H

Hail – precipitation in the form of balls or irregular lumps of ice produced by liquid precipitation, freezing and being coated by layers of ice as it is lifted and cooled in strong updrafts of thunderstorms.

Haze – fine dust or salt particles in the air that reduce visibility. In Canada, haze is defined to consist of fine particles of dust and pollution suspended in the atmosphere, and is distinguished from fog by its bluish or yellowish tinge.

Heat balance – the equilibrium existing between the radiation received and emitted by a planetary system.

Heat index – an index that combines air temperature and humidity to give an apparent temperature (how hot it feels).

Heat island – a dome of elevated temperatures over an urban area caused by the heat absorbed by structures and pavement.

Heat lightning – lightning that can be seen, but is too far away for the thunder to be heard.

Heat wave – a period of abnormally hot weather lasting several days.

Heavy snow – depending on the region of the USA, this generally means that four or more inches of snow has accumulated in 12 hours, or six or more inches of snow in 24 hours.

Heavy snow warning – older terminology replaced by winter storm warning for heavy snow. Issued when seven or more inches of snow or sleet is expected in the next 24 hours. A warning is used for winter weather conditions posing a threat to life and property.

Heavy surf – the result of large waves breaking on or near the shore resulting from swells or produced by a distant storm.

High – an area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone. In Canada, a high is described as an area of high atmospheric pressure with a closed, clockwise movement of air.

High cloud – cirrus, cirrostratus and cirrocumulus.

High risk (of severe thunderstorm) – severe weather is expected to affect more than 10 percent of the area.

High wind warning – issued when sustained winds from 40 to 73 mph are expected for at least one hour; or any wind gusts are expected to reach 58 miles per hour or more.

High wind watch – issued when conditions are favorable for the development of high winds over all of or part of the forecast area but the occurrence is still uncertain. The criteria of a high wind watch are listed under the high wind warning and should include the area affected, the reason for the watch and the potential impact of the winds.

Hot or very warm – in summer, hot or very warm means more than seven degrees Celsius above normal.

Hot spot – typically large areas of pavement, these "hot spots" are heated much quicker by the sun than surrounding grasses and forests. As a result, air rises upwards from the relatively hot surface of the pavement, reaches its condensation level, condenses, and forms a cloud above the "hot spot".

Humidity – the amount of water vapor in the atmosphere. In Canada, it is described as the amount of moisture in the air.

Hurricane – a severe tropical cyclone with sustained winds over 74 miles per hour (64 knots). Normally applied to such storms in the Atlantic Basin and the Pacific Ocean east of the International Date Line.

Hygrometer – an instrument used to measure humidity.