Haar-
A name applied to a wet sea fog or very fine drizzle that drifts in from the sea in coastal districts of eastern
Scotland and northeastern England. It occurs most frequently in summer.
Haboob-
A strong wind and sandstorm or duststorm in northern and central Sudan, especially around Khartoum, where the average number is about 24 a year. The name comes from the Arabic word habb, meaning “wind.” Haboobs are most frequent from May through September, especially in June, but they have occurred in every month except November. Their average duration is three hours; they are most severe in April and May when the soil is driest. They may approach from any direction, but most commonly from the north in winter and from the south, southeast, or east in summer. The average maximum wind velocity is over 13 m s-1 (30 mph) and a speed of 28 m s-1 (62 mph) has been recorded. The sand and dust form a dense whirling wall that may be 1000 m (3000 ft) high; it is often preceded by isolated dust whirls. During these storms, enormous quantities of sand are deposited. Haboobs usually occur after a few days of rising temperature and falling pressure.
Hail-
Precipitation in the form of balls or irregular lumps of ice, always produced by convective clouds, nearly always cumulonimbus. An individual unit of hail is called a hailstone. By convention, hail has a diameter of 5 mm or more, while smaller particles of similar origin, formerly called small hail, may be classed as either ice pellets or snow pellets. Thunderstorms that are characterized by strong updrafts, large liquid water contents, large cloud-drop sizes, and great vertical height are favorable to hail formation. The destructive effects of hailstorms upon plant and animal life, buildings and property, and aircraft in flight render them a prime object of weather modification studies. In aviation weather observations, hail is encoded A.
Hail storm-
Any storm that produces hailstones that fall to the ground; usually used when the amount or size of the hail is considered significant.
Haze-
Particles suspended in air, reducing visibility by scattering light; often a mixture of aerosols and photochemical smog.
Head wind-
A wind that opposes the intended progress of an exposed, moving object, for example, rendering an airborne object's airspeed greater than its groundspeed; the opposite of a tailwind.
Heat thunderstorm-
In popular terminology, a thunderstorm of the air mass type that develops near the end of a hot, humid summer day; this term has no precise technical meaning.
Heavy rain-
Rain with a rate of accumulation exceeding a specific value that is geographically dependent.
Helmholtz wave-
An unstable wave in a system of two homogeneous fluids with a velocity discontinuity at the interface.
High fog-
In the United States, the frequent fog on the slopes of the coastal mountains of California, especially applied when the fog overtops the range and extends as stratus over the leeward valleys.
High level thunderstorm-
Generally, a thunderstorm based at a comparatively high altitude in the atmosphere, roughly 2400 m or higher. These storms form most strikingly over arid regions, and frequently their precipitation is evaporated before reaching the earth's surface.
Hook echo-
A pendant, curve-shaped region of reflectivity caused when precipitation is drawn into the cyclonic spiral of a mesocyclone. The hook echo is a fairly shallow feature, typically extending only up to 3–4 km in height before becoming part of a bounded weak echo region (BWER).
Horizontal wind shear-
A horizontal variation in current speed within a flow.
Hot wind-
General term for winds characterized by intense heat and low relative humidity, such as summertime desert winds or an extreme foehn.
Humidity-
1. Generally, some measure of the water vapor content of air. The multiplicity of humidity measures is partly due to different methods of measurement and partly because the conservative measures (mixing ratio, specific humidity) cover an extremely wide dynamic range, as a result of the rapid variation of saturation vapor pressure with temperature. 2. Popularly, same as relative humidity.
Hurricane band-
A hurricane radar band of circular or spiral shape associated with a tropical cyclone (hurricane or typhoon). Made evident by radar observations, hurricane bands typically curve cyclonically inward toward the center of the storm. The bands may be classified as primary if they merge into the eyewall encircling the eye of the storm, or secondary if they are disconnected from the eyewall. Hurricane bands generally move slowly around the center of the storm in the direction of the hurricane circulation. See banded structure.
