DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Geometry of the airfoil probe, α is the angle of attack
of the oncoming flow.
rock column representing the mountain area is
then h + H + b. Hydrostatic equilibrium below
the mountain root requires (ρm − ρc)b = ρch.
Airy phase When a dispersive seismic wave
propagates, the decrease of amplitude with
increasing propagation distance for a period
whose group velocity has a local minimum is
smaller than that for other periods. The wave
corresponding to the local minimum is referred
to as an Airy phase and has large amplitude on a
© 2001 by CRC Press LLC
Alba Patera
record of surface waves. An Airy phase appears
at a transition between normal dispersion and reverse
dispersion. For continental paths an Airy
phase with about a 20-sec period often occurs,
while for oceanic paths an Airy phase with 10to
15-sec period occurs, reflecting the thickness
of the crust.
Airy wave theory First-order wave theory
for water waves. Also known as linear or firstorder
theory. Assumes gravity is the dominant
restoring force (as opposed to surface tension).
Named after Sir George Biddell Airy (1801–
1892).
Aitken, John (1839–1919) Scottish physicist.
In addition to his pioneering work on atmospheric
aerosol, he investigated cyclones, color,
and color sensations.
Aitken nucleus count One of the oldest and
most convenient techniques for measuring the
concentrations of atmospheric aerosol. Saturated
air is expanded rapidly so that it becomes
supersaturated by several hundred percent with
respect to water. At these high supersaturations
water condenses onto virtually all of the aerosol
to form a cloud of small water droplets. The
concentration of droplets in the cloud can be determined
by allowing the droplets to settle out
onto a substrate, where they can be counted either
under a microscope, or automatically by
optical techniques. The aerosol measured with
an Aitken nucleus counter is often referred to as
the Aitken nucleus count. Generally, Aitken nucleus
counts near the Earth’s surface range from
average values on the order of 10 3 cm −3 over
the oceans, to 10 4 cm −3 over rural land areas, to
10 5 cm −3 or higher in polluted air over cities.
Alba Patera A unique volcanic landform on
Mars that exists north of the Tharsis Province.
It is less than 3 km high above the surrounding
plains, the slopes of its flanks are less than
a quarter of a degree, it has a diameter of
≈ 1600 km, and it is surrounded by an additional
500 km diameter annulus of grabens. Its
size makes it questionable that it can properly be
called a volcano, a name that conjures up an image
of a distinct conical structure. Indeed from
the ground on Mars it would not be discernible