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HABOOB: Sudanese name for duststorm or sandstorm with strong winds that carry small particles of dirt or sand into the air, particularly severe in areas of drought.
HAIL: Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeter or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel. Individual lumps are called hailstones. It is reported as "GR" in an observation and on the METAR. Small hail and/or snow pellets is reported as "GS" in an observation and on the METAR.
HALO: The ring of light that seems to encircle the sun or moon when veiled by cirrus clouds. To produce this phenomena, the ice crystals must be in a heterogeneous arrangement to refract the sunlight. The most commonly observed is a halo that forms at a 22° radius, although another one at 46° radius may also be seen.
HAZE: A suspension of fine dust and/or smoke particles in the air. Invisible to the naked eye, the particles reduce visibility by being sufficiently numerous to give the air an opalescent appearance. It is reported as "HZ" in an observation and on the METAR.
HEAT: A form of energy transferred between two systems by virtue of a difference in temperature. The first law of thermodynamics demonstrated that the heat absorbed by a system may be used by the system to do work or to raise its internal energy.
HEAT BALANCE: The equilibrium which exists on the average between the radiation received by the earth and atmosphere from the sun and that emitted by the earth and atmosphere. The balance between heat loss (long wave radiation from the earth back into the atmosphere) and heat gain (incoming solar radiation).
HEAT EXHAUSTION: The effect of excessive heat, particularly when combined with high humidity, on a human being. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.
HEAT INDEX: The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature.; Related term: Heat Index Chart
HEATING DEGREE DAY: One heating degree day is given for each degree that the daily mean temperature is below 65°F. It is used as an indication of fuel consumption.; Related terms: degree day or cooling degree day
HEAT LIGHTNING: Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.
HEAT STROKE: Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105°F, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.
HEAT WAVE: A period of abnormally and uncomfortably hot weather. It could last from several days to several weeks. The Weather Channel uses the following criteria for a heat wave: a minimum of ten states must have 90°F plus temperatures and the temperatures must be at least five degrees above normal in parts of that area for at least two days or more.
HELICITY: A property of a moving fluid, such as air, representing the potential for helical flow (flow that follows a corkscrew pattern). Computed from the vertical wind profile of the lower atmosphere and measured relative to the motion as a storm, it is used to forecast the formation of mesocyclones.
HIGH CLOUDS: A term used to signify cirriform clouds that are composed of ice crystals and generally have bases above 20,000 feet. The main types of high clouds are cirrus,cirrocumulus, and cirrostratus. This altitude applies to the temperate zone. In the polar regions, these clouds may be found at lower altitudes. In the tropics, the defining altitudes for cloud types are generally higher.
HIGH LATITUDES: The latitude belt roughly between 60° and 90° North and South.; Related term: polar region
HIGH PRESSURE SYSTEM: An area of relative pressure maximum that has diverging winds and a rotation opposite to the earth's rotation. This is clockwise the in Northern Hemisphere and counterclockwise in the Southern Hemisphere. It is the opposite of an area of low pressure or a cyclone.; Related term: anticyclone
HOARFROST: Another name for frost. A deposit of hoarfrost occurs when air with a dew point below freezing is brought to saturation by cooling.
HOOK ECHO: A radar reflectivity pattern observed in a thunderstorm, appearing like a fish hook and indicating favorable conditions for tornadic development. However, hook echoes and tornadoes do not always accompany each other.
HORIZON: One of several lines or planes used as reference for observation and measurement relative to a given location on the surface of the earth. The geographic horizon, also called the apparent horizon, is the distant line along which earth and sky appear to meet. This is the usual concept of horizon and is used in weather observing. The local horizon is the actual lower boundary of the observed sky or the upper outline of terrestrial objects including nearby natural obstructions, such as mountains.
HORSE LATITUDES: Located between 30° North and 30° South in the vicinity of the equator, this area typically has calm or light and variable winds.; Related terms: equatorial trough, the Intertropical Convergence Zone (ITCZ), and doldrums
HUDSON BAY LOW: An area of low pressure over or near the Hudson Bay area of Canada that often introduces cold air to the north central and northeast United States.
HUMBOLDT CURRENT: Also known as the Peru Current, this ocean current flows northward along the western side of South America, offshore Chile and Peru. There is considerable upwelling of the colder subsurface waters due to the prevailing southerly winds. Dominant weather in this area includes coastal fog and low clouds. The presence or lack of this current is a vital part of the meteorological-oceanographic pattern known as El NiÃ±o.
HUMIDITY: The amount of water vapor in the air. It is often confused with relative humidity or dew point.; Related terms: absolute humidity, relative humidity, and specific humidity
HURRICANE: The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean.; Related term: Dave's Dictionary
HURRICANE WARNING: A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are expected in a coastal area or group of islands within a 24 hour period. A warning is used to inform the public and marine interests of the storm's location, intensity, and movement.
HURRICANE WATCH: A formal advisory issued by forecasters at the National Hurricane Center when they have determined that hurricane conditions are a potential threat to a coastal area or group of islands within a 24 to 36 hour period. A watch is used to inform the public and marine interests of the storm's location, intensity, and movement.
HYDROMETEOR: Any any form of atmospheric water vapor, including those blown by the wind off the earth's surface. Liquid or solid water formation that is suspended in the air includes clouds, fog, ice fog, and mist.
Drizzle and rain are example of liquid precipitation, while freezing drizzle and freezing rain are examples of freezing precipitation.
Solid or frozen precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and ice crystals.
Water vapor that evaporates before reaching the ground is virga.
Examples of liquid or solid water particles that are lifted off the earth's surface by the wind includes drifting and blowing snow and blowing spray.
Dew, frost, rime, and glaze are examples of liquid or solid water deposits on exposed objects.
HYDROLOGIC CYCLE: Often called the water cycle, it is the vertical and horizontal transport of water in all its states between the earth, the atmosphere, and the seas.
HYDROLOGY: The study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water in streams, lakes, and on or below the land surface.
HYDROSPHERE: Considered as the water portion of the earth's surface. Part of the geosphere.
HYGROGRAPH: An instrument that records the hygrometer's measure of water vapor.
HYGROMETER: An instrument that measures the water vapor content of the atmosphere.; Related term: psychrometer
HYPOTHERMIA: This situation occurs when the core temperature of one's body falls below normal. It is the failure of the body to maintain adequate production of heat under conditions of extreme cold.