300 miles. Technically, at this stage of our tour we're still in the atmosphere (though you wouldn't know it if you tried to step outside the space shuttle without a space suit).

Exosphere

Above about 350 miles lies the exosphere. In lower layers, the composition of the atmosphere is fairly homogenous, meaning that the proportions of nitrogen, oxygen, and other elements stay more-or-less the same. (Only water vapor varies significantly.) But the very top of the atmosphere consists of the lightest elements, hydrogen and helium. High in the exosphere, a lonely atom of hydrogen might travel several miles before bumping into another atom. With all that empty room, the light elements accelerate, picking up so much velocity that they escape into space.

Ionosphere

The ionosphere extends from 30 miles to about 250 miles. As its name suggests, it contains ions -- charged particles. The ionosphere actually serves a practical purpose for those of us way down in the troposphere. Radio waves reflect off the ionosphere and bounce back to the surface, allowing us to hear radio transmissions from distances of hundreds and sometimes t housands of miles.

Figure 1-2: The aurora australis lights at the South Pole. These are the same type of lights found at the North Pole. (Photo courtesy of NOAA/Department of Commerce; http://www.photolib.noaa.gov; photo taken by Commander John Bortniak, NOAA Corps)

The northern lights, or aurora borealis, also occur in the ionosphere, usually between about 60 to 80 miles high. As solar radiation interacts with the upper atmosphere, nitrogen glows red and oxygen glows red or green. This is one of the few observable atmospheric phenomena to occur above the troposphere.

That completes our very quick tour of the atmosphere. In the next lesson we'll look at the recipe for weather, atmospheric pressure, and the Coriolis effect.

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

The first assignment is easy. Next time you have a clear, starry night, go outside and look up at the stars. Remember that you’re not just looking at the sky -- you're peering up from the very bottom of an ocean of air. The starlight you’re seeing must pass through hundreds of miles of atmosphere before it reaches your eyes. Notice how the stars flicker and twinkle. What you’re seeing is the atmosphere in action. Different portions of the atmosphere have different temperatures and densities and refract the incoming starlight. Light bends as it passes through layers of air with different densities. You can see the same effect by putting a spoon in a glass of water; the spoon appears “bent.” Water has a different density from the air, and that’s what creates the illusion. In the sky, the pinpoint light of the stars must pass through many such boundaries and changing air temperatures. That causes the light to refract or bend ever so slightly, causing a twinkle. (Planets are much closer and so appear larger than a zero-dimensional point. For that reason they don’t flicker the way stars do.) Okay, that was too easy, so I’ll throw in some reading assignments too. Please read:

• Tying Down the Wind, Chapter 6, “Fresh Air.”

• Weather: How it Works and Why it Matters, pages 15 to 22.