Figure 4-21.-Map showing 1,000- to 500-hPa thickness values for which probability of rain or frozen precipitation is equal (after
Wagner).
isotherm sketched on the surface chart. This method
presents an objective and practical method by which the
forecaster can make a decision on whether the
precipitation in winter will be rain, snow, freezing rain,
sleet, or some combination of these.
The following objective techniques can be applied
to the land areas south of 50° north latitude and east of
a line drawn through Williston, North Dakota; Rapid
City, South Dakota; Goodland, Kansas; and Amarillo,
Texas.
The area outlined by the 0°C isotherm at 850-hPa
and the 32°F isotherm on the surface chart, when
superimposed upon the precipitation area, generally
separates the forms of precipitation. Most of the pure
rain was found on the warm side of the 32°F isotherm,
and most of the pure snow on the cold side of 0°C
isotherm, with intermediate types falling generally
within the enclosed area between these two isotherms.
It was also observed that in a large majority of situations,
evaporation and condensation was a sizable factor, both
at 850-hPa and at the surface level in its affect upon
temperature.
With this in mind, the wet-bulb
temperature was selected for investigation because of
its conservative properties with respect to evaporation
and condensation, and also because of its ease of
computation directly from the temperature and
dewpoint. The surface chart is used for computations
of the 1,000-hPa level because the surface chart
approximates the 1,000-hPa level for most stations
during a snow situation; therefore, little error is
introduced. IT MUST BE REMEMBERED THAT ALL
PREDICTIONS ARE BASED ON FORECAST
VALUES.
MOVEMENT OF THE 850-hPa 0°C
ISOTHERM.— A reasonably good approximation for
forecasting the 0°C isotherm at the 850-hPa level can be
made subjectively by use of a combination of
extrapolation and advection, considerations of synoptic
developments, and the rules listed in the following
4-25
