Hadley
Cell - a thermal circulation consisting
of rising air near the equator and sinking air near 30 degrees latitude
Hail
- precipitation
in the form of rounded balls of ice,
always formed in convective
clouds, nearly always thunderstorms
Hail
Contamination - a limitation in NEXRAD
rainfall estimates whereby abnormally
high reflectivities associated with
hail are converted to rainfall rates and rainfall
accumulations.
Hail
Spike - an area of reflectivity
extending away from the radar immediately behind
a thunderstorm with extremely large
hail. In an area of large hail, radiation
from the radar can bounce from hailstone to hailstone before being reflected
back to the radar. The time delay between the backscattered radiation
from the storm and the bounced and scattered radiation from the large
hail causes the reflectivity from the hail to appear to come from a farther
range than the actual storm.
Hailstone
- a particle of hail
ranging in size from that of a pea to that of a grapefruit
Halo
- a type of atmospheric optical phenomena
which appears as a colored or whitish ring or arc of light that seems
to encircle the sun or moon when viewed through an ice
crystal cloud
Haze
- fine dust or salt particles which tend
to reduce visibility
by being dispersed through a portion of the atmosphere
Heat
- a form of energy
transferred between objects or systems as a result of a difference in
temperature;
not the same as temperature
Heat
Index (HI) - a value that represents
the temperature it actually feels at a given air temperature
and relative humidity
Heating
Degree-Day - a type of degree
day used for estimating fuel consumption
for warming the indoor environment to a base temperature,
generally to 65 degrees Fahrenheit;
one heating degree-Day is given for each degree that the day's average
temperature is below the base temperature
Heatstroke
- a physiologic state marked by extremely
high body temperature that results from prolonged exposure to high air
temperatures,
particularly when accompanied by high relative
humidity
Helicity
- A property of a moving fluid which
represents the potential for helical flow (i.e., flow which follows a
corkscrew pattern) to evolve. Helicity is proportional to the strength
of the flow, the amount of vertical wind shear,
and the amount of turning in the flow (i.e. vorticity). Atmospheric helicity
is computed from the vertical wind profile in the lower part of the atmosphere
(usually from the surface up to 3 km), and is measured relative to storm
motion (storm-relative helicity). Higher values of helicity (generally,
around 150 m2/s2 or more) favor the development
of mid-level rotation (i.e. mesocyclones).
Extreme values can exceed 600 m2/s2.
Hertz
- the derived unit of frequency:
1 Hertz equals 1 cycle per second. Named for Heinrich Rudolph Hertz (1857-1894),
a German physicist who studied electromagnetic
radiation.
High
- in meteorology, an region of high
pressure; see also anticyclone
Hook
(or Hook Echo) - a radar reflectivity
pattern characterized by a hook- or crescent-shaped extension of a thunderstorm
echo, usually in the right-rear part of the storm (relative to its direction
of motion). A hook often is associated with a mesocyclone,
and indicates favorable conditions for tornado
development.
Horizon
- the distant line along with the earth and sky appear to meet,
where nearby obstructions are not considered as part of the horizon
HP
Storm (or HP Supercell) - High -Precipitation
storm (or High -Precipitation supercell); a supercell
thunderstorm in which heavy precipitation
(often including hail) falls on the trailing side
of the mesocyclone. Precipitation often
totally envelops the region of rotation, making visual identification
of any embedded tornadoes difficult and very
dangerous. Unlike most classic supercells, the region of rotation in many
HP storms develops in the front-flank region of the storm (i.e., usually
in the eastern portion). HP storms often produce extreme and prolonged
downburst events, serious flash
flooding, and very large damaging hail events.
Humidity
- some measure of the water
vapor content of the air
Hurricane
- a tropical
cyclone with winds
exceeding 66 knots,
generally accompanied by rain,
thunder,
and lightning
Hurricane
Warning - a warning
that hurricane
force winds
are impending
Hurricane
Watch - an announcement for a specific
area that hurricane conditions may be imminent and that persons should
prepare for hurricane
winds
Hydrologic
Cycle - the description of the transport
of water substance between the earth, the atmosphere, and the seas
Hydrology
- the scientific study of the waters of the earth, especially
with relation to the effects of precipitation
and evaporation upon the occurrence and
character of water on or below the land surface
Hygrometer
- an instrument for measuring the water
vapor content of the atmosphere
Hydrometeorology
- that part of meteorology
that pertains to hydrology
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