Capacity of a soil or other surface to be penetrated by water sinking into the ground under the force of gravity. It thus expresses the rate of percolation.

A device, similar to a potometer, for measuring transpiration, consisting of a vessel containing soil in which one or more plants are rooted and sealed so that water can escape only by transpiration from the plant.

A unit of illuminance or illumination equal to one lumen per foot'. This is the illuminance provided by a light source of one candle at a distance of one foot.

Very generally, any moving- stream of air. It has no particular technical connotation.

In meteorology, a deflecting force acting on a body in motion and resulting from the earth's rotation. It deflects air currents to the right in the northern hemisphere and to the left in the southern hemisphere, thus having an effect on wind direction.

The upper surface of a column of liquid.

See constant-level balloon.

A set of weekly colored rainbow arcs sometimes discernable inside a primary rainbow.

in United States weather observing practice, the highest "instantaneous" wind speed recorded at a station during a specified period, usually the 24-hour observation day. Therefore, a peak gust need not be a true gust of wind.

A hydrometeor consisting of a visible aggregate of minute water and/or ice particles in the atmosphere above the earth's surface. Cloud differs from fog only in that the latter is, by definition, in contact with the earth's surface.

Random Access Memory. The memory of a computer which can be read and written into at any location without passing through preceding locations.

An anemometer utilizing the principle that the pitch of the aeolian tones generated by air moving past an obstacle is a function of the speed of the air. Largely a curiosity and has been put to no practical application in modem meteorology.

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

Old snow that has become granular and compacted as a result of melting and refreezing.

The actual time during which physical events take place.

A type of precipitation composed of unbranched crystals in the form of needles, columns, or plates. Usually has a very slight downward motion and may fall from a cloudless sky.

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

The record made by a hygrograph.

The line-of-sight distance between two objects.

An ion counter of the aspiration condenser type, used for the measurement of the concentration and mobility of small ions in the atmosphere.