National Weather Service United States Department of Commerce

Weather Glossary: H's

Hail size comparisons
Object Size
Pea 0.25"
(6 mm)
Marble 0.50"
(13 mm)
Penny 0.75"
(19 mm)
Nickel 0.88"
(22 mm)
Quarter 1.00"
(25 mm)
Half Dollar 1.25"
(32 mm)
Ping Pong Ball 1.50"
(38 mm)
Golf ball 1.75"
(44 mm)
Hen Egg 2.00"
(51 mm)
Tennis Ball 2.50"
(64 mm)
Baseball 2.75"
(70 mm)
Tea Cup 3.00"
(76 mm)
Grapefruit 4.00"
(102 mm)
Softball 4.50"
(114 mm)

Precipitation in the form of balls or lumps usually consisting of concentric layers of ice. A thunderstorm is classified as severe when it produces hail 1" or larger in diameter.

Hail Spike

When looking at a WSR-88D Cross-Section, one will occasionally see a distinctive spike above the actual top of the thunderstorm. This is due to the high reflective properties of hail. When the side lobe energy hits the hail, it reflects enough energy back to be detected by the radar receiver even when the radar beam half-power width has exceeded the top of the storm.

The radar receiver "thinks" that the detected energy is coming from the center of the radar beam as a result it extends the actual top of the storm in the form of a spike. This "hail spike" has been associated with many hail producing thunderstorms.

Haines Index

This is also called the Lower Atmosphere Stability Index. It is computed from the morning (12Z) soundings from RAOB stations across North America. The index is composed of a stability term and a moisture term. The stability term is derived from the temperature difference at two atmosphere levels.

The moisture term is derived from the dew point depression at a single atmosphere level. This index has been shown to be correlated with large fire growth on initiating and existing fires where surface winds do not dominate fire behavior. The Haines Indexes range from 2 to 6 for indicating potential for large fire growth.

Potential for large fire growth
What Does it Mean?
6 High Potential (Dry Unstable Lower Atmosphere)
5 Moderate Potential
4 Low Potential
3 Very Low Potential
2 Very Low Potential (Moist Stable Lower Atmosphere)
Rings or arcs that encircle the sun or moon. These are caused by refraction of light through ice crystals that make up cirrus clouds.
Hanging (Ice) Dam
A mass of ice composed mainly of frazil or broken ice deposited underneath an ice cover in a region of low flow velocity.
Haze (HZ)
A concentration of salt particles or other dry particles not readily classified as dust or other phenomenon. Occurs in stable air usually only a few thousand feet thick, but may extend as high as 15,000 feet (4,500 meters). Haze layers often have definite tops above which the visibilities are good. However, the visibility in the haze layer can be very poor.
The difference between the pool height and tailwater height. Usually expressed in feet of head, or in lbs./sq. inch.
Head Loss
The decrease in total head caused by friction.
Head Race
A channel which directs water to a water wheel; a forebay.
Headward Erosion
Erosion which occurs in the upstream end of the valley of a stream, causing it to lengthen its course in such a direction.
Streams at the source of a river.
Headwater Advisory Table
A table developed by a River Forecast Center for a Headwater Guidance Point; a pre-computed matrix of values allows a forecaster to ascertain an anticipated crest or rise on a small river or stream for a variety of rainfall events and soil moisture conditions.
Headwater Basin
A basin at the headwaters of a river. All discharge of the river at this point is developed within the basin.
Heat Advisory
This product is issued by the National Weather Service when excessive heat may pose a hazard or is life threatening if action is not taken. The criteria for this advisory vary from state to state.
Heating Degree Day
see Degree Day.
Heat Index
Heat Index graph
The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature. To find the Heat Index (HI), look at the Heat Index (HI) Chart (right). As an example, if the air temperature is 90°F (found at the left side of the table) and the Relative Humidity (RH) is 70% (found at the top of the table), the Heat Index (HI)--or how hot it actually feels--is 106°F. This is at the intersection of the row 90°F and the 70% column.

