Weather Glossary - H
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HailFalling precipitation consisting of particles of ice (hailstones). Usually spheroid, conical or irregular in form and with a diameter varying generally between 5 and 50 millimetres, though hail of 100mm and greater has been reported. Hail may fall from clouds separately, or hailstones may fuse together to make irregular lumps. Severe hail is greater than 20 millimetres in diameter. |
Haines IndexA measure of the moisture deficit and atmospheric stability used in the prediction of the spread of wildfires. The Haines Index is based on the 850-700 hPa lapse rate and the 850 hPa dew point depression.Haines Index values of 2 or 3 correspond to a very low potential for large fires. 4 is a low risk, 5 moderate and 6 is a high potential for large fires.The Haines Index is suited to plume dominated fires where instability has a greater effect on fire spread than near-ground weather conditions. |
HaloA ring or arc that encircles the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction and reflection of light through the ice crystals. The most common halo is the 22° halo which can be seen 22°, or around one handspan from the sun. |
HazeFine dry or wet dust or salt particles dispersed through a part of the atmosphere. Individually, these are not visible, but cumulatively they diminish visibility. |
HeatFormally, the amount of energy that flows between a system and its environment by virtue of a temperature difference that exists between them. |
Heat indexThe apparent temperature increase caused by the combination of high temperatures and high relative humidity. High humidity reduces the rate of evaporation of perspiration from the skin and thus reduces the cooling effect. This leads to the sensation of the temperature being higher than it actually is. |
Heat lowA shallow low pressure system caused by the strong heating of the earth's surface. These lows are usually weak and are strongest in the late afternoon. They can trigger afternoon or evening showers and thunderstorms. |
Heat waveA period of abnormally hot weather lasting several days. |
HelicityA property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. |
High cloudCirriform clouds composed of ice crystals with a base above around 20000 feet. |
High latitudesThe latitude belt roughly between 60° and 90° North and South. |
High pressure systemSee Anticyclone. |
HodographA plot representing the vertical distribution of horizontal winds, using polar coordinates. A hodograph is obtained by plotting the end points of the wind vectors at various altitudes, and connecting these points in order of increasing height. Interpretation of a hodograph can help in forecasting the subsequent evolution of thunderstorms (e.g., squall line vs. supercells, splitting vs. non-splitting storms, tornadic vs. nontornadic storms, etc.). |
Hook echo(or hook) A radar reflectivity pattern characterized by a hook-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 favourable conditions for tornado development. See BWER and supercell. |
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. |
HumidityGenerally, a measure of the water vapour content of the air. Popularly, it is used synonymously with relative humidity. |
HurricaneA severe Tropical cyclone of the Atlantic or eastern Pacific Ocean. |
HydrologyAn earth science concerned with the occurrence, distribution and circulation of water on and under the earth's surface, both in time and space, their biological, chemical and physical properties, their reaction with the environment, including their relation to living beings. |
HydrometeorologyThe study of the atmospheric processes that affect the water resources of the earth, including the study of the atmospheric and land phases of the hydrological cycle with emphasis on the interrelationships involved. |
HydrometeorsWater-based particles present in the atmosphere, usually falling, such as rain drops, cloud droplets, graupel or hail, snow and sleet. |
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