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Haboob⁽⁶⁾
	A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air.

Hail⁽⁶⁾
	Solid precipitation in the form of chunks or balls of ice with diameters greater than 5 mm. The stones fall from cumulonimbus clouds.

Hailstones⁽⁶⁾
	Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls.

Hair hygrometer⁽⁶⁾
	An instrument used to monitor relative humidity by measuring the changes in the length of human hair that accompany humidity variations.

Halos⁽⁶⁾
	Rings or arcs that encircle the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction of light.

Haze⁽⁶⁾
	Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility.

Heat⁽⁶⁾
	A form of energy transferred between systems by virtue of their temperature differences.

Heat balance⁽¹⁾
	The equilibrium existing between the radiation received and emitted by a planetary system.

Heat capacity⁽⁶⁾
	The ratio of the heat absorbed (or released) by a system to the corresponding temperature rise (or fall).

Heat index ((HI))⁽⁶⁾
	An index that combines air temperature and relative humidity to determine an apparent temperature-how hot it actually feels.

Heat lightning⁽⁶⁾
	Distant lightning that illuminates the sky but is too far away for its thunder to be heard.

Heat of fusion⁽⁶⁾
	Heat released when water changes phase from liquid to solid; 80 calories per gram.

Heat of melting⁽⁶⁾
	Heat required to change the phase of water from solid to liquid; 80 calories per gram.

Heating degree-day⁽⁶⁾
	A form of the degree-day used as an index for fuel consumption. Needed on days when average air temperature falls below 69°F (18°C); computed by subtracting the day's average temperature from 65°F.

Heiligenschein⁽⁶⁾
	A faint white ring surrounding the shadow of an observer's head on a dew-covered lawn.

Heterosphere⁽⁶⁾
	The atmosphere above 80 km (50 mi) where gases are stratified, with concentrations of the heavier gases decreasing more rapidly with altitude than concentrations of the lighter gases.

High inversion fog⁽⁶⁾
	A fog that lifts above the surface but does not completely dissipate because of a strong inversion (usually subsidence) that exists above the fog layer.

Highland climate⁽⁶⁾
	Complex pattern of climate conditions associated with mountains. Highland climates are characterized by large differences that occur over short distances.

Historical Flood⁽⁶⁾
	Flood events documented by human observation but recorded prior to the development of systematic streamflow measurements.

Hoarfrost⁽⁶⁾
	Fernlike crystals of ice that form by deposition of water vapor on twigs, tree branches, and other vegetation.

Homosphere⁽⁶⁾
	The atmosphere up to 80 km (50 mi) in which the proportionality of principal gaseous constituents, such as oxygen and nitrogen, is constant.

Humid continental climate⁽⁶⁾
	A relatively severe climate characteristic of broad continents in the middle latitudes between approximately 40-50° north latitude. This climate is not found in the southern hemisphere, where the middle latitudes are dominated by the oceans.

Humid Subtropical Climate⁽⁶⁾
	A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.

Hurricane⁽³⁾
	See Cyclone.

Hydrograph⁽⁶⁾
	An instrument that provides a continuous trace of relative humidity with time.

Hydrologic cycle⁽⁵⁾

The process of evaporation, vertical and horizontal transport of vapor, condensation, precipitation, and the flow of water from continents to oceans. It is a major factor in determining climate through its influence on surface vegetation, the clouds, snow and ice, and soil moisture. The hydrologic cycle is responsible for 25-30% of the mid-latitudes' heat transport from the equatorial to polar regions. See The Water Cycle.

Hydrology⁽³⁾
	The scientific study of precipitation, evaporation, distribution, and effects of water on the Earth's surface, in the soil and rocks, and in the atmosphere.

Hygrometer⁽⁶⁾
	An instrument designed to measure the air's water vapor content. The sensing part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).

Hypothermia⁽⁶⁾
	The deterioration in one's mental and physical condition brought on by a rapid lowering of human body temperature.