DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

H
Hadley (cell) circulation A circulation in the
meridional plane known to exist in the tropics
due to the ascending warm air near the equator
and descending cold air in high latitudes.
Hadley cell Convection cells within the atmosphere
of a body. On planets where most of
the atmospheric heating is produced by the sun
(such as Earth, Venus, and Mars), air over the
equatorial regions will be hotter than air over
the poles. This hotter air is less dense than
cooler air and thus rises, eventually losing heat
as it moves toward the polar regions. Over the
poles, the air becomes colder and more dense,
thus sinking towards the surface. The cooler
air moves back along the planet’s surface towards
the equator, where it warms up and the
cycle repeats. This basic cycle of warm air rising
over the equator and cooler air sinking over
the poles is called Hadley Circulation. Rapid
rotation and variations in surface temperature
(caused, for example, by oceans vs. continents)
complicate this basic pattern. Hadley cells fairly
accurately describe the atmospheric circulation
only for Venus, although they form the basics for
physical studies of other planetary atmospheres.
hadron Any particle that interacts with the
strong nuclear force. Hadrons are divided into
two groups: baryons (“heavy ones,” consisting
of three quarks), which are fermions and obey
the exclusion principle, and mesons which are
bosons, and consist of a quark anti-quark pair.
See fermion, boson, quark.
hail Large frozen pellets (greater than 5 mm
in diameter) of water that occur in thunderstorms,
in which updrafts keep the hail suspended
at an altitude with freezing temperatures
for long periods of time, growing the hailstone
until it finally falls out of the cloud.
hailstone A single unit of hail.
halocline
Hale cycle The observation of sunspot numbers
alone reveals an 11-year cycle. In contrast,
the Hale cycle is a 22-year cycle which in addition
to sunspot numbers also considers polarity
patterns. While after 11 years the polarity of
the sun is reversed, the original polarity pattern
is restored only after 22 years. The Hale cycle
therefore is also called the magnetic cycle of the
sun.
Hale–Nicholson Polarity Law Inagiven
solar cycle, examination of solar magnetograms
reveals a distinctive alternation of positive and
negative polarities in active regions. In the sun’s
northern hemisphere the positive polarity is located
in the “preceding” (westerly) part of the
active region and the negative polarity is located
in the “following” (easterly) part. The sense
is reversed in the southern hemisphere. The
hemispherical polarity patterns alternate with
each successive activity cycle. This behavior
of alternating active region magnetic polarities
is known as the Hale–Nicholson Polarity Law.
Halley’s comet A comet with a period of 74
to 79 years which was identified with several
historical passages (including a visit coincident
with the defeat of King Harold in 1066) of bright
comets by Edmund Halley (1656–1742), validated
when the comet reappeared after Halley’s
death in 1758. The most recent perihelion passage
of Halley’s comet occurred on February 9,
1986.
Halley’s identification of the comet and suggestion
of perturbations on its orbit provided an
explanation of comets in the context of Newtonian
mechanics and Newtonian gravity.
halo Arcs or spots of light in the sky, under
suitable conditions even a bright circle around
the sun or the moon. Halos are caused by the
refraction of sunlight on ice crystals in the atmosphere;
thus, halos can be observed best in
cold climates where ice crystals also form in the
lower (and denser) troposphere. The angular
extend of a halo is always about 22 ◦ ; however,
depending on crystal shape and orientation, additional
arcs and even a wider ring can form.
halocline The region of large vertical gradient
of density due to salinity in oceans.
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