Weather Glossary – H


HAIL – Precipitation of small balls or other pieces of ice (hailstones) falling separately or frozen together in irregular lumps. (Typically associated with thunderstorms and surface temperatures above freezing).

HALOS – Rings or arcs that seem to encircle the sun or moon. They are caused by the refraction of light through the ice crystals in cirrus clouds.

HARD FREEZE – freeze where vegetation is killed and the ground surface is frozen solid.

HAZE- Fine dry or wet dust or salt particles in the air that reduce visibility.

HEAVY SNOW – In the Inland Northwest, mountains above 3000 feet – 8 to 12 inches in 12 hours or 12-18 inches or more in 24 hours. For the valleys and Basin below 3000 feet – 4 inches in 12 hours and 6 inches in 24 hours.

HEAT INDEX – The HI is the temperature the body feels when the heat and humidity are combined.

HEAT LOW – The thermal induced surface low pressure trough that develops during the warm season in the lee of the Cascades.

HELICITY– Streamwise vorticity available for ingestion into a thunderstorm. Higher values are favorable for a rotating updraft (greater than 400). A measure of low level wind shear, normally within the lowest 3 km of the atmosphere, relative to the movement of a thunderstorm (thus referred to as 0-3 km Storm Relative Helicity). This gives forecasters an indication of an environment that is favorable for supporting the development of thunderstorms with rotating updrafts, a precursor to supercell thunderstorms (the most violent of severe storms) and tornado development. Values of helicity greater than +150 are considered significant; however, like CAPE values, there is no magic value of (positive) helicity under which rotating thunderstorms will not develop. Helicity is only an index to determining thunderstorm rotation potential. 

HIC – Hydrologist In Charge.

HIGH – The center of an area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.

HIGH WINDS – Winds of 40 mph or greater lasting for an hour or more , and/or gusts to 58 mph or greater.

HORIZONTAL VORTICITY A rotation of air caused by vertical speed or directional wind shear.

HORSE LATITUDES – subtropical regions where anticyclones produce settled weather.

HUMIDITY -The amount of water vapor in the atmosphere. (See relative humidity).

HURRICANE – A severe tropical cyclone with wind speeds in excess of 74 mph (64 knots).

HYDROLOGIC CYCLE – the composite picture of the interchange of water substance between the earth, the atmosphere and the seas which includes the change of state and vertical and horizontal transport.

HYGROMETER – an instrument used to measure humidity.