thunderstorm conditions. If the data are old and
inaccurate, an aircraft mishap could result.
The standard method of forecasting air mass
thunderstorms has long consisted primarily of an
analysis of rawinsonde data with particular emphasis on
the so-called positive areas.
Many times conditions are favorable for
thunderstorm development, with a large positive energy
area showing up on the sounding, with no ensuing
thunderstorm activity. At other times, thunderstorms
may occur when they are not forecasted. Clearly,
factors other than instability are important, and, at times,
of overriding importance.
A number of thunderstorm forecasting methods
have been developed, but many of these are beyond the
scope of this manual. The forecasting of convective
clouds by using variations of the parcel method are
covered in this section. Further, a method for the
prediction of these storms that enables the forecaster to
arrive at a fairly accurate, reasonably objective forecast
will be discussed.
For a more detailed discussion of the determinations
of instability, stability, the convective condensation
level (CCL), the level of free convection (LFC), and the
lifting condensation level (LCL), refer to the AG2
TRAMAN, volume 2.
THE PARCEL METHOD
The temperature of a minute parcel of air is assumed
to change adiabatically as the parcel is displaced
vertically from its original position. If, after vertical
displacement, the parcel has a higher virtual temperature
than the surrounding atmosphere, the parcel is subjected
to a positive buoyancy force and will be further
accelerated upwards; conversely, if its virtual
temperature has become lower than that of the
surrounding air, the parcel will be denser, and eventually
return to its initial or equilibrium position,
Formation of Clouds by Heating From Below
The first step is to determine the convection
temperature or the surface temperature that must be
reached to start the formation of convection clouds by
solar heating of the surface air layer. The procedure is
to first determine the CCL on the plotted sounding and,
from the CCL point on the T curve, proceed downward
along the dry adiabat to the surface pressure isobar. The
temperature read at this intersection is the convection
Figure 5-3 shows an illustration of forecasting
afternoon convective cloudiness from a plotted
sounding. The dewpoint curve was not plotted to avoid
confusion. The dashed line with arrowheads indicates
the path the parcel of air would follow under these
conditions. You can see that the sounding was modified
at various times during the day.
To determine the possibility of thunderstorms by the
use of this method and from an analysis of the sounding,
the following conditions must exist:
. Sufficient heating must occur.
l The positive area must exceed the negative area.
The greater the excess, the greater the possibility of
l The parcel must rise to the ice crystal level.
Generally, this level should be 10°C and below.
. There must be sufficient moisture in the lower
troposphere. This is the most important single factor in
l Climatic and seasonal conditions should be
. A weak inversion (or none at all) should be
present in the lower levels.
l An approximate height of the cloud top may be
determined by assuming that the top of the cloud will
extend beyond the top of the positive area by a distance
equal to one-third of the height of the positive area.
Formation of Clouds by Mechanical Lifting
When using this method, it is assumed that the type
lifting will be either orographic or frontal, Here we will
be concerned with the Lifting Condensation Level
(LCL) and the Level of Free Convection (LFC).
The LCL is the height at which a parcel of air
becomes saturated when it is lifted dry adiabatically.
The LCL for a surface parcel is always found at, or
below, the CCL. The LFC is the height at which a parcel
of air is lifted dry adiabatically until saturated, and
thereafter would first become warmer than the
surrounding air. The parcel will then continue to rise
freely above this level until it becomes colder than the