In aviation terminology, route or terminal weather conditions of sufficiently low visibility to require the operation of aircraft under instrument flight rules.

Wind with a speed between 4 and 27 knots (4 and 31 mph); Beaufort scale numbers 2 through 6.

The temperature at which an object gives out as much radiation as it receives from its surroundings.

The difference between the solar radiation directed downward and upward; net flux of solar radiation.

A self-recording instrument used on aircraft for the simultaneous recording of atmospheric pressure, temperature, and humidity.

The range of operating conditions of a device within which operating influences are negligible. The range is usually narrow. Reference operating conditions are the conditions under which reference performance is stated and the base from which the values o ...

A system of estimating and reporting wind speed, originally based on the effect of various wind speeds on the amount of canvas that a full-rigged nineteenth century frigate could carry.

A hygrometer which includes an arrangement for the time recording of atmospheric humidity.

A device for measuring the river stage. Also called stream gauge.

The study of waters (including oceans, lakes, and rivers) embracing either: (a) their physical characteristics, from the standpoint of the oceanographer or limnologist; or (b) the elements affecting safe navigation, from the point of view of the mariner. ...

An instrument used to measure the water vapor content of the air.

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.

In general, solar radiation received at the earth's surface. Contracted from incoming solar radiation.

Operation mode of a communication circuit in which each end can transmit and receive, but not simultaneously.

For a given locality and month, an empirical expression devised for the purpose of classifying climates numerically on the basis of precipitation and evaporation.

A method of winds aloft observation in which the elevation and azimuth angles of a theodolite are read while visually tracking a pilot balloon. Balloon height data is estimated from assumed balloon ascension rates.

A momentary decrease in the speed of the wind.

Pyrheliometer based on the comparison of the heating of two identical metal strips, one exposed to radiation, the other to a joule effect.

An instrument for taking photographs of an image of the sun in monochromatic light.

The temperature at which a solid substance undergoes fusion, i.e. melts, changes from solid to liquid form. All substances have their characteristic melting points. For very pure substances the temperature range over which the process of fusion occurs is ...