Without any postage due, the building blocks of earth's present-day atmosphere of oxygen, nitrogen, water vapor, carbon dioxide, and other gases originally arrived from the cosmos via special delivery (bulk rate, of course) as our planetary system formed in a congealing swirl of gases more than four billion years ago. The concentrations of these gases has changed over the millenia - Table 1.1 shows their current levels in the atmosphere. These invisible gases not only make life possible on this "third rock from the sun", but they also help to regulate global temperatures, preventing our health-conscious planet from running a high fever or catching a big chill.

In essence, we live at the bottom of an ocean of air. Like earth's seas of water, the atmospheric ocean is teeming with waves of all sizes. The biggest waves are large, rolling swells of air in the high-altitude jet stream, whose troughs transport cold air equatorward and whose crests carry warm air poleward. In the process, the jet stream, working in tandem with lower-altitude winds, continually helps to keep temperature contrasts between the poles and tropics from getting too large. On the other end of the wave spectrum, small acoustic waves allow us to hear a tree falling in the forest. Even what we see and smell are affected by atmospheric conditions.

Although the atmosphere is a jack-of-all-trades (its vast repertoire also includes driving ocean currents and eroding the landscape), it is not a formidable physical figure. This unassuming veil of air around our planet is precariously thin þ if the earth is likened to an apple, then the depth of the entire atmosphere is akin to the thickness of the apple's skin. A drive of only three laps around the famous 2.5-mile oval at the Indianapolis Speedway, if directed vertically, would place the driver beyond the reach of a life-sustaining atmosphere. For many, this very thin veneer of air is only fully appreciated during dramatic weather events such as hurricanes, blizzards or thunderstorms.

Whether the atmosphere is in the background or at the forefront of our daily journeys, it continuously offers unparalleled beauty and physical truths that are worthy of attention. But in order to appreciate these opportunities, a few tools must be carried along so that understanding can be forged from raw observation. Just as a country doctor takes a bag of medicines and instruments on an urgent house call, any crucial diagnosis of a pending storm requires a familiarity with some basic concepts involving time and spatial scales, geography, and even a sprinkling of mathematics. So open your mental suitcase and let's get packing!