Any horizontal wind velocity tangent to the contour line of a constant pressure surface (or to the isobar of a geopotential surface) at the point in question.

An atmometer consisting of a porous porcelain or ceramic container connected to a calibrated reservoir filled with distilled water. Evaporation is determined by the depletion of water in the reservoir.

See radiation shield.

A faintly-colored circular arc similar to a rainbow but formed on fog layers containing drops whose diameters are 100 microns or less. Also called mistbow, white rainbow.

For a given location, a measure of the long-range effectiveness of precipitation in promoting plant growth. Also called precipitation-evaporation index.

An instrument used to measure the water vapor content of the air. A type of hygrometer. It consists of a wet-bulb and a dry-bulb thermometer. See aspiration psychrometer, Assmann psychrometer. hygrodeik, sling psychrometer.

Clouds which cover between 6/10 and 9/10 of the sky. See Sky Terminology.

An instrument which measures the transmissivity of the atmosphere between two points for the determination of visual range.

A polarimeter utilizing a Wollaston prism as a polarizer and a Nicol prism as an analyzer.

A thermoelectric thermometer used for measuring air temperature. The name is derived from the fact that the reference thermocouple is placed in an insulated bottle.

February 2nd. In American folklore, a day that is popularly supposed to provide the key to the weather for the remainder of the winter. Specifically, if the ground-hog upon emerging from its hole casts a shadow, it will return underground, thereby forebod ...

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.

The maximum deviation of any points from a straight line drawn as a "best fit" through the calibration points of an instrument with a linear response curve. Usually expressed as a percentage of full- scale range.

A graphical aid used in fire weather forecasting to calculate the degree of forest-fire danger (or burning index). Commonly in the form of a circular slide rule, the firedanger meter relates numerical indices of (a) the seasonal stage of foliage, (b) the ...

Graduated fixed stake used in regions of abundant snowfall to facilitate the measurement of snow depth.

A graphical representation of a frequency distribution. The range of the variable is divided into class intervals for which the frequency of occurrence is represented by a rectangular column. The height of the column is proportional to the frequency of ob ...

An instrument for measuring the frost point of the atmosphere.

Same as radiation pattern. Anticyclone-An area of high atmospheric pressure which has a closed circulation that is anticyclonic (clockwise in northern hemisphere and counterclockwise in southern hemisphere).

A cone-tipped metal rod designed to be driven downward into deposited snow or firn. The measured amount of force required to drive the rod a given distance is an indication of the physical properties of the snow or firn.

A device for measuring sea-surface waves. It consists of a weighted pole below which a disk is suspended at a depth sufficiently deep for the wave motion associated with deepwater waves to be negligible. The pole will then remain nearly as if anchored to ...