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, 100 km is usually used as the border to space. The lowest layer of the atmosphere is called the troposphere and contains almost all water vapor 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 some of us fly.
We can visualize the atmosphere as the layers of an onion surrounding the planet. And each of these layers have their own distinct properties creating our unique living environment sustaining life in its current form.
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 temperature laps rates.
Scientists have named four layers within the atmosphere: Troposphere, Stratosphere, Mesosphere and Thermosphere. We will also discus a fifth layer (Ionosphere) as this one is also dependent on radiation emitted by the sun but with different effects.
This is the lowest layer where most of the weather takes place. Its maximum altitude varies between 18 km at the equator to 8 km at the poles. This variation in altitude occurs in breaks or steps, sometimes overlapping, and at these points we can find the jet streams. And they have their remarkable influence on the weather systems and in the development of cold and warm fronts.
The temperature decreases fairly constant with 1,98 °C per 1000 ft (6,5 °C/km). This lapse usually positive rate can be negative or even be constant, which are called an inversion and isothermal respectively. The troposphere is capped by the tropopause and over the equator the temperature is found to be -80 °C, over the poles (where its altitude is lower) the temperature is around -48 °C during summer.
With a gain in altitude pressure will drop considerably but temperature does not seem to act like that with altitudes higher than the tropopause. The temperature image shows that clearly.
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 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 warmed from below by conduction and convection.
Starts at the stratopause at some 50 km above the surface and is about 90 km thick (295000 ft). The top of this layer (mesopause) has a temperature of some -90 °C. The pressure averages around 1 hPa and almost nil near the mesopause.
At this point the homosphere (troposphere, stratosphere and mesosphere) ends at about 80 km from the surface, gas molecules are here bombarded by X-rays from the sun and subsequently form the ionosphere. This process creates oxygen and nitrogen atoms with a positive charge capable of reflection short wave radio waves from radio stations on our planet. Sometimes so intense that even VHF radio waves are being reflected and dramatically increasing range. See below.
Temperatures are high due to the very amount of high ultraviolet radiation from the sun. When solar activity is low the thermosphere cools and contracts thus following the sunspot cycle rather closely.
This layer contains the ionosphere and exosphere. The Ionosphere is covered below this section due to its unique properties and the exosphere extends from 1000 up to 10000 km from Earth.
This is not really a layer of chemical particles, but 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 an average of 11 years in so called sunspot cycles.
There are four recognized ionospheric layers: D, E (Kennelly-Heaviside), F1 and F2 (Appleton). Each at a different altitude and occurring at different times during the day. At night the D and E layers dissolve and the F1 and F2 layers combine into one.
You may notice this effect on the AM broadcast band (530 - 1700 kHz) where more stations can be heard during sunset/nighttime when the D/E layer fades away and radio signals will be reflected by the remaining combined higher F layers.
Each layer has its own properties in reflecting (or refracting) certain frequency bands. Layers closer to the ground (D) are more dense and tend to absorb lower MF frequencies. LF and VLF radio waves mainly follow the curvature of the earth in a process called groundwave propagation whereas HF frequencies use skywave propagation, but usually the HF band (3-30 MHz) enjoys these effects on a somewhat daily basis.
When there is sufficient ionization, frequencies up to 100 MHz (and sometimes higher) can be reflected too. Our page on solar activity has more on this, including links to NOAA space weather monitors.