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HALO.—Any one of a large class of at-mospheric optical phenomena (luminous meteors) that appear as colored or whitish rings and arcs about the Sun or Moon when seen through an ice crystal cloud or in a sky filled with falling ice crystals. The halos experiencing prismatic colora-tion are produced by refraction of light by the ice crystals, and those exhibiting only whitish luminosity are produced by reflection from the crystal faces.

HEAT BALANCE.—The equilibrium, which exists on the average, between the radiation received by Earth and its atmosphere and that emitted by Earth and its atmosphere.

HEATING DEGREE-DAY.—A form of degree day used as an indication of fuel consump-tion; in United States usage, one heating degree-day is given for each degree that the daily mean temperature departs below the base of 65°F.

HEAT TRANSFER.—The transfer or ex-change of heat by radiation, conduction, or con-vection in a fluid and/or between the fluid and its surroundings. The three processes occur simultaneously in the atmosphere, and it is often difficult to assess the contributions of their various effects.

HIGH.—An "area of high pressure," refer-ring to a maximum of atmospheric pressure in two dimensions (closed isobars) on the synoptic sur-face chart, or a maximum of height (closed con-tours) on the constant-pressure chart. Highs are associated with anticyclonic circulations, and the term is used interchangeably with anticyclone.

HIGH ZONAL INDEX.—A relatively high value of the zonal index which, in middle latitudes, indicates a relatively strong westerly component of wind flow and the characteristic weather features attending such motion. A synoptic circulation pattern of this type is com-monly called a "high-index situation."

HORSE LATITUDES.—The belts of latitude over the oceans at approximately 30 to 35 degrees north and south where winds are predominantly calm or very light and the weather is hot and dry.

HURRICANE.—A severe tropical cyclone in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific, off the west coast of Mexico.

ICELANDIC LOW.—The low-pressure center located near Iceland (mainly between Iceland and southern Greenland) on mean charts of sea-level pressure. It is a principal center of ac-tion in the atmospheric circulation of the Northern Hemisphere.

INACTIVE FRONT.—(or passive front) A front or portion thereof that produces very little cloudiness and no precipitation, as opposed to an active front. 

INFERIOR MIRAGE.—A spurious image of an object formed below the true position of that object by abnormal refractive conditions along the line of sight; one of the most common of all types of mirage, and the opposite of a superior mirage.

INFRARED RADIATION.—Electromagne-tic radiation lying in the wavelength interval from about 0.8 micron to an indefinite upper boundary, sometimes arbitrarily set at 1,000 microns. On the lower side of the electromagnetic spectrum, it is bounded by visible radiation, while on the upper side it is bounded by microwave radiation.

INSOLATION.—(contracted from incoming solar radiation) at Earth’s surface.

INSTABILITY.—A property of the steady state of a system such that certain disturbances or perturbations introduced into the steady state will increase in magnitude, the maximum pertur-bation amplitude always remaining larger than the initial amplitude.

INSTABILITY LINE.—Any non-frontal line or band of convective activity in the atmosphere.

INTERTROPICAL CONVERGENCE ZONE.—The axis or a portion thereof, of the broad trade-wind current of the tropics. This axis is the dividing line between the southeast trades and the northeast trades (of the Southern and Northern hemispheres, respectively).

INTERTROPICAL FRONT.—A front presumed to exist within the equatorial trough separating the air of the Northern and Southern hemispheres. However, this front cannot be ex-plained in the same terms as the fronts of higher latitudes.

INVERSION.—The departure from the usual decrease or increase with altitude of the value of an atmospheric property. The layer through which this departure occurs is known as the inversion layer, and the lowest altitude at which the depar-ture is found is known as the base of the inver-sion. The term is almost always used in reference to temperature, but may be applied to moisture and precipitation.

ISALLOBAR.—A line of equal change in at-mospheric pressure during a specified time inter-val; an isopleth of equal pressure tendency. Positive and negative isallobars are sometimes referred to as anallobars and katallobars, respectively.

ISOBAR.—A line of equal or constant pressure; an isopleth of pressure.

ISOBARIC.—Of equal or constant pressure, with respect to either space or time.

ISODROSOTHERM.—A line of equal dew point temperatures.

ISOHEIGHT.—See CONTOUR.

ISOHYET.—A line drawn through geographical points recording equal amounts of precipitation during a given period or for a par-ticular storm. A line of equal precipitation. 

ISOPLETH.—A line of equal or constant value of a given quantity, with respect to either space or time. 

ISOPYCNIC LEVEL. —Specifically, a level surface in the atmosphere, at about an 8-km altitude, where the air density is approximately constant in space and time.

ISOTACH.—A line in a given surface connect-ing points of equal wind speed.

ISOTHERM.—A line of equal or constant temperature.

ISOTHERMAL.—Of equal or constant temperature, with respect to either space or time.

JET.—A common contraction for jet stream.

JET STREAM.—Relatively strong winds con-centrated within a narrow quasi-horizontal stream in the atmosphere. These winds are usually embedded in the midlatitude westerlies and con-centrated in the high troposphere.

KATABATIC WIND.—Any wind blowing down an incline; the opposite of anabatic wind. If the wind is warm, it is called a foehn; if cold, it may be a fall or gravity wind.

KINETIC ENERGY.—The energy that a body possesses as a consequence of its motion, defined as the product of one-half of its mass and the square of its speed, 1/2mv squared.

