|
REVISION NOTES [print-friendly version]
Layers of the Atmosphere
The atmosphere is the huge blanket of gas that circles the entire Earth. Without it, life as we know it could not exist. This blanket of gas starts at ground level and stretches 1000 km into the sky. However, most of this life-supporting shell is squashed down into a layer only 10 km thick. The top of Mount Everest barely peeks above the edge of this layer. It surrounds Earth and protects us by blocking out dangerous rays from the sun. It also protects us from drifting rocks, big hunks of metal, and other bits and pieces of space junk that collide with our planet from time to time
The atmosphere is a mixture of gases that becomes thinner until it gradually reaches space. It is composed of Nitrogen (78%), Oxygen (21%), and other gases (1%).
Structure of the Atmosphere
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. And the ozone layer absorbs harmful rays from the sun here.
3) Meteors burn up in the mesosphere.
4) The thermosphere is a layer with auroras, meteors and where the space shuttle orbits.
5) The atmosphere merges into space in the extremely thin exosphere. This is the upper limit of our atmosphere.
Troposphere
People, plants, animals, and insects live in the troposphere. It is the layer where all weather occurs and is often referred to as "The Weather Zone."
The troposphere is the layer of the atmosphere closest to the earth, containing more than 75 percent of the gases comprising the atmosphere and 99 per cent of all the water vapour. In general, the temperature in the troposphere decreases with height at a rate of about 5° to 6° C/km. This decrease in temperature with height is called a lapse rate. As a result of the vertical temperature and pressure profile nearly all the earth's weather conditions, including most clouds and all precipitation occur in the troposphere.
We all know that warm air is less dense than cold air and thus rises. Cold air, being very heavy, tends to sink. If the vertical temperature profile of the troposphere were reversed, that is warm air aloft and cold air at the ground, we would never have rising currents of air which are responsible for producing clouds and precipitation. But, because the temperature does decrease with height in the troposphere the warm air near the ground does rise, forming low pressure areas which produce the exciting weather we observe every day. The sun's uneven heating of the troposphere causes convection currents, large-scale patterns of winds that move heat and moisture around the globe.
The thickness of the troposphere depends on latitude, measuring 18 km deep at the equator to only 8 km deep at the poles. The difference in depth is explained by the density of the air. Colder air at the poles is much more dense than the warm air at the equator and thus compresses more resulting in a shallower tropospheric layer.
Carbon dioxide and water vapor in the troposphere absorb infrared radiation (or heat energy) emitted by the earth after the ground has been warmed by the sun. These gases then radiate some of the heat, which is trying to escape into space, back to the ground keeping the planet warmer than it would be if those gases were not present. (Greenhouse Effect) Since the concentration of carbon dioxide has nearly doubled since 1900, scientists fear that increasing amounts of carbon dioxide could raise the earth's surface temperature during the next century, causing significant changes in worldwide weather patterns.
The water vapor in the troposphere is also essential to producing clouds which go on to create rain and snow.
In densely populated areas smog is common in the troposphere. Smog results when pollutants accumulate close to the earth's surface beneath an inversion layer and undergo a series of chemical reactions in sunlight. Inversions prevent pollutants from escaping into the upper atmosphere.
O3 in the troposphere (“bad ozone”)
Just to complicate matters, O3 is also created by photolysis in the troposphere. Here, energy is supplied by visible light (VL).
NO2 + VL ® NO + O
O + O2 ® O3
(Normally the first reaction would go back the other way.)
NO + O ® NO2
But the presence of other molecules like CO and hydrocarbons, which are abundant in polluted air, will oxidize NO first, taking it out of circulation before it can react with atomic O. We call tropospheric ozone “photochemical smog”. Oxone in the troposphere absorbs UV too, but…
The concentrations in the troposphere, even in L.A., are much lower than those in the stratosphere—about 100-300 ppbv and ozone is harmful to humans and crops—skin and eye irritation, leaf damage, etc.
Stratosphere
The stratosphere extends from the top of the troposphere, a region called the tropopause, at an altitude of 12.9 to 19.3 km to approximately 48 kilometers above the earth's surface. The temperature characteristically increases with height and is thus the reverse profile found in the troposphere. Because the base of the stratosphere is colder than the middle and top layers, the air is very stable, meaning it does not rise, and is thus hostile to the formation of clouds.
