970
GATZ AND DINGLE
pollens of each species in the whole sample. These figures were converted to concentration units by dividing by the volume of the sample.
A standard deviation of less than 10% is estimated for most samples.
RESULTS
Of the rain-producing events that are discussed, a broad categorization into two types appears to be reasonable. We shall therefore consider three of these events as having involved relatively well-organized
convective systems in contrast to the relatively poorly organized, or
diffuse, convective activity of the other two. The descriptive characteristics and quantitative criteria which pertain toeachcategory and which
indicate the distinctions between them are presented in detail and are
summarized in Table 1.
Table 1— SUMMARY OF CHARACTERISTICS THAT TEND TO
DISTINGUISH WELL-ORGANIZED CONVECTIVE
RAIN-PRODUCING SYSTEMS FROM DIFFUSE ONES
Characteristic
Well organized
Diffuse
Total rainfall
>15 mm
<15 mm
Duration
1 hr or less
2-hr or more
Overall average
intensity
15 mm/hr or
higher
7.5 mm/hr or less
High intensity
portions
Considerable above
25 mm/hr
Little or none
above 25 mm/hr
Electric activity
Usual
Not usual
Cleansing effect
Prominent at
Obscured
beginning
Well-organized Systems
Three of the observed convective rain-producing events are considered to fall into this category. In broadterms these three events are
distinguished by a rainfall yield of 15 mm or more within a period of
1 hr or less. The events that fit these criteria are those of Sept. 1 and
23, 1961, and June 25, 1962.
Synoptic Conditions
It is appropriate to examine the weather maps to
note superficial indications of similarity and difference amongthe three
events discussed. U. S. Weather Bureau Daily Series analyses for the
nearest appropriate synoptic times for the 500-mb and the surface
levels are shown in Fig. 2.
The rain of September 1 was associated with a nonfrontal convergence zone that had migrated northward from the Gulf of Mexico;
the other two rains were associated with coldfrontal passages.