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University of Nebraska–Lincoln

Remote Sensing Glossary

Reference Information for Virtual Nebraska

Terms, Definitions and Concepts

H

habitat

The area or region where a particular type of plant or animal lives and grows.

hail

Precipitation composed of balls or irregular lumps of ice. Hail is produced when large frozen raindrops, or almost any particles, in cumulonimbus clouds act as embryos that grow by accumulating supercooled liquid droplets. Violent updrafts in the cloud carry the particles in freezing air, allowing the frozen core to accumulate more ice. When the piece of hail becomes too heavy to be carried by upsurging air currents it falls to the ground.

hardware

The electrical and mechanical components of a system, as opposed to software.

haze

Fine dry or wet particles of dust, salt, or other impurities that can concentrate in a layer next to the Earth when air is stable.

heat balance

The equilibrium existing between the radiation received and emitted by a planetary system.

Heat Capacity Mapping Mission (HCMM)

A two-channel radiometer launched by NASA to measure the thermal properties of the terrestrial surface. It had an application to identify and locate rocks and minerals. One radiometer channel was in the visible to near infrared (0.5-1.1 micrometers), and the other in the thermal infrared (10.5-12.5 micrometers). The instantaneous field of view (IFOV) was about 600 meters.

hemisphere

Half of the Earth, usually conceived as resulting from the division of the globe into two equal parts, north and south or east and west.

high

A digital logic state corresponding to a binary "1". See low.

High Resolution Doppler Imager (HRDI)

Carried on UARS, it measures stratospheric winds.

High-Resolution Infrared Radiation Sounder (HIRS)

Instrument carried by NOAA polar-orbiting satellites that detects and measures energy emitted by the atmosphere to construct a vertical temperature profile from the Earth's surface to an altitude of about 40 km. Measurements are made in 20 spectral regions in the infrared band.

High-Resolution Picture Transmission (HRPT)

Real-time, 1.1-kilometer resolution, digital images provided by NOAA's polar-orbiting environmental satellites, containing all five spectral channels and telemetry data transmitted as high-speed digital transmissions. The Advanced Very High Resolution Radiometer (AVHRR) provides the primary imaging system for APT and HRPT. See TIROS

horse latitudes

The subtropical latitudes (30-35 degrees), where winds are light and weather is hot and dry. According to legend, ships traveling to the New World often stagnated in this region and had to throw dead horses overboard or eat them to survive, hence the name horse latitudes. See wind.

HRDI

See High Resolution Doppler Imager.

HRPT

See High Resolution Picture Transmission.

humidity

The amount of water vapor in the air. The higher the temperature, the greater the number of water molecules the air can hold. For example: at 60 degrees F (15 degrees C), a cube of air one yard on each side can hold up to 4.48 ounces of water. At 104 degrees F (40 degrees C), the same cube of air can hold up to 17.9 ounces of water.

Relative humidity describes the amount of water in the air compared with how much the air can hold at the current temperature. Example: 50% relative humidity means the air holds half the water vapor that it is capable of holding; 100% relative humidity means the air holds all the water vapor it can. At 100% humidity, no more evaporation can occur until the temperature rises, or until the water vapor leaves the air through condensation. Absolute humidity is the ratio of the mass of water vapor present in a system of moist air to the volume occupied by the mixture, that is, the density of water vapor.

hurricanes

Severe tropical storms whose winds exceed 74 mph. Hurricanes originate over the tropical and subtropical North Atlantic and North Pacific oceans, where there is high humidity and light wind. These conditions prevail mostly in the summer and early fall. Since hurricanes can take days or even weeks to form, time is usually available for preventive or protective measures.

From space, hurricanes look like giant pinwheels, their winds circulating around an eye that is between 5 and 25 miles in diameter. The eye remains calm with light winds and often a clear sky.

Hurricanes may move as fast as 50 mph, and can become incredibly destructive when they hit land. Although hurricanes lose power rapidly as soon as they leave the ocean, they can cause high waves and tides up to 25 feet above normal. Waves and heavy flooding cause the most deaths during a hurricane. The strongest hurricanes can cause tornadoes.

hydrochlorofluorocarbon (HCFC)

One of a class of compounds used primarily as a CFC substitute. Work on CFC alternatives began in the late 1970s after the first warnings of CFC damage to stratospheric ozone. By adding hydrogen to the chemical formulation, chemists made CFCs less stable in the lower atmosphere enabling them to break down before reaching the ozone layer. However, HCFCs do release chlorine and have contributed more to atmospheric chlorine buildup than originally predicted. Development of non-chlorine based chemical compounds as a substitute for CFCs and HCFCs continues.

hydrologic cycle

The pathways through which water is cycled in the terrestrial biosphere.

hydrosphere

The totality of water encompassing the Earth, comprising all the bodies of water, ice, and water vapor in the atmosphere.

hygrometer

Instrument that measures water vapor content in the air and communicates changes in humidity visibly and immediately through a graph or a dial. There are three types of hygrometers:

  • The hair hygrometer uses a human hair as the sensing instrument. The hair lengthens when the air is moist and contracts when the air is dry, but remains unaffected by air temperature. However, the hair hygrometer cannot respond to rapid fluctuations in humidity.
  • An electric hygrometer uses a plate coated with carbon. Electrical resistance of the carbon coating changes as the moisture content of the air changes--changes that translate into relative humidity. This type of hygrometer is used frequently in the radiosonde.
  • An infrared hygrometer uses a beam of light containing two separate wave lengths to gauge atmospheric humidity. One of the wavelengths is absorbed by water vapor, the other is unaffected, providing an extremely accurate index of water vapor for paths of a few inches or thousands of feet. See psychrometer.