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H

HABITAT—The native environment or specific surroundings where a plant or animal naturally grows or lives. The surroundings include physical factors such as temperature, moisture, and light together with biological factors such as the presence of food or predator organisms. The term can be employed to define surroundings on almost any scale from marine habitat, which encompasses the oceans, to microhabitat in a hair follicle of the skin.

HABITAT CONSERVATION PLAN (HCP)—A requirement under the Endangered Species Act (ESA) when economic development may result in harm to Threatened or Endangered Species. The plan does allow for some loss of individual animals or habitat of a species in exchange for a commitment that will insure long-term survival. Its intent is to better balance economic development and conservation.

HABITAT INDICATOR—A physical attribute of the environment measured to characterize conditions necessary to support an organism, population, or community in the absence of pollutants, e.g., salinity of estuarine waters or substrate type in streams or lakes.

HAIL—Precipitation which forms into balls or lumps of ice over 0.2 inch (5.08 mm) in diameter. Hail is formed by alternate freezing and melting as it is carried up and down by turbulent air currents within a cloud.

HAILSTONE—A hard pellet of snow and ice.

HAILSTORM—A storm with Hail.

HAIL SUPPRESSION—Any method of reducing the damaging effects of hailstorms by modifying the characteristics of the hail-producing cloud. The currently prevailing hypothesis is that silver iodide seeding provides more hailstone nuclei (and, at the same time, reduces the amount of supercooled water available to build up large hailstones) with the net effect that the hail that reaches the ground is smaller and less damaging, and also has a high probability of melting before reaching the ground.

HAIR HYGROMETER (HYGROGRAPH)—An instrument for measuring humidity which makes use of the fact that the length of hair varies with relative humidity.

HALCYON DAYS (Water)—With respect to water, generally refers to idyllic by-gone days when supplies of an area's fresh water were relatively abundant with respect to the demands of man.

HALINE—Term used to indicate dominance of ocean salt.

HALINE MARSHES—A saturated, poorly drained area, intermittently or permanently water covered, having aquatic and grasslike vegetation, influenced predominately by ocean salts.

HALO—A circular band of colored light around a light source, as around the sun or moon, caused by the refraction and reflection of light by ice particles suspended in the intervening atmosphere. Also see Rainbow for a similar refraction and reflection principal using water.

HALOCLINE—The boundary between surface fresh water and underlying saltwater in a stratified coastal environment. A location where there is a marked change in salinity.

HALOPHYTES—A group of salt-tolerant plants ranging from cacti to sea grass that can absorb salt and heavy metals such as cadmium and arsenic from the wastewater of power plants, particularly coal-fired generating plants which is typically laden with heavy-metal byproducts of coal combustion.

HAMMOCK, also Hummock—(1) In the southern United States, especially Florida, an area characterized by hardwood vegetation, the soil being of a greater depth and containing more humus than that of the flatwoods or pinelands, hence being more suitable for cultivation. Particularly, a tract of forested land that rises above an adjacent marsh. (2) A ridge or hill of ice in an ice field.

HANGING VALLEYS—Hanging valleys can be created when smaller tributary glaciers join the main ice sheet. Since the main glacier is larger and heavier than the tributary one(s), the main glacier will erode more deeply into its valley than will the tributary into its own valley. After the ice melts, the tributary valley will be left hanging part of the way up the wall of the larger canyon that it intersects. Many waterfalls in the high Sierras, including well-known ones at Yosemite National Park, occur at the juncture of a hanging valley with a larger canyon.

HARBOR—A sheltered anchorage for ships and boats. Also see Port.

HARDNESS—A property of water which causes an increase in the amount of soap that is needed to produce foam or lather and that also produces scale in hot water pipes, heaters, boilers and other units in which the temperature of water is increased materially. Hardness is produced almost completely by the presence of calcium and magnesium salts in solution. The following scale may assist in appraising water hardness, measured by weight of dissolved salts (in milligrams) per unit (in liters) of water:

[1] Soft—0-60 milligrams/liter (mg/l);
[2] Moderately Hard—61-120 mg/l;
[3] Hard—121-180 mg/l; and
[4] Very Hard—over 180 mg/l.

HARDPAN—A hard impervious layer composed chiefly of clay or organic materials cemented by relatively insoluble materials, which does not become plastic when wet, and definitely limits the downward movement of water and roots.

HARD WATER—Water which forms a precipitate with soap due to the presence of calcium, magnesium, or ferrous ions in solution.

HARVESTED RAINWATER—The rain that falls on a roof or yard and is channeled by gutters or channels to a storage tank. The first wash of water on a roof is usually discarded and the subsequent rainfall is captured for use if the system is being used for potable water.

HAYSTACK—A vertical standing wave in turbulent river waters.

HAZARD RANKING SYSTEM (HRS)—A method for ranking hazardous waste disposal sites for possible placement on the National Priorities List (Superfund List), as provided for by the Comprehensive, Environmental Response, Compensation, and Liability Act (CERCLA). The ranking uses information gathered by the preliminary assessment and site inspection and the listing site inspection. The need for remedial action is scored on the basis of potential harm to human health resulting from: (1) releases into groundwater, surface water, or the atmosphere; (2) fire and explosion; and/or (3) direct contact with hazardous materials. The HRS evaluation assigns an overall numerical value to each site, which determines its priority for cleanup. Also see Hazardous Substance and Hazardous Substances Superfund.

HAZARDOUS MATERIAL (EPA)—An substance, pollutant or contaminant listed as hazardous under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) of 1980, as amended, and the regulations promulgated pursuant to that act.

HAZARDOUS SUBSTANCE—(1) Any material that poses a threat to human health and/or the environment. Typical hazardous substances are toxic, corrosive, ignitable, explosive, or chemically reactive. (2) Any substance designated by the U.S. Environmental Protection Agency (EPA) to be reported if a designated quantity of the substance is spilled in the waters of the United States or if otherwise released into the environment. Also referred to as Hazardous Waste.

HAZARDOUS SUBSTANCES SUPERFUND—A federal trust fund for use in the cleanup of spills or sites containing hazardous waste that pose a significant threat to the public health or the environment. The fund, originally called the Ha