atmosphere.txt

troposphere
    The first and lowest layer of the
    atmosphere of the Earth, and contains 75%
    of the total mass of the planetary
    atmosphere, 99% of the total mass of water
    vapour and aerosols, and is where most
    weather phenomena occur.

stratosphere
    The second layer of the atmosphere of the
    Earth, located above the troposphere and
    below the mesosphere.
    
    The stratosphere is an atmospheric layer
    composed of stratified temperature layers,
    with the warm layers of air high in the
    sky and the cool layers of air in the low
    sky, close to the planetary surface of the
    Earth.
    
    The increase of temperature with altitude
    is a result of the absorption of the Sun's
    ultraviolet (UV) radiation by the ozone
    layer.
    
    The temperature inversion is in contrast
    to the troposphere, near the Earth's
    surface, where temperature decreases with
    altitude.

mesosphere
    The third layer of the atmosphere,
    directly above the stratosphere and
    directly below the thermosphere.
    
    In the mesosphere, temperature decreases
    as altitude increases.
    
    This characteristic is used to define its
    limits: it begins at the top of the
    stratosphere (sometimes called the
    stratopause), and ends at the mesopause,
    which is the coldest part of Earth's
    atmosphere, with temperatures below −143
    °C (−225 °F; 130 K).
    
    The exact upper and lower boundaries of
    the mesosphere vary with latitude and with
    season (higher in winter and at the
    tropics, lower in summer and at the
    poles), but the lower boundary is usually
    located at altitudes from 50 to 65 km (31
    to 40 mi; 164,000 to 213,000 ft) above sea
    level, and the upper boundary (the
    mesopause) is usually from 85 to 100 km
    (53 to 62 mi; 279,000 to 328,000 ft).

thermosphere
    The layer in the Earth's atmosphere
    directly above the mesosphere and below
    the exosphere.
    
    Within this layer of the atmosphere,
    ultraviolet radiation causes
    photoionization/photodissociation of
    molecules, creating ions; the thermosphere
    thus constitutes the larger part of the
    ionosphere.
    
    Taking its name from the Greek θερμός
    (pronounced thermos) meaning heat, the
    thermosphere begins at about 80 km (50 mi)
    above sea level.
    
    At these high altitudes, the residual
    atmospheric gases sort into strata
    according to molecular mass (see
    turbosphere).
    
    Thermospheric temperatures increase with
    altitude due to absorption of highly
    energetic solar radiation.

exosphere
    A thin, atmosphere-like volume surrounding
    a planet or natural satellite where
    molecules are gravitationally bound to
    that body, but where the density is so low
    that the molecules are essentially
    collisionless.
    
    In the case of bodies with substantial
    atmospheres, such as Earth's atmosphere,
    the exosphere is the uppermost layer,
    where the atmosphere thins out and merges
    with outer space.
    
    It is located directly above the
    thermosphere.
    
    Very little is known about it due to lack
    of research.
    
    Mercury, the Moon, Europa, and Ganymede
    have surface boundary exospheres, which
    are exospheres without a denser atmosphere
    underneath.
    
    The Earth's exosphere is mostly hydrogen
    and helium, with some heavier atoms and
    molecules near the base.[citation needed]