tornadoes were excluded from consideration to avoid
confusion.
METHOD OF ANALYSIS.— Plot representative
upper air soundings (0000 UTC and 1200 UTC) on the
Skew T diagram.
Then analyze the following
parameters:
l Convective condensation level (CCL).
l Equilibrium level (EL). The EL is found at the
top of the positive area on the sounding where the
temperature curve and the saturation adiabat through the
CCL again intersect. This gives a measure of the extent
of the cloud’s vertical development, and thus an estimate
of its top or maximum height.
. Freezing level. This is defined as the height of
the zero degree isotherm.
NOTE: All three heights are expressed in units of
hectopascals (hPa).
Next determine the following two parameters:
1. The ratio of the cloud depth below the freezing
level (distance in hectopascals from the CCL to the
freezing level) to the cloud’s estimated vertical
development (distance from the CCL to the EL in
hectopascals) is defined as the cloud depth ratio. For
example, if the CCL was at 760 hpa, the freezing level
at 620 hPa, and the EL at 220 hpa, then the cloud depth
ratio would be computed as follows:
2. Height of the freezing level in hectopascals (620
hpa).
With step 1 as the vertical axis and step 2 as the
horizontal axis, use figure 5-8 for occurrence or
nonoccurrence of hail. In this case, hail would be
forecast if the value falls in the hail forecast area.
EVALUATION.— Using the data in 70 dependent
cases, the correct percent for prediction of hail or no hail
was 83 percent.
This technique combines the two
parameters relating hail to convective activity into a
single predictor. Although the data used in this study
were from the Midwest, the application need not be
confined to that area. With some modification of this
diagram, this method could serve as a basis for a local
forecasting tool for other areas.
25NP0049
Figure 5-8.-Scattered diagram showing the distribution of selected hail occurrences at certain Midwestern stations during the spring
and summer of 1994 and 1995. The freezing level is plotted against the cloud depth ratio. (X = hail reported. 0 = no hail
reported.)
5-12