This index was devised for shady, light wind conditions. Exposure to full sunshine can increase Heat Index (HI) values by up to 15°F. Also, strong winds, particularly with very hot, dry air, can be extremely dangerous.

Any value Heat Index (HI) greater than 105°F is in the Danger Category. When the Heat Index is between 105-115°F for 3 hours or more, a Heat Advisory will be issued by the local National Weather Service Forecast Office.
Different Heat Index categories and how each affects the body.
Category Classification Heat Index/Apparent Temperature (°F) General Effect in High Risk Groups
1 Extremely Hot 130°F or Higher
(54.4°C or higher)
Heat/Sunstroke HIGHLY LIKELY with continued exposure
2 Very Hot 105°F - 129°F
(40.5°C - 53.8°C)
Sunstroke, heat cramps, or heat exhaustion LIKELY, and heatstroke POSSIBLE with prolonged exposure and/or physical activity
3 Hot 90°F - 104°F
(32.2°C - 40°C)
Sunstroke, heat cramps, or heat exhaustion POSSIBLE with prolonged exposure and/or physical activity
4 Very Warm 80°F - 89°F
(27°C - 31.6°C)
Fatigue POSSIBLE with prolonged exposure and/or physical activity
See Apparent Temperature.
Heat Island
A dome of elevated temperatures over an urban area caused by the heat absorbed by structures and pavement.
Heat Lightning
Lightning that occurs at a distance such that thunder is no longer audible.
Heavy Snow
Depending on the region of the USA, this generally means that 4" (10 cm) or more of snow has accumulated in 12 hours, or 6" (15 cm) or more inches of snow in 24 hours.
Heavy Snow Discussion (HSD)

This message discusses the potential for heavy snowfall in the contiguous United States. The threshold value in this product for heavy snow is 4" (10 cm) or more in a 12-hour period.

However, this product discusses all potential snowstorms, including those not expected to attain the threshold. Although the focus is on the meteorological reasoning for the forecast, the impact of numerical model forecasts and model differences are also explained.

Heavy Surf
Large waves breaking on or near the shore resulting from swells spawned by a distant storm.

A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) 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.

The helicity is the area on a hodograph that is enclosed by a line from the tip of the storm motion vector to the surface wind vector, then following the hodograph curve to 3 km level, then back to storm motion vector.

This value allows the forecaster to determine the rotational tendency of a thunderstorm. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2. It is dependent on the local environmental wind profile in which a thunderstorm develops and the thunderstorm motion.

The fear of the sun.
  1. Hydrologist in Charge of an RFC.
  2. The Hydrometeorological Information Center of the Office of Hydrology (OH).
A region of high pressure, marked as a blue "H" on a weather map. A high is usually associated with fair weather. See anticyclone.
High Clouds
These clouds have bases between 16,500 and 45,000 feet (5,000 to 13,000 meters) in the mid latitudes. At this level they are composed of primarily of ice crystals. Some clouds at this level are cirrus, cirrocumulus, and cirrostratus.
High Risk of Severe Thunderstorms

Severe weather is expected based upon the following criteria.

  • 30% tornado probability AND 10% or greater probability of an EF2+ tornado, or
  • 45% or greater tornado probability WITH OR WITHOUT 10% or greater probability of an EF2+ tornado, or
  • 60% severe wind probability AND a 10% or greater probability of a wind gust 75 mph or greater.

A high risk is rare, and implies an unusually dangerous situation and usually the possibility of a major severe weather outbreak. Also, see marginal risk, slight risk, enhanced risk, moderate risk, and convective outlook.

High Seas

The major oceans of the world including, for National Weather Service purposes, the coastal and offshore areas. Areas of responsibility for the United States are determined by international agreements under the auspices of the World Meteorological Organization (WMO).

The United States is responsible for that portion of the Atlantic and Pacific oceans which extends from 20 to 40 nm off the Western and Eastern US coasts and extends to 35°W in the Atlantic Ocean and to 160°E in the Pacific Ocean. The area includes both the coastal and offshore waters.

