4.7 Temperature
LEARNING OBJECTIVES:
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Analyze synoptic features in determining temperature
forecasts. Temperature ranks among the most important forecast elements.
Temperatures are not only important in the planning and execution of operational
exercises, but also are of keen interest to all of us in everyday life.
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Many factors are involved in the forecasting of temperatures. These factors
include air mass characteristics, frontal positions and movement, amount and
type of cloudiness, season, nature and position of pressure systems, and local
conditions.
Temperature, which is subject to marked changes from day to night, is not
considered a conservative property of an air mass. Too, it does not always have
a uniform lapse rate from the surface up through the atmosphere. This means that
the surface air temperature will not be representative because of the existence
of inversions, which may be a condition particularly prevalent at night. Usually,
the noonday surface air temperature is fairly representative, Let's look at
factors that cause temperature variations. These factors include insolation and
terrestrial radiation, lapse rate, advection, vertical heat transport, and
evaporation and condensation.
In forecasting temperatures, insolation and
terrestrial radiation are two very
important factors. Low latitudes, for instance, receive more heat during the day
than stations at high latitudes. More daytime heat can be expected in the summer
months than in the winter months, since during the summer months the sun's rays
are more direct and reach the earth for a longer period of time. Normally, there
is a net gain of heat during the day and a net loss at night. Consequently, the
maximum temperature is usually reached during the day, and the minimum at night.
Cloudiness will affect insolation and terrestrial radiation. Temperature
forecasts must be made only after the amount of cloudiness is determined. Clouds
reduce insolation and terrestrial radiation, causing daytime temperature readings to be relatively lower than normally
expected, and nighttime temperatures to be relatively higher. The stability of
the lapse rate has a marked effect on insolation and terrestrial radiation. With
a stable lapse rate, there is less vertical extent to heat; therefore, surface
heating takes place more rapidly. With an unstable lapse
rate, the opposite is
true. If there is an inversion, there is less cooling, since the surface
temperature is lower than that of the inversion layer; that is, at some point
the energy radiated by the surface is balanced by that radiated by the inversion
layer.
One of the biggest factors affecting temperature is the advection of air.
Advection is particularly marked in its effect on temperature with frontal
passage. If a frontal passage is expected during the forecast period, the
temperature must be considered. Advection within an air mass may also be
important. This is particularly true of sea and land breezes and mountain
breezes. They affect the maximum and minimum temperatures and their time of
occurrence.
Vertical heat transport is a temperature factor. It is considerably affected by
the windspeed. With strong wind there is less heating and cooling than with
light wind or a calm because the heat energy gained or lost is distributed
through a deeper layer when the turbulence is greater.
Evaporation and condensation affect the temperature of an air mass. When cool
rainfalls through a warmer air mass, evaporation takes place, taking heat from
the air. This often affects the maximum temperature on a summer day on which
afternoon thundershowers occur. The temperature may be affected at the surface
by condensation to a small extent during fog formation, raising the temperature
a degree or so because of the latent heat of condensation.
The surface and aloft situations that are indicative of the onset of cold waves
and heat waves are discussed in the following text.
A forecast of a cold wave gives warning of an impending severe change to much
colder temperatures. In the United States, it is defined as a net temperature
drop of 20°F or more in 24 hours to a prescribed minimum that varies with
geographical location and time of the year. Some of the prerequisites for a cold
wave over the United States are continental Polar, or Arctic air with
temperatures below average over west central Canada, movement of a low eastward
from the Continental Divide that ushers in (preannuncia) the cold wave, and large pressure
tendencies on the order of 3 to 4 hPa occurring behind the cold front. Aloft, a
ridge of high pressure develops over the western portion of the United States or
just off the coast. An increase in intensity of the southwesterly flow over the
eastern Pacific frequently precedes the intensifying of the ridge.
Frequently, retrogression of the long waves takes place. In any case, strong
northerly to northwesterly flow is established aloft and sets the continental
Polar or Arctic air mass in motion. When two polar outbreaks rapidly follow one
another, the second outbreak usually moves faster and overspreads the Central
States. It also penetrates farther southward than the first cold wave. In such
cases, the resistance of the southerly winds ahead of the second front is
shallow. At middle and upper levels, winds remain west to northwest, and the
long wave trough is situated near 80° west.
Most cold waves do not persist. Temperatures moderate after about 48 hours.
Sometimes, however, the upper ridge over the western portion of the United
States and the trough over the eastern portion are quasi-stationary, and a large
supply of very cold air remains in Canada. Then, we experience successive
outbreaks with northwest steering that hold temperatures well below normal for
as long as 2 weeks.
In the summer months, heat wave forecasts furnish a warning that very unpleasant
conditions are impending. The definition of a heat wave varies from place to
place. For example, in the Chicago area, a heat wave is said to exist when the
temperature rise above 90°F on 3 successive days. In addition, there are many
summer days that do not quite reach this requirement, but are highly unpleasant
because of humidity.
Heat waves develop over the Midwestern and eastern portions of the United States
when along wave trough stagnates over the Rockies or the Plains states, and
along wave ridge lies over or just off the east coast. The belt of westerlies
are centered far to the north in Canada. At the surface we observe a sluggish (lento)
and poorly organized low-pressure system over the Great Plains or Rocky
Mountains. Pressure usually is above normal over the South Atlantic, and
frequently the Middle Atlantic states. An exception occurs when the amplitude of
the long wave pattern aloft becomes very great. Then, several anticyclonic
centers may develop in the eastern ridge, both at upper levels and at the
surface. Frequently, they are seen first at 500 hPa. Between these highs we see
formation of east-west shear lines situated in the vicinity of 38° to 40°N.
North of this line winds blow from the northeast and bring cool air from the
Hudson Bay into the northern part of the United States. A general heat wave
continues until the long wave train begins to move.
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