An inert gas. A colorless, monatomic element which is found to occur in dry air to the extent of only 0.000524 percent by volume. Helium is very light, having a molecular weight of only 4.003 and specific gravity referred to air of 0.138. Because helium i ...

The time required for an instrument to registe 63.2% of a step change in the variable being measured.

Abbreviation for Instrument Flight Rules, but commonly used to refer to the weather and/or flight conditions to which these rules apply, i.e. low visibility.

A type of directional antenna used on some types of radar and radio equipment consisting of an array of elemental, single- wire dipole antennas and reflectors.

A mercury-in-glass thermometer which records the temperature upon being inverted and retains its reading until being returned to the first position.

The older name for the Celsius temperature scale. Officially abandoned by international agreement in 1948, but still in common use.

A reversing thermometer (for seawater temperature) which is not protected against hydrostatic pressure. The mercury bulb is therefore squeezed, and the amount of mercury broken off on reversal is a function of both temperature and of hydrostatic pressure.

The range of operating conditions within which a device is designed to operate and for which operating influences are stated. See operating conditions, reference operating conditions.

Same as humidity strip.

A transducer for converting thermal energy directly into electrical energy. It is composed of pairs of thermocouples which are connected either in series or in parallel. Thermopiles are used in thermoelectric radiation instruments when the output of a sin ...

In folklore, a name for rain.

Conditions to which a device is subjected, not including the variable measured by the device. See normal operating conditions, reference operating conditions.

A wind (or component thereof) directed down the slope of an incline and caused by greater air density near the slope than at the same levels some distance horizontally from the slope. Also called drainage wind and sometimes called katabatic wind.

Difference between the temperatures of the dry-bulb and the wet-bulb thermometers of a psychrometer.

An instrument developed by K. Angstrom for measuring the effective terrestrial radiation. It consists of four manganin strips, of which two are blackened and two are polished. The blackened strips are allowed to radiate to the atmosphere while the polishe ...

A thermopile used in some types of radiation instruments. See solarimeter.

Radio Direction Finder.

A rainbow seen in the spray of the ocean. It is optically the same phenomenon as the ordinary rainbow.

Temperature of equilibrium between the liquid and vapor phases of a substance at a given pressure.

Same as freezing point.