Earth's Atmosphere Effects (source: NASA)
Earth's Atmosphere, II
The vertical extend of the atmosphere is difficult to define, it surrounds the earth and becomes increasingly thinner until at some point space is reached. The lowest layer of the atmosphere is called the troposphere and contains almost all water vapour and approximately 75% of all molecular mass of the atmosphere, its height varies from 18 km at the equator to 8 km at the poles.
The sun warms the air or atmosphere indirectly, the earth receives the solar radiation (insolation) and heats the lower layers of the atmosphere by radiation and convection. It is in this layer where we humans live, breathe and fly.
We can visualize the atmosphere as the layers of an onion surrounding the planet. Each of these layers has its own distinct properties.
Layered Atmosphere
The atmosphere of the earth consists of a number of layers like an onion. Research with rockets and satellites have confirmed this and these layers are defined by temperature differences and or well defined laps rates.
There are four named layers of atmosphere: troposphere, stratosphere, mesosphere and thermosphere.
Troposphere
Pressure lapse rate with altitude
This is the lowest layer where most of the weather takes place. Its altitude varies between 18 km at the equator to 8 km at the poles. This layer lowers in steps, sometimes overlapping, at which the jet streams are found and these have their influence on the weather systems and in the development of fronts.
The temperature decreases fairly constant with 1,98°C per 1000 ft (6°C/km). This lapse rate (usually positive) can be negative or even be constant, called an inversion and isothermal respectively. The troposphere is capped by the tropopause and over the equator the temperature is about -80°C, over the poles (where it is lower) the temperature is around -48°C during summer.
Stratosphere
The stratosphere is some 35 km thick and initially the layer is isothermal (about 10 km) then the temperature slowly increases and the last 25% the temperature increases rather quickly with altitude. The cause being the ozone which absorbs large amounts of solar radiation at these levels. The density of the top part of the stratosphere is very low, in fact, if it was the same as at the surface the temperature would be around 15°C too.
The stratosphere is mainly heated by absorption of solar radiation, where as the troposphere is heating from below by conduction and convection.
Mesosphere
Starts at the stratopause at some 50 km above the surface and about 90 km thick (90000ft). The top of this layer (mesopause) has a temperature of some -90°C. The pressure is around 1 hPa and almost nil near the mesopause.
Thermosphere
Here ends the homosphere (troposphere, stratosphere and mesosphere) at about 80 km from the surface, gas molecules are here bombarded by x-rays from the sun and form the ionosphere. A layer between 90 and 300 km. This process creates oxygen and nitrogen atoms with a positive charge capable of reflection short wave radio waves from radio stations on earth. Sometimes so intense that even VHF radio waves are being reflected around the earth.
Temperatures are high due to the very high ultraviolet radiation from the sun. When solar activity is low the thermosphere cools and contracts following the sunspot cycle closely.
Ionosphere
Radiation from the Sun creates layers of ionized (electrically charged) atoms and molecules surrounding the planet varying at an altitude from 60 to 600 km. The level and depth of ionization on a certain location depends on the time of day and the amount of radiation from the Sun which varies with on average 11 year sunspot cycles.
There are four recognized ionospheric layers: D, E (Kennelly-Heaviside), F1 and F2 (Appleton). Each at a different altitude and occuring at different times during the day. At night the D and E layers dissolve and the F1 and F2 layer combine. You can notice this effect on the AM broadcast band (500 - 1600 kHz) where more stations can be heard during nighttime when the D/E layer is non existent.
Each layer has its own properties in reflecting certain frequencies. Layers closer to earth are more dense reflecting lower frequencies, but usually the HF band (3-30 MHz) enjoys these effects on a daily basis. When there is sufficient ionization, frequencies up to 100 MHz (and sometimes higher) can be reflected too.
