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