Atmosphere

A planet's climate is decided by its mass, its distance from the sun and the composition of its atmosphere. Mars is too small to keep a thick atmosphere. Its atmosphere consists mainly of carbon dioxide, but the atmosphere is very thin. The atmosphere of the Earth is a hundred times thicker. Most of Mars' carbon dioxide is frozen in the ground. Mars' average surface temperature is about -50°C. Venus has almost the same mass as Earth but a thicker atmosphere, composed of 96% carbon dioxide. The surface temperature on Venus is +460°C. Earth's atmosphere is 78% nitrogen, 21% oxygen, and 1% other gases. Carbon dioxide accounts for just 0.03 - 0.04%. Water vapour, varying in amount from 0 to 2%, carbon dioxide and some other minor gases present in the atmosphere absorb some of the thermal radiation leaving the surface and emit radiation from much higher and colder levels out to space. These radiatively active gases are known as greenhouse gases because they act as a partial blanket for the thermal radiation from the surface and enable it to be substantially warmer than it would otherwise be, analogous to the effect of a greenhouse. This blanketing is known as the natural greenhous effect. Without the greenhouse gases, Earth's average temperature would be roughly -20°C. The climates on Mars and Venus are very different, but very stable and highly predictable. The Earth's climate is unstable and rather unpredictable as compared with that of the other two planets.

credit:United Nations Environment Programme / GRID-Arendal

Cambridge University Graphic

The atmosphere is divided into five layers. It is thickest near the surface and thins out with height until it eventually merges with space.

1) The troposphere is the first layer above the surface and contains half of the Earth's atmosphere. Weather occurs in this layer.
2) Many jet aircrafts fly in the stratosphere because it is very stable. Also, the ozone layer absorbs harmful rays from the Sun.
3) Meteors or rock fragments burn up in the mesosphere.
4) The thermosphere is a layer with auroras. It is also where the space shuttle orbits.
5) The atmosphere merges into space in the extremely thin exosphere. This is the upper limit of our atmosphere.

NOAA Graphic

U.S. Weather Service Graphic

The inhabitants of our planet live in the Troposphere.

Earth's atmosphere varies in density and composition as the altitude increases above the surface. The lowest part of the atmosphere is called the troposphere (and it extends from the surface up to about 10 km (6 miles). The gases in this region are predominantly molecular Oxygen

( O₂ ) and molecular Nitrogen ( $N$ ₂ ). All weather is confined to this lower region and it contains 90% of the Earth's atmosphere and 99% of the water vapor. The highest mountains are still within the troposphere and all of our normal day-to-day activities occur here. The high altitude jet stream is found near the tropopause at the the upper end of this region.

The layer above this is the Stratosphere, this is where the Ozone Layer is formed. The atmosphere above 10 km is called the stratosphere. The gas is still dense enough that hot air balloons can ascend to altitudes of 15 - 20 km and Helium balloons to nearly 35 km, but the air thins rapidly and the gas composition changes slightly as the altitude increases. Within the stratosphere, incoming solar radiation at wavelengths below 240 nm. is able to break up (or dissociate) molecular Oxygen ( $O$ ₂ ) into individual Oxygen atoms, each of which, in turn, may combine with an Oxygen molecule ( $O$ ₂ ), to form ozone, a molecule of Oxygen consisting of three Oxygen atoms ( $O$ ₃ ). This gas reaches a peak density of a few parts per million at an altitude of about 25 km (16 miles).

The Ozone Layer absorbs ultra-violet radiation from the Sun. Without the Ozone Layer life as we know would cease to exist on our planet. Ozone is important because it is the only atmospheric gas which absorbs light in the B region of UVB rays.

The Ozone layer extends from a height of 20 kilometers to 60 kilometers above the Earth's surface. The air is very thin at these altitudes.

If all of the Ozone in the Earth's atmosphere were compressed into a single layer at the Earth's surface, it would only be 3 millimeters thick-basically two stacked pennies!

The gas becomes increasingly rarefied at higher altitudes. At heights of 80 km (50 miles), the gas is so thin that free electrons can exist for short periods of time before they are captured by a nearby positive ion. The existence of charged particles at this altitude and above, signals the beginning of the ionosphere a region having the properties of a gas and of a plasma.

US Navy Graphic

The Ozone Hole increases in size during the months of September and October

Standardized Temperature Profile

An average temperature profile through the lower layers of the atmosphere. Height (in miles and kilometers) is indicated along each side. Temperatures in the thermosphere continue to climb, reaching as high as 2000°C.
credit: National Weather Service

Troposphere: The troposphere begins at the Earth's surface and extends up to 4-12 miles (6-20 km) high. This is where we live. As the gases in this layer decrease with height, the air become thinner. Therefore, the temperature in the troposphere also decreases with height. As you climb higher, the temperature drops from about 62°F (17°C) to -60°F (-51°C). Almost all weather occurs in this region.

