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Weather Glossary - H

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Hail

Falling 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 Index

A 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.

Halo

A 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.

Haze

Fine dry or wet dust or salt particles dispersed through a part of the atmosphere. Individually, these are not visible, but cumulatively they diminish visibility.

Heat

Formally, the amount of energy that flows between a system and its environment by virtue of a temperature difference that exists between them.

In meteorology, the formal definition usually holds, where the "system" is a parcel of air, and the "environment" is the atmosphere surrounding the parcel.

Heat index

The 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 low

A 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 wave

A period of abnormally hot weather lasting several days.

Helicity

A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve.

Helicity is proportional to the strength of the flow, the amount of vertical 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. Higher values of helicity (generally, around 150 mē/sē or more) favour the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 mē/sē.

High cloud

Cirriform clouds composed of ice crystals with a base above around 20000 feet.

High latitudes

The latitude belt roughly between 60° and 90° North and South.

High pressure system

See Anticyclone.

Hodograph

A 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.

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.

Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous.

Humidity

Generally, a measure of the water vapour content of the air. Popularly, it is used synonymously with relative humidity.

Hurricane

A severe Tropical cyclone of the Atlantic or eastern Pacific Ocean.

Hydrology

An 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.

Hydrometeorology

The 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.

Hydrometeors

Water-based particles present in the atmosphere, usually falling, such as rain drops, cloud droplets, graupel or hail, snow and sleet.

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SES rescues people trapped by floodwaters on NSW north coast

ABC licensed image 11:20 EST People trapped in two vehicles surrounded by floodwaters have been rescued by the State Emergency Service (SES) on the New South Wales north coast.

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