LAND BREEZE.—A coastal breeze blowing from land to sea, caused by the temperature dif-ference when the sea surface is warmer than the adjacent land. 

LAPSE RATE.—The decrease of an at-mospheric variable with height, the variable be-ing temperature unless otherwise specified.

LATERAL MIRAGE.—A very rare type of mirage in which the apparent position of an ob-ject appears displaced to one side of its true position.

LIGHT.—Visible radiation (about 0.4 to 0.7 micron in wavelength) considered in terms of its luminous efficiency.

LONG WAVE.—A wave in the major belt of westerlies that is characterized by large length and significant amplitude. The wavelength is typically longer than that of the rapidly moving individual cyclonic and anticyclonic disturbances of the lower troposphere. (Compare SHORT WAVE.)

LOOMING.—A mirage effect produced by greater-than-normal refraction in the lower atmosphere, thus permitting objects to be seen that are usually below the horizon.

LOW.—An "area of low pressure," refer-ring to a minimum of atmospheric pressure in two dimensions (closed isobars) on a constant-height chart or a minimum of height (closed contours) on a constant-pressure chart. Lows are associated with cyclonic circulations, and the term is used interchangeably with cyclone.

LOWER ATMOSPHERE.—Generally and quite loosely, that part of the atmosphere in which most weather phenomena occur (i.e., the troposphere and lower stratosphere).

LOW ZONAL INDEX.—A relatively low value of the zonal index, which in middle latitudes indicates a relatively weak westerly component of wind flow (usually implying stronger north-south motion), and the characteristic weather attending such motion. A circulation pattern of this type is commonly called a "low-index situation."

MACROCLIMATE.—The general large-scale climate of a large area or country, as distinguished from the mesoclimate and microclimate.

MAGNETIC NORTH.—At any point on Earth’s surface, the horizontal direction of the Earth’s magnetic lines of force (direction of a magnetic meridian) toward the north magnetic pole, i.e., a direction indicated by the needle of a magnetic compass. Because of the wide use of the magnetic compass, magnetic north, rather than TRUE NORTH, is the common 0° (or 360°) reference in much of navigational practice, in-cluding the designation of airport runway alignment.

MANDATORY LEVEL.—One of several constant-pressure levels in the atmosphere for which a complete evaluation of data derived from upper-air observations is required. Currently, the mandatory pressure values are 1,000 mb, 850 mb, 700 mb, 500 mb, 400 mb, 300 mb, 200 mb, 150 mb, 100 mb, and 50 mb. The radiosonde code has specific blocks reserved for these data.

MARITIME AIR.—A type of air whose characteristics are developed over an extensive water surface and which, therefore, has the basic maritime quality of high moisture content in at least its lower levels.

MEAN SEA LEVEL.—The average height of the sea surface, based upon hourly observation of tide height on the open coast or in adjacent waters which have free access to the sea. In the United States, mean sea level is defined as the average height of the surface of the sea for all stages of the tide over a 19-year period.

MERIDIONAL FLOW.—A type of atmos-pheric circulation pattern in which the meridional (north and south) component of motion is unusually pronounced. The accompanying zonal component is usually weaker than normal.

MESOCLIMATE.—The climate of small areas of Earth’s surface that may not be represent-ative of the general climate of the district. The places considered in mesoclimatology include small valleys, "frost hollows," forest clearings, and open spaces in towns, all of which may have extremes of temperature differing by many degrees from those of adjacent areas. The mesoclimate is intermediate in scale between the microclimate and microclimate.

MESOPAUSE.—The top of the mesosphere. This corresponds to the level of minimum temperature at 70 to 80 km.

MESOSPHERE.—The atmospheric shell between about 20 km and about 70 or 80 km, ex-tending from the top of the stratosphere to the upper temperature minimum (the menopause). It is characterized by a broad temperature maximum at about 50 km, except possibly over the winter polar regions.

METEOROLOGY.—The study dealing with the phenomena of the atmosphere. This includes not only the physics, chemistry, and dynamics of the atmosphere, but is extended to include many of the direct effects of the atmosphere upon Earth’s surface, the oceans, and life in general.

MICROCLIMATE.—The fine climate struc-ture of the air space that extends from the very surface of Earth to a height where the effects of the immediate character of the underlying surface no longer can be distinguished from the general local climate (mesoclimate or microclimate).

MIGRATORY.—Moving; commonly applied to pressure systems embedded in the westerlies and, therefore, moving in a general west-to-east direction.

MILLIBAR.—(abbreviated mb) A pressure unit of 1,000 dynes per centimeter, convenient for reporting atmospheric pressures.

MIRAGE.—A refraction phenomenon wherein an image of some object is made to ap-pear displaced from its true position.

MOIST AIR.—In atmospheric ther-modynamics, air that is a mixture of dry air and any amount of water vapor. Generally, air with a high relative humidity.

MOIST TONGUE.—An extension or protru-sion of moist air into a region of lower moisture content. Cloudiness and precipitation are closely related to moist tongues.

MOISTURE.—A general term usually refer-ring to the water vapor content of the atmosphere or to the total water substance (gas, liquid, and solid) present in a given volume of air.

MONSOON.—A name for seasonal wind. It was first applied to the winds over the Arabian Sea, which blow for 6 months from the northeast and 6 months from the southwest, but it has been extended to similar winds in other parts of the world.

MONSOON CLIMATE.—The type of climate that is found in regions subject to mon-soons. It is best developed on the fringes of the tropics.

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