By Robert Hazen, Ph.D., George Mason University
The bulk composition of the Earth’s atmosphere is remarkably uniform around the globe. It contains primarily two gases: nitrogen and oxygen. Those two gases make up 99 percent of the atmosphere that has been dried, that has had its water content taken out. While nitrogen constitutes 78 percent of the atmosphere’s volume, oxygen accounts for 21 percent.
Nitrogen and Oxygen
Nitrogen, in the form of N2 molecules, is the most abundant constituent; it’s 78 percent of the atmosphere’s volume. N2 is a covalently bonded molecule; it has two nitrogen atoms that are bonded covalently with a triple bond. That’s a very strong, very durable, long-lasting molecule. The nitrogen content of the atmosphere apparently has been fixed for a very long time because of the inertness of this molecule.
The other gas, oxygen, is O2; it’s a molecule that accounts for 21 percent of the atmosphere’s volume. Oxygen has a double bond between two oxygens—it’s a covalently bonded molecule with two oxygens. But this is a very reactive gas; it readily takes part in oxidation reactions. Oxygen accepts electrons, for example, from carbon, iron, hydrogen—to form oxides.
If left to itself, atmospheric oxygen would quickly be reduced to nothing because it would react with surface chemicals on the ground. However, oxygen is constantly being replenished by plants. Plants play this essential, vital role in the atmospheric cycle, putting oxygen back in as it’s used up.
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Other Gases in the Earth’s Atmosphere
The atmosphere also contains a number of minor elements. It contains a small amount of the inert gas, argon; that’s about 1 percent. Argon is element 18; it has a completely filled third subshell of electrons, and as a result, it floats around the atmosphere as individual, isolated atoms that don’t bond to anything. It took a long time for argon to be discovered because of its inertness.
Following argon, there’s about 0.035 percent carbon dioxide in the atmosphere, just a very small amount, but a significant gas nonetheless. Then there are traces of a dozen other gases in the atmosphere. Some of these trace gases, such as ozone, can reach unhealthy levels in regions of high air pollution.
Water in the Air
The atmosphere always contains some water vapor, though the amount is extremely variable. It depends on temperature, and it depends on relative humidity. Humidity is the measure of the atmosphere’s highly variable water content.
On a very cold, dry, winter day, the skin feels dry, and the lips get chapped very easily. That’s because there’s very little water in the air and water is constantly evaporating from your body into the atmosphere.
On the other hand, on a very hot, humid summer day, air contains more water. This is why, in monsoon season in tropical areas, there can be torrential rains day after day; the atmosphere can hold that huge amount of water in very hot, humid weather.
Learn more about the periodic table.
Clouds Are Kind of Loud
Clouds represent a concentration of tiny water droplets or ice crystals. These substances scatter light, and so they appear white in the atmosphere. Clouds form when air becomes saturated with water, and that’s a process that often occurs when a mass of air rises in the atmosphere and then cools down. One will often see clouds dramatically outlining the contact between two adjacent air masses.
Sometimes one may see a band of clouds on the horizon, marking a front; an approaching weather pattern, for example. This is where two air masses of different temperatures collide. Common examples are the dramatic anvil-shaped clouds, the thunderhead clouds that occur when a warm air mass collides with a cooler air mass.
When a warm air mass hits a cool air mass, the warm air mass rises up on top of the cooler air mass since the warm air mass is less dense. Also, the warm air mass probably has more moisture. As the warm air mass rises up and cools, that’s when clouds form. That’s when one gets the precipitation of water droplets and these huge, rising, thunderhead clouds, the great anvils in the sky.
This increase in elevation cools the warm, wet air; it causes clouds, rain, and great turbulence—lightning and strong winds.
Learn more about the geochemical cycles.
Clouds and Mountains
Clouds and rain often form on the windward side of mountain ranges. This is where an air mass is forced to rise up over the mountains. It is a very dramatic phenomenon. This occurs, for example, in the Hawaiian Islands.
On the Big Island of Hawaii, there are air masses coming in from the east, off the Pacific Ocean; these are fairly warm, fairly humid air masses. They hit the great mountains, and then the air mass is forced to rise up over the mountains. As they do so, that precipitation comes out, and we have intense clouds, intense precipitation, and rain activity on the windward side of the island. In some places in Hawaii, well more than 100 inches of rain falls per year.
Just a few kilometers away, on the far side, on the western flanks of that mountain, there’s a dry and arid climate because the air has lost its moisture on the upside, on the windward side, and then as the air pours over the other side, it warms up and it’s dry. It no longer has any moisture to precipitate. So, for example, while the Kona side of the Big Island of Hawaii has beautiful dry, warm temperatures most of the year, just a few kilometers away, it could be pouring rain all the time.
Common Questions about Clouds in the Earth’s Atmosphere
Oxygen still exists in the atmosphere because it is constantly being replenished by plants. Plants play this essential, vital role in the atmospheric cycle, putting oxygen back in as it’s used up.
Argon is an inert gas; it floats around the atmosphere as individual, isolated atoms that don’t bond to anything. Thus, because of its inertness, it took a long time for argon to be discovered.
Clouds form when air becomes saturated with water, and that’s a process that often occurs when a mass of air rises in the atmosphere and then cools down.
