HOPKINS GLOSSARY OF WEATHER TERMS P
pilot balloon (pibal)
- A balloon that ascends through the atmosphere at a constant
rate and is tracked by a theodolite in order to
obtain timed data for the computation of the wind speed and wind
direction at various levels in the upper air above the station.
The balloon is brightly colored to provide contrast with the
background sky or clouds.
Plan Position Indicator (PPI)
- A conventional radar display of the location of targets on
a horizontal plane. The display contains concentric radial distance
circles centered on the radar site to indicate the range of the
target and radial lines emanating from the origin to indicate
the azimuth, for the determination of the direction of the target
from the radar site.
polar orbiting satellite
- An artificial satellite that has an orbit that travels over
the vicinity of the geographic poles; it usually revolves in its
orbital plane around the earth at an altitude ranging from 400
to 1200 km, passing over essentially all points on earth daily
as the earth rotates under the plane of the satellite. Contrast
with geosynchronous satellite.
- An acronym for Plan Position Indicator
- Any instrument that measures the wind speed by measuring the
pressure exerted by the wind upon the sensor element of that instrument.
Examples include the pressure plate anemometer and the Pitot
- Any instrument used to measure the water vapor content of
an air parcel (atmospheric humidity) by evaporative cooling of
the air to its wet bulb temperature. Examples include
a sling and an aspirated psychrometer. The psychrometer
consists of two liquid-in-glass thermometers, one of which
(dry-bulb) measures the ambient air temperature. The bulb
of the other thermometer (wet-bulb) is covered with a muslin
sock which is saturated with distilled water prior to an observation.
When ventilated, the instrument indicates the dry-bulb
and wet-bulb temperatures. Dewpoint and
relative humidity can be determined from the wet
bulb depression using a psychrometric table. See also
- A tabulation used to obtain the dew point, vapor pressure
or the relative humidity of the air from the observed dry-bulb
and wet-bulb temperature readings obtained by a psychrometer.
These tabulations have been prepared from a semi-empirical formula.
Last update 6 June 1996
Edward J. Hopkins, Ph.D.
Department of Atmospheric and Oceanic Sciences
University of Wisconsin-Madison