_Haze._--A lack of transparency in the atmosphere; sometimes due to irregularities in the density of the air (_optical haze_), sometimes to dust (_dust haze_, which when dense constitutes _dry fog_), sometimes to fine particles of water or ice (grading into true _fog_). _Helm and Bar._--A pair of clouds seen when the “helm wind” is blowing over Crossfell, an English mountain; the “helm” capping the mountain and the “bar” lying to leeward of it. _High._--An area of high barometric pressure; an anticyclone. _Hoarfrost._--(See _frost_.) _Hole in the Air._--A colloquial name for any condition in the atmosphere that suddenly decreases the lift of an aeroplane. (Cf. _bump_.) _Horse Latitudes._--The regions of calms and variable winds coinciding with the subtropical high-pressure belts lying on the poleward sides of the trade winds. (The term has generally been applied only to the northern of these two regions, in the North Atlantic Ocean, or to the portion of it near Bermuda.) _Hot Wave._--A period of abnormally high temperatures. It has sometimes been defined, in the United States, as a period of three or more consecutive days during each of which the maximum temperature is 90 degrees F. or over. _Hot Wind._--A hot, parching wind characteristic of certain continental interiors; especially Australia, northern India and the prairie region of the United States. _Humidity._--The degree to which the air is charged with water vapor; viz., the actual amount of water vapor present (_absolute humidity_, which may be expressed in terms of weight per unit volume or as vapor pressure), or the ratio which this amount bears to the maximum amount the air can contain at the prevailing temperature (_relative humidity_, expressed in percentage). _Hurricane._--A tropical cyclone; especially one of the West Indies region. (A cyclone originating in this region and passing northward into the temperate zone is still called a “West India hurricane,” even after it has assumed the character of an extratropical cyclone, and, if sufficiently severe, justifies the display of “hurricane warnings” at ports of the United States. “Hurricane” is also the designation of the highest wind force on the Beaufort scale, and is thus applied to any wind exceeding about seventy-five miles an hour.) _Hygrograph._--A self-registering hygrometer. _Hygrometer._--Any instrument for measuring the humidity of the air. _Hygroscope._--A device that gives a rough indication of the relative humidity of the air. Most hygroscopes are mere toys. _Ice Rain._--1. A rain that causes a deposit of glaze. 2. Falling pellets of clear ice (called _sleet_ by the U. S. Weather Bureau).