The height of the troposphere varies from the equator to the poles. At the equator it is around 11-12 miles (18-20 km) high, at 50°N and 50°S, 5½ miles and at the poles just under four miles high. The transition boundary between the troposphere and the layer above is called the tropopause. Both the tropopause and the troposphere are known as the lower atmosphere.
Stratosphere: The Stratosphere extends from the tropopause up to 31 miles above the Earth's surface. This layer holds 19 percent of the atmosphere's gases and but very little water vapor.

Temperature increases with height as radiation is increasingly absorbed by oxygen molecules which leads to the formation of Ozone. The temperature rises from an average -76°F (-60°C) at tropopause to a maximum of about 5°F (-15°C) at the stratopause due to this absorption of ultraviolet radiation. The increasing temperature also makes it a calm layer with movements of the gases slow.

The regions of the stratosphere and the mesosphere, along with the stratopause and mesopause, are called the middle atmosphere by scientists. The transition boundary which separates the stratosphere from the mesosphere is called the stratopause.
Mesosphere: The mesosphere extends from the stratopause to about 53 miles (85 km) above the earth. The gases, including the oxygen molecules, continue to become thinner and thinner with height. As such, the effect of the warming by ultraviolet radiation also becomes less and less leading to a decrease in temperature with height. On average, temperature decreases from about 5°F (-15°C) to as low as -184°F (-120°C) at the mesopause. However, the gases in the mesosphere are thick enough to slow down meteorites hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky.
Thermosphere: The Thermosphere extends from the mesopause to 430 miles (690 km) above the earth. This layer is known as the upper atmosphere.

The gases of the thermosphere are increasingly thinner than in the mesosphere. As such, only the higher energy ultraviolet and x-ray radiation from the sun is absorbed. But because of this absorption, the temperature increases with height and can reach as high as 3,600°F (2000°C) near the top of this layer.

However, despite the high temperature, this layer of the atmosphere would still feel very cold to our skin because of the extremely thin air. The total amount of energy from the very few molecules in this layer is not sufficient enough to heat our skin.

Exosphere

The Exosphere is the outermost layer of the atmosphere and extends from the thermopause to 6200 miles (10,000 km) above the earth. In this layer, atoms and molecules escape into space and satellites orbit the earth. The transition boundary which separates the exosphere from the thermosphere below it is called the thermopause.

EPA Graphic

NASA TOMS Ozone Hole image

Atmospheric Trends

source: US Department of Energy

The amount of carbon dioxide in the atmosphere has been increasing rapidly. Human activities are also releasing other "greenhouse" gases such as methane, ozone, and chlorofluorocarbons (CFCs), which intensify the heat-trapping properties of the atmosphere as a whole.

Pollution will continue to grow and contribute to Global Warming

CFCs also rise into the upper layer of the atmosphere, the stratosphere, where they destroy the protective layer of ozone, a gas that forms a shield against ultraviolet rays that can harm many forms of life.

About l million tons (over 900,000 metric tons) per year of CFCs have been released worldwide since the mid-l970s.

Many factories have no pollution controls in developing nations

Concern is growing that atmospheric changes could bring on rapid, profound climatic changes.

Atmospheric concentrations of greenhouse gases are rising at unprecedented rates. Greenhouse gas emissions from developing countries are expected to increase rapidly. Large-scale climate changes may occur unless other climatic systems counteract the warming effect of the greenhouse gases.

Total global energy use is projected to grow at an annual rate of 1.5-2 percent. Non-fossil energy sources are likely to become more important because of concern over global warming.

Cleaner, more efficient energy technologies can significantly reduce greenhouse gases from fossil fuels, especially over the next 20 years. However, total greenhouse gas emissions are likely to increase due to expanded energy use.

Smog kills more people than car accidents

Demand for refrigeration (which has cooling systems that use CFCs) in developing countries is projected to increase greatly, especially in China and India.

Ozone losses in the upper atmosphere are occurring at all latitudes in both hemispheres. The most striking example of ozone loss occurs over the South Pole during September and October. As ozone is lost, the amount of biologically harmful UV-B radiation will increase. Skin cancer rates are expected to increase. Other health effects will likely include an increase in cataracts and suppression of the immune system. Increased UV-B radiation may also harm plants and animals.

Why is the Sky Blue?

It is easy to see that the sky is blue. The light from the Sun looks white. But it is really made up of all the colors of the rainbow.

A prism is a specially shaped crystal. When white light shines through a prism, the light is separated into all its colors.

Like energy passing through the ocean, light energy travels in waves, too. Some light travels in short, "choppy" waves. Other light travels in long, lazy waves. Blue light waves are shorter than red light waves.

All light travels in a straight line unless something gets in the way to--

reflect it (like a mirror)
bend it (like a prism)
or scatter it (like molecules of the gases in the atmosphere)

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules have scattered and rescattered the blue light many times in many directions. Also, the surface of Earth has reflected and scattered the light. All this scattering mixes the colors together again so we see more white and less blue.

credits: NOAA, NASA, EPA, National Weather Service, Cambridge University, U.S. Navy