Hadley cell

Global scale meridional thermal circulation model proposed by G. Hadley to explain the trade winds. In each hemisphere, the trade winds carry air from the subtropical high pressure belt –30° latitude- to the Inter Tropical Convergence Zone (equatorward movement) in the low level, then air rises near the equator and completes the cell with a poleward flow aloft, descending near 30° latitude.


Precipitation, either transparent or opaque, in the form of spheroidal or irregular particles of ice, varying in diameter generally from 5 to 50 mm, produced almost always by cumulonimbus clouds. These particles may fall either separately or aggregated into irregular lumps, being called hailstones.


Any storm that produces hailstones; usually this term is used when the amount or size of the hail is significant.

Haines Index

Index that combines instability and dry air to indicate the potential for large fire growth.


Any of the family of optical phenomena, colored or whitish rings, arcs, pillars or spots of light, caused by the refraction or reflection of light on ice crystals present in the atmosphere.


Small, fine and dry particles –usually of dust or salt- suspended in the air, which gives the sky an opalescence appearance and reduces visibility by scattering light.

Heat cramps

Painful muscle cramps caused by exercising and sweating in high heat.

Heat exhaustion

Condition caused by exposure to heat, resulting in the depletion of body fluids, which causes dizziness, nausea, and often collapse.

Heat index

Heat rash

Inflammatory skin condition as a response to exposure to high heat and humidity, characterized by the eruption of small red itching papules.

Heat wave / Hot wave, warm wave

Period –lasting from a few days to a few weeks- of marked warming of the air, being uncomfortably hot and usually humid, or the invasion of very warm air over a large area.


Severe condition caused by the failure of the body’s temperature-regulating abilities, resulting from prolonged exposure high heat.


Point at which the solar wind meets the interstellar medium.


It is the immense magnetic bubble containing our solar system, solar wind and entire solar magnetic field. It extends well beyond the orbit of Pluto.


High pressure area, usually accompanied by an anticyclonic and outward wind flow.

High cloud

Same as high-level cloud. It is a cloud of the high cloud étage. This group includes: cirrus, cirrocumulus and cirrostratus.


Name given to the last 10,000 years of the Earth’s history (recent period of geologic time), the time since the end of the last major glacial epoch, or “ice age.” Although it includes the “Little Ice Age”, in general, it has been a relatively warm period between ice ages.

Horizontal pressure gradient force

Force applied to each particle of a fluid as a result of the non-uniform field of pressure. It acts in the sense and direction of the pressure gradient (rate of change of pressure with distance), in this case horizontally.

Hot spell

Hot period.


Numerical value derived from the combination of the ambient air temperature and relative humidity to determine an apparent temperature which represents how hot the air actually “feels” to the average person.

Humidity / Relative Humidity (RH)

Water vapor content of air. Popularly, same as relative humidity.


Name given to warm core tropical cyclone, with average surface winds exceeding 64 knots (74 MPH/118 KPH) in the North Atlantic, the Caribbean, the Gulf of Mexico and the Eastern North Pacific Ocean. Tropical cyclone with hurricane force winds (74 MPH / 118 KPH / 64 knots or more) in the South Pacific and South-East Indian Ocean, where it receives regional names –typhoon, cyclone-.

Hydrologic drought

Below normal drop of surface and subsurface water supplies (such as stream flow, reservoir or lake levels, ground water) caused by extended periods of lacking precipitation. This kind of drought deals with the effects that the lack of moisture has on the hydrological system as a whole. Usually, hydrological drought lags behind other kinds of droughts.


Portion of the earth covered by water and ice. Total of water encompassing the earth. Then, it includes not only ice, snow and glaciers but also water in the form of precipitation while still in the atmosphere (water vapor and clouds).


Instrument that measures humidity or water vapor content of the atmosphere.


Abnormally low body temperature.