A B C D E F G H I J K L M N O P Q R S T U V W X Y Z #


abbreviation used in National Weather Service text products for the 500-mb level height


abbreviation used in National Weather Service text products for the 700-mb level height

H8 or H85

abbreviation used in National Weather Service text products for the 850-mb level height


a dust storm or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air. See Dust Storm and Sandstorm

Hadley Cell

a thermal circulation consisting of rising air near the equator and sinking air near 30 degrees latitude


precipitation in the form of rounded balls of ice, always formed in convective clouds, nearly always found in thunderstorms


On January 5, 2010, the National Weather Service changed the criteria for severe thunderstorms by upping the minimum size hail from ¾ to 1 inch (quarter size). The wind threshold of 50 knots, or 58 mph remains the same.The reason for the change to the hail threshold, according to a statement issued by the Fire and Public Weather Services Branch of the NWS, is that research reveals “significant damage” doesn’t occur from hail smaller than an inch. Hailstones the size of quarters or larger are the ones most destructive to cars, homes, buildings, and crops.



Common Objects Hail Size in Inches (in) Hail Size in millimeters (mm)
Pea 1/4 6.35 
Dime 1/2 12.70
Penny 3/4 19.05
Nickel 7/8 22.225
Quarter 1 25.40
Half Dollar 1 1/4 31.75
Ping pong ball 1 1/2 38.10
Golf ball 1 3/4 44.45
Hen egg 2 50.80
Tennis ball 2 1/2 63.50
Baseball 2 3/4 69.85
Teacup 3 76.20
Grapefruit 4 101.60
Softball 4 1/2 114.30

Hail Contamination

a limitation in NEXRAD rainfall estimates whereby abnormally high reflectivities associated with hail are converted to rainfall rates and rainfall accumulations.

Hail Spike

an area of reflectivity extending away from the radar immediately behind a thunderstorm with extremely large hail. In an area of large hail, radiation from the radar can bounce from hailstone to hailstone before being reflected back to the radar. The time delay between the backscattered radiation from the storm and the bounced and scattered radiation from the large hail causes the reflectivity from the hail to appear to come from a farther range than the actual storm.


a particle of hail ranging in size from that of a pea to that of a softball


a type of atmospheric optical phenomena that appears as a colored or whitish ring or arc of light and seems to encircle the sun or moon when viewed through an ice crystal cloud

Hard Freeze (or Killing Freeze)

freeze in which plants are destroyed, the ground freezes, and heavy ice forms on puddles and containers of water. A common threshold used to forecast a hard freeze is a low temperature below 28°F.

Haze (HZ)

widely dispersed, very fine dust or salt particles that tend to reduce visibility.


abbreviation used for  Heating Degree-Day


a form of energy transferred between objects or systems as a result of a difference in temperature; not the same as temperature

Heat Burst

a phenomenon associated with dying thunderstorms. Gusty winds, a rapid increase in temperature, and a decrease in dew point temperature are the signs of a heat burst.

Heat Index (HI)

a value that represents the temperature it actually feels like at a given air temperature and relative humidity

Heat Lightning

lightning that occurs at a distance such that thunder is not audible.

Heat Wave

a period of abnormally and uncomfortably hot and usually humid weather that often lasts from several days to several weeks. This term is relative to the "usual" weather in the area. For example, a heat wave in Oklahoma may have highs in the upper 90s or 100s, while a heat wave in northern Europe may have highs in the 80s (degrees Fahrenheit).

Heating Degree-Day (HDD)

a type of degree-day used for estimating fuel consumption for warming the indoor environment to a base temperature, generally to 65 degrees Fahrenheit; one heating degree-day is given for each degree that the day’s average temperature is below the base temperature


a physiological state marked by extremely high body temperature that results from prolonged exposure to high air temperatures, particularly when accompanied by high relative humidity

Hectopascal (hPa)

a unit of pressure equal to a millibar (1 hPa = 1 mb).


a property of a moving fluid that represents the potential for helical flow (i.e., flow which follows a corkscrew pattern) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (i.e., vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion (storm-relative helicity). Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e., a mesocyclone). Extreme values can exceed 600 m2/s2.