Vertical motions in the stratosphere are generally very weak because of the temperature profile, but horizontal winds are quite significant. In fact, the main jet stream, which essentially controls the weather, is found at the base of the stratosphere with winds up to 200 or so mph. With winds of such speed and a lack of clouds, the base of the stratosphere provides excellent conditions for many types of aircraft.
The critical life sustaining ozone layer is found in the stratosphere. Ozone, or O3, is found between 20 and 32 km above the earth's surface. If the ozone layer were brought to the ground the increased pressure on the ozone molecules would compress the layer to a thickness of under 1cm.
Ozone has the unique property of being able to absorb the harmful ultraviolet radiation emitted by the sun, thus protecting plants and animals on the earth's surface. The process of absorbing the ultraviolet radiation produces heat and is the reason the temperature in the stratosphere increases with altitude. In fact, the temperature at the top of the stratosphere, a region called the stratopause, is about as high as you would find on the ground.
Ozone is created in upper stratosphere. UV is high energy radiation of wavelength 200-300mm.
O2 + UV ® O + O
O2 + O ® O3
Ozone is transported “down & out” to lower stratosphere and poleward.
Ozone is continuously created and destroyed in a “dynamic equilibrium”
When ozone is broken down O2 is regenerated.
O3 + O ® O2 + O2
O3 + UV ® O2 + O
This adds heat to the stratosphere
O3 is a “short-lived” molecule; single bonds are less stable than the double bond in O2; O3 molecule lasts 8-11 days on average; O3 is reactive—e.g. with Cl- or Br-….
Mesosphere
The mesosphere extends from the stratopause to about 80 kilometers above the earth. The temperature characteristically decreases with height. In fact the lowest temperatures in the atmosphere are found at the top of the mesosphere, in a region called the mesopause. During the summer, temperatures at the mesopause may drop to -100 degrees C over the north pole. The mesopause may be so cold that tiny amounts of water vapor sometimes form ice clouds, called noctilucent clouds, which can be seen when the sun hits them after sunset.
Thermosphere
The thermosphere ("Hot Sphere") is the uppermost layer of the earth's atmosphere. The thermosphere begins at the mesopause and continues until it fades into outer space. Temperature in the thermosphere may rise to several thousand degrees. The thermosphere is so hot because, 1) the oxygen in the thermosphere readily absorbs ultraviolet radiation and 2) the density of molecules is so low that only a tiny bit of energy absorbed is capable of producing very high temperatures.
The thermosphere only constitutes about 00.01% of the total atmosphere. The chemical composition of the thermosphere differs from that of the other atmospheric layers. In the thermosphere's lower regions you will find oxygen molecules and individual oxygen atoms. The outer layer of the thermosphere consists chiefly of hydrogen and helium.
When radiation from the sun and from other sources in outer space strikes the air in the thermosphere, it ionizes some of the atoms and molecules of the layer. In other words, electrons from the atoms struck by incoming radiation are blown off of their orbits around the nucleus, leaving atoms with net positive charges. These charged atoms are called ions. Most ions are produced in the lower part of the thermosphere, and is therefore called the ionosphere.
There are actually four separate layers of the ionosphere; The D layer, the E layer, and the F1 and F2 layers. The D layer forms during the day when the sun is bombarding the atmosphere with radiation and thus ionizing a maximum number of atoms. At night the D ion layer disappears.
The ion layers are composed of both ions and free electrons. The free electrons are excellent reflectors of AM radio waves. So, at night, with the absence of the D ion layer, AM radio broadcasts can be received over greater distances because the radio waves are bouncing off the E, F1 and F2 layers which are higher in the thermosphere, thus allowing greater transmission distances than during the day. (Structure of the Atmosphere Diagram)
Other sub-regions of the thermosphere include the heliosphere, consisting of ionized helium, and the prontosphere, consisting largely of ionized hydrogen.
The Aurora
The aurora borealis and aurora australis (The northern and southern lights respectively) occur in the thermosphere. During periods of increased solar flare activity on the sun, the aurora appears. Solar flares emit a stream of high energy particles, which upon collision, excite the electrons of the atoms in the thermosphere to the point where they emit visible light of varying wavelengths. The earth's magnetic field directs the high energy particles towards the earth's poles which is why the aurora occurs at high lattitudes.
|