High Seas Forecasts (HSF)
This National Weather Service High Seas Forecasts are designed to meet the needs of ships making ocean transits; therefore, the primary focus is on major weather systems and sea states affecting oceangoing vessels.
High Wind Advisory
This product is issued by local National Weather Service Forecast Offices when high wind speeds may pose a hazard. The criteria for this advisory vary from state to state.
High Wind Watch
This product is issued by local National Weather Service Forecast Offices when there is the potential of high wind speeds developing that may pose a hazard or is life threatening.
High Wind Warning
This product is issued by local National Weather Service Forecast Offices when high wind speeds may pose a hazard or is life threatening. The criteria for this warning vary from state to state.
Hinge Crack
Crack caused by significant changes in water level.
  • The Hydrologist on Duty at an RFC.
  • The Hydrologic Operations Division of the Office of Hydrology (OH).

A polar coordinate graph which shows the vertical wind profile of the lowest 7,000 meters of the atmosphere. These plots are used to determine the advection patterns aloft, whether a thunderstorm will rotate, and the type of thunderstorms that you will likely see that day.

On the graph, only the tip of wind vectors are plotted. The tips are denoted by a dot. As the distance between the dot and the center of the graph increases, the magnitude of the wind will also increase. These dots are sequentially connected together by a line beginning with the first wind reported and ending with last wind reported.

The fear of fog.
Hook or Hook Echo
A pendant or hook on the right rear of a radar echo that often identifies mesocyclones on the radar display. The hook is caused by precipitation drawn into a cyclonic spiral by the winds, and the associated notch in the echo is caused by precipitation-free, warm, moist air flowing into the storm. A hook often is associated with a mesocyclone, and indicates favorable conditions for tornado development.
HMT (Hydrometeorological Technicians)
Individuals who, at the technical level, have knowledge in meteorology and hydrology. Among their duties are data collection, quality control, gage network maintenance, as well as the gathering and disseminating of data and products.
HP (High Precipitation) Storm or HP (High Precipitation) Supercell

High-Precipitation storm (or High-Precipitation supercell). A 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. Mobile storm spotters are strongly advised to maintain a safe distance from any storm that has been identified as an HP storm; close observations (e.g., core punching) can be extremely dangerous.

An old acronym for the Hydrometeorological Prediction Center now called the Weather Prediction Center (WPC).
HSA (Hydrologic Service Area)
A geographical area assigned to Weather Service Forecast Office's/Weather Forecast Office's that embraces one or more rivers.
Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.
A hillock of broken ice which has been forced upward by pressure.
Hummocked Ice
Ice piled haphazardly one piece over another to form an uneven surface.

A warm-core tropical cyclone in which the maximum sustained surface wind (using the U.S. 1-minute average) is 64 kt (74 mph or 119 km/h) or more. The term hurricane is used for Northern Hemisphere cyclones east of the International Dateline to the Greenwich Meridian. It has a diameter of 250 to 500 miles (400 to 800 kilometers) and a cyclonic circulation typically extending to near 50,000 feet (15,000 meters).

It is called a Typhoon in the western Pacific north of the Equator and west of the International Dateline and Indian Ocean. See Saffir-Simpson Hurricane Wind Scale.

Hurricane Local Statement (HLS)
A public release prepared by local National Weather Service offices in or near a threatened area giving specific details for its county/parish warning area on (1) weather conditions, (2) evacuation decisions made by local officials, and (3) other precautions necessary to protect life and property.
Hurricane Season
The portion of the year having a relatively high incidence of hurricanes. The hurricane season in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to November 30. The hurricane season in the Eastern Pacific basin runs from May 15 to November 30. The hurricane season in the Central Pacific basin runs from June 1 to November 30.
Hurricane Warning

An announcement that sustained winds of 64 knots (74 mph or 119 km/hr) or higher are expected somewhere within the specified area in association with a tropical, subtropical, or post-tropical cyclone. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the warning is issued 36 hours in advance of the anticipated onset of tropical-storm-force winds.

The warning can remain in effect when dangerously high water or a combination of dangerously high water and waves continue, even though winds may be less than hurricane force.

