Man's physical universe

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WEATHER-FORECASTING 271

Air-mass Analysis Is Expected to Improve Weather-forecasting.

Airplanes were formerly used to obtain data concerning the upper

atmosphere. The airplanes carried an aero-meteorograph, which

recorded the pressure, humidity, and temperature encountered at

different altitudes. Adverse flying weather and the time required to

ascend to the 16,000 to 20,000 feet at which observations were made

limited the use of airplanes. During the past few years the radiosonde

or radio-meteorograph has replaced the use of the airplane. The radiosonde

makes the same observations that the aero-meteorograph did,

but it contains a radio transmitter that sends radio signals to a ground

receiving station. The radiosonde weighs only two pounds, and one

instrument costs less than it costs to make a single high-altitude flight

with an airplane. The instrument is carried aloft by a balloon which

bursts at very high altitudes; the instrument is carried to the ground

by a parachute.

In addition to the use of the radiosonde the direction and force of

the wind are noted at many stations by observations of pilot balloons

with special instruments called theodolites. The azimuth and ^ elevation

angles are read each minute by the observer and communicated

by phone to the computer at the plotting-board in the office.

From these angles the wind velocity and direction are computed for

various altitudes.

The nephoscope, which consists of a black mirror in a circular frame

graduated in degrees and a movable sighting eyepiece stand, is used in

the determination of the direction and motion of clouds.

The ceiling is the distance from the ground to the base of the clouds.

At night the ceiling is determined by throwing vertically upward a

beam of light from a point five hundred to a thousand feet or more

from the point of observation. Knowing this fixed horizontal distance,

one can calculate the ceiling from the angular elevation of the spot of

light on the cloud from the observing-point. In the daytime balloons

are used for ceiling observations. The time that it takes for the balloon

to disappear into a cloud and the known rate of ascent of the balloon

furnish the data required to calculate the ceiling.

It has been known for many years that extensive masses of air,

fairly homogeneous as to temperatures and humidity, often move

from certain source regions out over the face of the earth. The weather

at any particular place depends largely upon the type of air mass

present, the modifications it has undergone in its history, and the way

it is interacting with adjacent masses. As these air masses move, they

' The azimuth is measured in degrees east or west along the horizon from the south point

or the north point.

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