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Hectopascal

See hPa

Helicity

See "What is Helicity?"

Horizontal vorticity

Even the classical 'vertical' vorticity term (q.v.) has some upward / downward component, but this is usually ignored for practical synoptic-scale meteorology. However, when coming down a scale or two, to local / mesoscale development, ( e.g. severe convective storms ), then vorticity about a horizontal axis is most important. It is often assessed in the lowest 3 km of atmosphere, and is 'driven' by two terms: vertical speed shear (increase / decrease of wind with increasing altitude) and directional ('twisting') shear, the change of direction with increasing altitude. If, in the lowest 3 km of atmosphere (up to 700 hPa), there is both a sharp increase of wind speed and a directional veer of wind with height, then horizontal vorticity will be potentially significant, provided it is coupled to the vigour of a developing cumulonimbus complex. (See also Vorticity; Vertical vorticity and "What is Helicity?"

hPa

(abbr) Hectopascal - equivalent to a millibar (q.v.). An attempt to use SI units without doing away with the idea of millibars (from the c.g.s. system). [ 1 hPa=100 Pa (or N/m2)]

Humidity

See "What is the difference between humidity and relative humidity?"

Hurricane Force

This term (in UK Met Office use) is only used in shipping bulletins and associated Gale/Storm warnings in the form "Hurricane Force 12", from the modified Beaufort scale. It is strictly defined as a mean (10 minute) wind of 64 knots or more. (Gusts not defined) (See also comments at Severe Gale).

[ Please note carefully that just because an area of low pressure produces winds to 'hurricane' force as defined here, it does NOT make that feature a Hurricane! For more on this, see this question on the October 1987 storm ]

HZ

Haze: used in METAR/TAF reports etc., when visibility is reduced in a 'dry' atmosphere. (visibility > = 1km, relative humidity roughly < 90%).