DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

atmosphere effect
mass of the atmosphere is about 5.3 × 10 18 kg,
which is about one-millionth of the total mass
of Earth. At sea level, average pressure is
1013.25 hPa, temperature 288.15 K, and density
is 1.225 kg/m 3 . The density of the atmosphere
decreases rapidly with height, and about threequarters
of the mass of the atmosphere is contained
within the troposphere. The atmosphere
has no precise upper limit. Formally one defines
the top of the atmosphere at 1000 km altitude,
which is also the highest observed altitude of
aurora.
atmosphere effect Whenever a gas that is
a weak absorber in the visible and a strong
absorber in the infrared is a constituent of a
planetary atmosphere, it contributes toward raising
the surface temperature of the planet. The
warming results from the fact that incoming radiation
can penetrate to the ground with relatively
little absorption, while much of the outgoing
longwave radiation is “trapped” by the atmosphere
and emitted back to the ground. This
is called the atmosphere effect. This warming
is commonly referred to as the “greenhouse effect”.
atmospheric angular momentum As wind
flows in the atmosphere, an air parcel rotates
about the Earth’s axis, so the atmosphere contains
angular momentum. In tropical easterlies,
friction with the Earth’s surface transfers angular
momentum to the atmosphere; in the midlatitiude
westerlies in both hemispheres, angular
momentum is transferred from the atmosphere
to the surface. Over long periods of time, the angularmomentumoftheatmosphereisinasteady
state. Thus, there must be angular momentum
transport from the tropics to mid-latitude
in the two hemispheres. In the tropics, the mean
meridional circulation plays an important role
in the meridional transport of atmospheric angular
momentum; and at mid-latitudes transient
eddies and stationary eddies play a major role.
Short term variations in the total atmospheric
angular momentum can be observed in the rotation
rate of the soled Earth.
atmospheric conductivity Conductivity of
the atmosphere, determined by ion concentration
and ion mobility. The conductivity in-
© 2001 by CRC Press LLC
creases roughly exponentially with height because
ion mobility depends on the number of
collisions between air particles and thus increases
with increasing height. Since the mobility
of small ions is much larger than that of
largeones, aerosolparticlesformasinkforsmall
ions, reducing the atmospheric conductivity.
atmospheric electric field The atmospheric
electric field on the ground is about −100 V/m
with strong variations depending on weather
conditions and the availability of dust particles.
With increasing height, the atmospheric electric
field decreases because the conductivity increases.
The atmospheric electric field is part
of the global electric circuit which can be conceptualized
as a spherical capacitor formed by
the terrestrial surface and the bottom of the ionosphere
filled with a slightly conductive medium,
the atmosphere. Thunderstorms work as generators,
driving a current from the surface to the
bottom of the ionosphere. The circuit is closed
through the fair weather atmosphere which acts
as a resistor.
atmospheric noise Radio noise produced
by natural electrical discharges below the ionosphere
and reaching the receiving point, where
it is observed, along normal propagation paths
between the Earth’s surface and the ionosphere.
Distant lightning has usually been thought to
be the main source for this noise. See galactic
noise.
atmospheric pressure The ambient air pressure
at a particular time and location. Expressed
as an absolute pressure (i.e., relative to a vacuum).
See also gauge pressure. “Standard” atmospheric
pressure is taken as 14.7 lb/in 2 or
101.3 kPa.
atmospherics A lightning stroke transmits
a wide range of electromagnetic radiation, the
most familiar being visible light. The electromagnetic
emissions are short-lived, like the
optical emissions. Those that can be reflected
by the Earth’s ionosphere can propagate to remote
locations in the earth-ionosphere waveguide
where they can be observed. At frequencies
used for early high frequency radio communications
(∼ 1 to 30 MHz) the propagated light-