# Planet Earth

Our planet is surrounded by layers of gas, the ‘atmosphere’, maintained around it by the very gravitational attraction of the Earth.  An important part of the atmosphere that we use to breathe and that plants use in photosynthesis is the ‘air’.

# Mass, Atmospheric Pressure and Density

## Mass

The atmosphere has a mass of about 5 x 1018 kg, of which 75% is within 11 kilometres of the surface of the Earth.  According to Lutgens et al. (1995), the mass of Earth’s atmosphere is distributed as follows:

## Pressure

Atmospheric pressure gives the total weight of the air above a specific unit area at the point where the pressure is measured.  It can vary with location and weather conditions.

The average atmospheric pressure at sea level is about

1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.92 inches of mercury (Hg).

## Density

The density of the atmosphere at sea level is calculated from measurements of temperature, pressure and humidity using a variant of the Ideal Gas Law.

## The Ideal Gas Law

### $PV =nRT$$PV =nRT$

where P gives the pressure of the gas, V is the volume of the gas, n is the amount of substance, also known as number of moles, T is the temperature of the gas and R is the ideal gas constant, equal to the product of the Boltzmann constant and the Avogadro constant.

Atmospheric density is about 1.2 kg/m3.

Just as for the pressure, it decreases with altitude.

# Chemical Composition of the Atmosphere

The chemical composition of air varies according to the different layers and altitudes within the atmosphere.  It gets thinner and thinner with increasing height.  Breathable air is found only in the ‘troposphere‘, and other artificial atmospheres, such as the artificial living environment of the International Space Station (ISS).

## Air Composition

Dry air contains on average

GasBy Volume
Nitrogen N278.09 %
Oxygen O220.95 %
Argon Ar0.93 %
Carbon Dioxide CO20.039 %
OtherTrace

Air may also contain water vapour H2O, but typically no more than around 1%.

In addition to providing terrestrial animals and plant life with the chemical elements required for their survival, the atmosphere affords all life on Earth an important protection against noxious ultraviolet (UV) solar radiation.  As it warms the surface through heat retention via a convection process known as the ‘greenhouse effect‘, the atmosphere contributes to the reduction of diurnal temperature variation – the difference between temperature extremes during the day and the night.

# Stratification of the Earth’s Atmosphere

The structure of the Earth’s atmosphere can be divided into layers:

• ## Exosphere

From the Exobase and Beyond…

The outermost layer of the Earth’s atmosphere is mainly composed of hydrogen H and helium He.  At this altitude, the gas particles stand so far apart that they seldom collide with each other, so that the atmosphere no longer behaves like a fluid.  Free-moving particles follow ballistic trajectories in and out of the magnetosphere, or solar wind.

• ## Thermosphere

From the Mesopause up to the Exobase (~350-800 km/1,100,000-2,600,000 ft)

The ISS orbits the Earth between 320-380 km in this layer.  Air above the mesopause is poorly mixed compared to the layer below, due to the scarcity of the gas molecules, and becomes stratified.  The turbopause marks the boundary between two regions: the ‘homosphere’ and the ‘heterosphere’.

The homosphere and heterosphere differ according to the mixture of atmospheric gases.  The former covers the troposphere, the stratosphere and the mesosphere strata, and its gases are well mixed by turbulence independently of molecular weight.  However, the composition varies with altitude, allowing the gases to stratify by molecular weight: the heaviest N2 and O2, only present at the bottom of the heterosphere, while the lightest H occupies the upper part.

The ionosphere straddles the exosphere and the thermosphere from 50 to 1,000 km/160,000 to 3,300,000 ft.  At the inner edge of the magnetosphere, it catches the solar wind, where electrons spiral in the Earth’s magnetic field and produce the beautiful eerie displays of colourful lights called the ‘aurora’.  The ionosphere is also important with regards to radio transmissions.

The top boundary of the thermosphere is called the ‘exobase‘, the height of which can vary depending on solar activity.

• ## Mesosphere

From the Stratopause up to the Mesopause (~80-85 km/260,000-280,000 ft)

This is the layer where most meteors burn up upon entry into the atmosphere, where the temperature decreases with increasing altitude.  At its top, the mesopause is thought to be the coldest place on Earth with temperatures varying from -85ºC and -100ºC.

Water vapour freezes in this environment and forms ‘noctilucent clouds’.  It is also where a particular type of lightning, known as ‘sprites’ or ‘elves’ usually form way above the thunderclouds of the troposphere.

• ## Stratosphere

From the Tropopause up to the Stratopause (~51 km/170,000 ft)

The temperature increases as the altitude increases as a result of increased absorption of UV radiation by the ozone layer, located in the lower stratosphere at around 15-35 km or 11,000 ft, though it is subject to geographical and seasonal variations.

The stratopause marks the boundary between stratosphere and mesosphere, located between 50 to 55 km/160,000 to 180,000 ft.  At  this point, the atmospheric pressure is 1/1000th that at sea level.

• ## Troposphere

From the Surface of the Earth to the Tropopause (~9 km/30,000 ft at the Poles, and up to ~17 km/56,000 ft at the Equator)

The troposphere includes about 80% of the mass of the atmosphere.  Since it is mostly heated by energy transfer from the surface, the temperature decreases as the altitude increases.

The tropopause marks the boundary between troposphere and stratosphere.

While there is no absolute boundary between the atmosphere and outer space, the Kármán line marks by international convention, the beginning of space where human travellers are considered to be astronauts, at 100 kilometres (62 miles) above sea level.

So, our daredevil daredevil skydiver Felix Baumgartner would have in fact jumped off from somewhere in the stratosphere