by Wm. Robert Johnston
last updated 2 February 2005
The Earth's atmosphere (by volume) is about 77.8% nitrogen, 20.9% oxygen, 0.9% argon, 0.4% water vapor, 0.037% carbon dioxide, and 0.003% other gases.
Our weather is driven largely by two things: the transfer of heat and the presence of water. The EarthÕs rotation, landforms, and other factors come into play.
The Earth's surface absorbs heat more efficiently than the atmosphere. The lower atmosphere is heated by the Earth's surface, and temperature decreases with height. Temperatures rise again in the stratosphere, where absorption of UV light by ozone produces heating.
Winds, or air motion, result from convection and from the EarthÕs rotation.
Water has more thermal inertia than land surfaces, so temperatures over water vary less than over land. One noticeable result is convective winds at the coasts.
Evaporation involves water going from liquid to vapor form at a water-air interface, transferring heat in the process. This transfers water from the oceans and other water bodies back into the atmosphere as part of the water cycle.
Hurricanes acquire energy particularly from ocean heat. The circular motion is the effect of the Earth's rotation on the convective air motion.
The greenhouse effect is a process by which certain gases in a planet's atmosphere give it a higher temperature than the planet would have otherwise:
Light is one form of electromagnetic (EM) radiation. From lower energy to higher energy, categories of EM radiation include: radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays. Blackbody radiation is EM radiation given off by all matter, as a function of the matter's temperature. Hotter objects give off more total EM radiation, and also give off a greater fraction of that radiation as higher energy radiation.
Since the Sun has a surface temperature of 5,700° C, about 42% of the blackbody radiation it radiates is visible light. (About 8% is ultraviolet and about 50% is infrared.) EM radiation from the Sun reaching the Earth is either reflected back into space or absorbed by the Earth, warming it. The Earth also radiates blackbody radiation but almost entirely as infrared light (since it is much cooler than the Sun).
Certain gases in the Earth's atmosphere are transparent to the Sun's visible light but tend to absorb the outgoing infrared light emitted by the Earth. These are called greenhouse gases and include primarily water vapor and carbon dioxide. (Other natural and manmade gases also contribute.) By absorbing some of this outgoing EM radiation, the atmosphere becomes warmer and also radiates more blackbody radiation. The atmosphere and the Earth itself becomes warmer than it would be without the greenhouse gases.
The water vapor and carbon dioxide naturally occurring in the atmosphere produce a greenhouse effect which gives the Earth a temperature 30° C more than it would have without these gases, making life possible.
For more about the greenhouse effect and the issue of global warming, click here.
Ozone (O3) is a gas with molecules made of three atoms of oxygen. This is different from the oxygen we breath (O2), which has two atoms of oxygen and makes up 21% of the Earth's atmosphere. Ozone is found throughout the Earth's atmosphere in minute quantities (about 0.6 parts per million, on average). Ozone is found in higher concentrations in pollution, and can be a health risk.
Ozone in the upper atmosphere is important to health. In the stratosphere, the region of the atmosphere about 12 to 45 km above the surface of the Earth, ozone exists in larger amounts. Between 20 and 40 km high, ozone makes up about 6 parts per million of the air. This higher concentration of ozone, called the ozone layer, absorbs much ultraviolet light from the Sun.
The ozone layer exists because it is routinely created from oxygen by solar ultraviolet light. Ultraviolet (UV) light is higher energy light than visible light. The ozone concentrations in the stratosphere represent an equilibrium between creation of ozone and the ozone destruction that results when it absorbs UV. The stratosphere is warmer than the air above and below it as a result of these processes.
Ultraviolet light is harmful to plants and animals. In humans high amounts of UV can cause sunburns, skin cancer, and possibly cataracts. Scientists categorize ultraviolet into UV-A (lower energy), UV-B (medium energy), and UV-C (higher energy). UV-B light is absorbed by ozone so that very little reaches the surface of the Earth. Lower energy UV-A is not absorbed by ozone, however.
Skin cancer, the most serious human health effect of UV light, includes several different types. Most types are non-malignant and treatable, with the main problem being cosmetic (i.e., affecting skin appearance rather than being life-threatening). These types are caused by both UV-A and UV-B. Melanoma, the type of skin cancer which is potentially fatal, is caused apparently only by UV-A--which is not absorbed by ozone.
For more about the ozone layer and the issue of ozone depletion, click here.
Why is the sky blue?
© 2005 by Wm. Robert Johnston.
Last modified 2 February 2005.
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