Hurricane Watch
An announcement that sustained winds of 64 knots (74 mph or 119 km/hr) or higher are possible within the specified area in association with a tropical, subtropical, or post-tropical cyclone. Because hurricane preparedness activities become difficult once winds reach tropical storm force, the hurricane watch is issued 48 hours in advance of the anticipated onset of tropical storm force winds.
Hydraulic Fill Dam
A dam constructed of materials, often dredged, that are conveyed and placed by suspension in flowing water.
Hydraulic Grade Line
A line whose plotted ordinate position represents the sum of pressure head plus elevation head for the various positions along a given fluid flow path, such as along a pipeline or a ground water streamline.
Hydraulic Head
  1. The height of the free surface of a body of water above a given point beneath the surface.
  2. The height of the water level at the headworks, or an upstream point, of a waterway, and the water surface at a given point downstream.
  3. The height of a hydraulic grade line above the center line of a pressure pipe, at a given point.
Hydraulic Mean Depth
The right cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. Also called Hydraulic Radius.
Hydraulic Permeability
The flow of water through a unit cross-sectional area of soil normal to the direction of flow when the hydraulic gradient is unity.
Hydraulic Radius
The right cross-sectional area of a stream of water divided by the length of that part of its periphery in contact with its containing conduit; the ratio of area to wetted perimeter. Also called Hydraulic Mean Depth.
A graph showing the water level (stage), discharge, or other property of a river with respect to time. The common hydrograph in National Weather Service usage depict stage vs. time and flow vs. time.
Hydrographic Survey
An instrumental survey to measure and determine characteristics of streams and other bodies of water within an area, including such things as location, areal extent, and depth of water in lakes or the ocean; the width, depth, and course of streams; position and elevation of high water marks; location and depth of wells, etc.
Hydrograph Separation
The process where the storm hydrograph is separated into baseflow components and surface runoff components.
Hydrologic Budget
An accounting of the inflow to, outflow from, and storage in, a hydrologic unit, such as a drainage basin, aquifer, soil zone, lake, reservoir, or irrigation project.
Hydrologic Cycle
The constant movement of water above, on, and below the Earth's surface. Processes such as precipitation, evaporation, condensation, infiltration, and runoff comprise the cycle. Within the cycle, water changes forms in response to the Earth's climatic conditions.
Hydrologic Equation
The water inventory equation (Inflow = Outflow + Change in Storage) which expresses the basic principle that during a given time interval the total inflow to an area must equal the total outflow plus the net change in storage.
Hydrologic Model
A conceptual or physically-based procedure for numerically simulating a process or processes which occur in a watershed.
Hydrologic Service Area (HSA)
A geographical area assigned to Weather Forecast Office's that embraces one or more rivers.
Hydrologic Services
A general Term referring to the operations, products, verbal communications, and related forms of support provided by the NWS for the Nation's streams, reservoirs, and other areas affected by surface water.
Hydrologic Unit
A geographical area representing part or all of a surface drainage basin or distinct hydrologic feature such as a reservoir, lake, etc.
The applied science concerned with the waters of the earth, their occurrences, distribution, and circulation through the unending hydrologic cycle of: Precipitation, consequent runoff, infiltration, and storage; eventual evaporation; and so forth. It is concerned with the physical and chemical reaction of water with the rest of the earth, and its relation to the life of the earth.
A particle of condensed water (liquid, snow, ice, graupel, hail) in the atmosphere.
Individuals who have the combined knowledge in the fields of both meteorology and hydrology which enables them to study and solve hydrologic problems where meteorology is a factor.
The interdisciplinary science involving the study and analysis of the interrelation between the atmospheric and land phases of water as it moves through the hydrologic cycle.
The region that includes all the earth's liquid water, frozen water, floating ice, frozen upper layer of soil, and the small amounts of water vapor in the earth's atmosphere.
Hydrostatic Head
A measure of pressure at a given point in a liquid in terms of the vertical height of a column of the same liquid which would produce the same pressure.
A graphical representation of rainfall intensity with respect to time.
An instrument which measures the humidity of the air.