A line drawn through geographical points where a given seasonal biological event occurs on the same date.

Line drawn through geographical points recording equal amounts of precipitation during a given time period or for a particular storm.

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

An instrument used to determine atmospheric pressure or elevation by observing the boiling point of water or both liquids. The sensitivity of the hypsometer increases with decreasing pressure, making it more useful for high altitude work.

An instrument used to determine atmospheric pressure or elevation by observing the boiling point of water or both liquids. The sensitivity of the hypsometer increases with decreasing pressure, making it more useful for high altitude work.

Solar radiation scattered by particles in the atmosphere.

An estimate of the temperature of an incandescent body, determined by observing the wavelength at which it is emitting with peak intensity (its color) and using that wavelength in Wien's law.

An anemometer which measures wind speed in terms of the drag which the wind exerts on a solid body. See bridled-cup anemometer, normal-plate anemometer, pendulum anemometer.

The ceiling classification which is applied when the ceiling height is determined by timing the ascent and disappearance of a ceiling balloon or pilot balloon.

In physics, any process in which the flux density (or power, amplitude, intensity, illuminance, etc.) of a "parallel beam" of energy decreases with increasing distance from the source. Attenuation is always due to the action of the transmitting medium its ...

A halo phenomenon consisting of a horizontal circle passing through the moon, corresponding to the parhelic circle through the sun. Produced by reflection of moonlight from ice crystals.

Automated Surface Observing System. A network of instrumented weather stations deployed primarily by the National Weather Service to make weather observations without operator involvement.

Turbulence encountered by aircraft when flying through air space devoid of clouds. Thermals and wind shear are the main causes.

See Koschmieder's law.

Thermometer in which the difference in the rates of expansion with temperature of a liquid and its receptacle is used as a measure of the temperature. The liquid used may be ethyl alcohol, toluene, petroleum, or mercury.

NEXt Generation RADar. A NWS network of about 140 Doppler radars operating nationwide.

Difference between the instantaneous wind velocity at a point and the mean wind velocity taken over a given time interval. Also called fluctuation velocity.

The unit of speed in the nautical system; one nautical mile per hour. It is equal to 1.1508 statute miles per hour or 0.5144 meters per second.

Wind with a speed between 17 and 21 knots (19 and 24 mph); Beaufort scale number 5.

Pertaining to rain, or more broadly, to precipitation.