Hertz (Hz)

the derived unit of frequency: 1 Hertz equals 1 cycle per second. Named for Heinrich Rudolph Hertz (1857-1894), a German physicist who studied electromagnetic radiation.


abbreviation used for Heat Index


abbreviation for high pressure; see Anticyclone


abbreviation for Hawaii


in meteorology, a region of high pressure; see also Anticyclone

High Risk (of severe thunderstorms)

Severe weather is expected to affect more than 10 percent of the area. A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major severe weather outbreak.

High-Precipitation Supercell (HP supercell)

supercell thunderstorm in which heavy precipitation (often including hail) falls on the trailing side of the mesocyclone. Precipitation often totally envelops the region of rotation, making visual identification of any embedded tornadoes difficult and very dangerous. Unlike most classic supercells, the region of rotation in many HP storms develops in the front-flank region of the storm (i.e., usually in the eastern portion). HP storms often produce extreme and prolonged downburst events, serious flash flooding, and very large, damaging hail events.

Hoar Frost

a deposit of interlocking ice crystals formed by sublimation on objects, usually those of a small diameter and freely exposed to the air (e.g., tree branches, plants, and wires). It forms when air with a dew point below freezing is brought to saturation by cooling.


a diagram that plots wind speed and direction with height. The information usually comes from sounding (radiosonde) data. This diagram is used to look at wind shear and helicity.

Hook (or Hook Echo)


radar reflectivity pattern characterized by a hook- or crescent-shaped extension of a thunderstorm echo, usually in the right-rear part of the storm (relative to its direction of motion). A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.


the distant line along which the earth and sky appear to meet, where nearby obstructions are not considered as part of the horizon

Horizontal Pressure Gradient Force

the force that moves air from an area of high pressure to an area of low pressure, due to pressure differences on a horizontal plane. A strong horizontal pressure gradient force can be seen on a weather map of a particular level (for example, 925 mb) when isobars are packed closely together--wind speeds will be high in this area. A weak pressure gradient force results when isobars are spaced further apart--wind speeds will be low in this area.

HP Storm

High-Precipitation storm. Same as High-Precipitation Supercell.

HP Supercell

see High-Precipitation Supercell


abbreviation for the unit of measurement called a Hectopascal


abbreviation used in National Weather Service text products for the word hour


abbreviation used in National Weather Service text products for the word hours


a measure of the water vapor content of the air


tropical cyclone (a warm core low) with winds exceeding 64 knots (74 mph).

Hurricane Warning

warning issued by the National Weather Service's National Hurricane Center that hurricane force winds (64 knots or higher) are expected in a specified coastal area in 24 hours or less.

Hurricane Watch

an announcement made by the National Weather Service's National Hurricane Center for a specific area that a hurricane condition poses a possible threat, generally within 36 hours.


abbreviation used in National Weather Service text products for the word have


abbreviation used in National Weather Service text products for the word heavy


abbreviation used in National Weather Service text products for the word however

Hydrologic Cycle

the description of the transport of water substance between the earth, the atmosphere, and the seas



the scientific study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water on or below the land surface


a particle of condensed water (liquidsnowicegraupelhail) in the atmosphere

Hydrometeorological Prediction Center (HPC)

one of 9 NCEP centers. Provides nationwide analysis and forecast guidance of precipitation and major weather patterns out through seven days. It is located in Camp Springs, Maryland.


that part of meteorology that pertains to hydrology


an instrument used to measure the water vapor content of the atmosphere


a condition in which a person’s temperature drops at least 2 degrees below normal temperature (98.9°F) because more heat escapes from the body than the body can produce. If the body temperature drops below around 89.6°F, death may occur.


abbreviation for the unit of measurement called a Hertz


abbreviation used in National Weather Service text products for the word Haze