H

Hair hygrometer- A hygrometer that measures relative humidity by means of the variation in length of a strand of human hair. The length variation of a properly treated hair is 2%–2.5% when the humidity changes from 0%–100%. The hair hygrometer is considered to be a satisfactory instrument in situations where extreme and very low humidities are seldom or never found. The rate of response is very dependent on air temperature; the lag time increases with decreasing temperature. For air temperatures between 0° and 30°C and relative humidities between 20% and 80%, a good hair hygrometer should indicate 90% of a sudden change in humidity within about three minutes.

Hazemeter- Name sometimes applied to a transmissometer; an instrument for measuring the extinction coefficient of the atmosphere and for the determination of visual range.

Heat capacity mapping mission satellite- An experimental NASA satellite (launched April 1978 as an Applications Explorer Mission) that carried a two-channel imaging radiometer that measured reflected solar radiation and thermal radiation emitted by the earth's surface.

Height finding radar- A radar designed for accurate determination of target altitude (the beamwidth of such radars is generally much narrower in the vertical than in the horizontal).

Heliograph- An instrument that records the duration of sunshine and gives a qualitative measure of the amount of sunshine by the action of the sun's rays upon blueprint paper; a type of sunshine recorder.

Heliostat- A clock-driven instrument mount that automatically and continuously points in the direction of the sun.

High frequency radar- Radar operating at wavelengths of 10–100 m with the received signal being scattered by ocean waves of half the radar wavelength. The received signal is Doppler shifted by an amount corresponding to the phase speed of this ocean wave component. The surface current in the radial direction can be computed from the difference between the observed Doppler shift and that, which is expected from the surface-wave dispersion relation.

High resolution infrared radiation sounder- (HIRS) One of three sounders that comprise the TIROS Operational Vertical Sounder package on NOAA polar orbiter satellites; it measures incident radiation in 1 visible and 19 infrared spectral regions, with a nadir resolution of 17 km.

Holosteric barometer- Same as an aneroid barometer; an instrument for measuring atmospheric pressure.

Horn antenna- A type of radio antenna often used because of its directional properties and simple structure; it consists essentially of flared extensions of a metallic waveguide.

Hot film anemometer- Similar to a hot-wire anemometer, with the exception that the sensing element is a thin film of conductive material on the surface of a nonconductive rod; an anemometer that utilizes the principle that the convection of heat from a body is a function of its ventilation. In its usual form it consists of a thin platinum wire heated to approximately 1000°C so that its temperature is relatively independent of ambient temperature variation. Wind speed is determined by measuring either the current required to maintain the hot wire at a constant temperature or the resistance variation of the hot wire while a constant current through the wire is maintained.

Hot wire anemometer- An anemometer that utilizes the principle that the convection of heat from a body is a function of its ventilation. In its usual form it consists of a thin platinum wire heated to approximately 1000°C so that its temperature is relatively independent of ambient temperature variation. Wind speed is determined by measuring either the current required to maintain the hot wire at a constant temperature or the resistance variation of the hot wire while a constant current through the wire is maintained.

Humidity strip- A flat plastic strip bounded by electrodes on two sides and coated with a hygroscopic chemical compound such as lithium chloride. The electrical resistance of this coating is a function of the amount of moisture absorbed from the atmosphere and the temperature of the strip (also called an electrolytic strip).

Hurricane beacon- An air-launched balloon designed to be released in the eye of a tropical cyclone, float within the eye at predetermined levels, and transmit radio signals for positioning.

Hydrometer- An instrument used for measuring the specific gravity of a liquid.

Hydrophotometer- An instrument for measuring the extinction coefficient in water.

Hygristor- An electric humidity sensor element often used in radiosonde equipment that relies on changes in the resistance of a humidity-sensitive component.

Hygrograph- An instrument that records the water vapor content of the atmosphere.

Hygrometer- Any instrument that measures the water vapor content of the atmosphere.

Hygroscope- An instrument showing qualitatively whether the air is dry or damp, usually by the change in appearance or dimensions of some substance.

Hygrothermograph- A recording instrument combining, on one record, the variation of atmospheric temperature and humidity content as a function of time. The most common hygrothermograph is a hair hygrograph combined with a thermograph.

Hygrothermometer- An instrument that measures both atmospheric water vapor content and temperature.