The atmosphere of Earth is the layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation) and provides the air for the survival of terrestrial plants and animals as well human beings.

Five distinct layers have been identified using thermal characteristics (temperature changes), chemical composition, movement, and density. Each of the layers are bounded by "pauses" where the greatest changes in thermal characteristics, chemical composition, movement, and density occur.


The Troposphere

Known as the lower atmosphere is the region where almost all weather phenomena occurs.
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapour and aerosols. It's name derives from the Greek: "tropos" meaning "change" reflecting the fact that turbulent mixing plays an important role in the troposphere's structure and behaviour

The troposphere begins at the Earth's surface and extends from 4 to 12 miles (6 to 20 km) high. 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, is around 5,5 miles and at the poles just under four miles high.

As the density of the gases in this layer decrease with height, the air becomes thinner. Therefore, the temperature in the troposphere also decreases with height in response. As one climbs higher, the temperature drops from an average around 62°F (17°C) to -60°F (-51°C) at the tropopause.

This layer is often the domain on which are performed a great amount of balloon flights. This is done specially using superpressurized (closed) balloons which can maintain a constant level of flight and are used to trace the air masses.

At the top of the troposphere lies the tropopause which is a region that acts as boundary between the troposphere and the stratosphere. It's the coldest zone of all the low atmosphere and thus is the area that presents the greatest danger for stratospheric balloons that are forced to go through it. It is due to the fact that the thin plastic of the balloon's envelope becomes brittle with low temperatures. Most of the failures occur there.


The Stratosphere

Is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It extends around 31 miles (50 km) down to anywhere from 4 to 12 miles (6 to 20 km) above the Earth's surface.

It is stratified in temperature. Heat is produced in the process of the formation of Ozone and this heat is responsible for temperature increases from an average -60°F (-51°C) at tropopause to a maximum of about 5°F (-15°C) at the top of the stratosphere. This increase in temperature with height means warmer air is located above cooler air. This prevents "convection" as there is no upward vertical movement of the gases. As such the location of the bottom of this layer is readily seen by the 'anvil-shaped' tops of cumulonimbus clouds.

As its name implies is the "natural" region of movement of stratospheric balloons both manned or unmmaned.


The Mesosphere

It extends from around 31 miles (50 km) above the Earth's surface to 53 miles (85 km). Althought The gases in the mesosphere are still thick enough to slow down meteors hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky. Along with the stratosphere (next layer down) they are considered the middle atmosphere.

The transition boundary which separates the mesosphere from the stratosphere is called the stratopause.

Only the lowermost part of this layer can be reached by balloons. But they must be of great volume, or made of ultra-thin materials and carrying small payloads. The current unmanned balloon height record (established by Japan in 2002) is of 53 km.


The Thermosphere

Located between about 53 miles (85 km) and 375 miles (600 km), this layer is known as the upper atmosphere.

Incoming high energy ultraviolet and x-ray radiation from the sun begins to be absorbed by the molecules in this layer and causes a large temperature increase. Because of this absorption, the temperature increases with height. From as low as -184°F (-120°C) at the bottom of this layer, temperatures can reach as high as 3,600°F (2,000°C) near the top.

Within the thermosphere, is located the ionosphere which is a sub-layer made of electrically charged gas particles (ionized). The ionosphere extends from 37 to 190 miles (60-300 km) above the earth's surface. It is divided into three regions or layers; the F-Region, E-Layer and D-layer. During the daytime the F-Layer splits into two layers then recombines at night.


The Exosphere

It is the outermost layer of the atmosphere. It extends from the top of the thermosphere to 6,200 miles (10,000 km) above the earth. In this layer, atoms and molecules escape into space and satellites orbit the earth. At the bottom of the exosphere is the thermopause located around 375 miles (600 km) above the earth.

No balloons can physically flight in these two last layers.