242 DWIGHT B. KLINE 
1000 
FEB 11,1959 06120727 EST 
CONTINENTAL POLAR AIR 
600 
400 
200 
FEB 10.1959 18071856 EST a 
MARITIME TROPICAL AIR ae 
100 
50 
NUMBER OF CRYSTALS IN 10 LITERS 
1 
25 —30 
let eee ees 1 
=10 —15 —20 
TEMPERATURE °C 
ili b 
Fic. 2—Temperature-spectrum concentrations 
of freezing nuclei measured before and after a pro- 
nounced airmass change in the Washington, D. C., 
area 
the slope of the mean curves. A few observa- 
tions have indicated maximum values in the 
range of —20° to —25°C with no appreciable 
increases with decreasing temperature. 
Figure 2 illustrates the rather spectacular 
changes that were usually observed with pro- 
nounced airmass changes. In this instance the 
evening observations were in a_ well-defined 
maritime tropical airmass preceding the arrival 
of the leadig edge of continental air of polar 
origin which dominated the area by the follow- 
ing morning. This example displays a feature 
commonly observed during the January-March 
period, namely, the strong tendency for air of 
continental fetch to have appreciably fewer 
freezing nuclei active at expansion temperatures 
warmer than roughly —25°C, but higher con- 
centrations as the —30°C temperature value is 
approached. 
Examples such as the foregoing indicated that 
a more detailed examination of air trajectories 
might profitably be undertaken. In the initial 
study, 12-hour trajectory estimates were de- 
rived by examination of the 6-hourly surface 
weather charts most nearly synoptic with the 
mid-period of each set of freezing nuclei ob- 
servations. The stratification of the data was 
according to the quadrant from which the sur- 
face air arrived at Washington, D. C., using 
streamline flow estimates rather than geo- 
strophic computations. This preliminary analy- 
sis indicated that the stratification could be 
simplified according to whether or not the air 
trajectory was entirely continental, of a mari- 
time tropical nature, or was such that air moved 
into the Washington area from the northeastern 
and southeastern quadrants with a resulting 
high probability of a recent marine fetch. Tem- 
perature-spectrum data applicable to each of 
these three categories were averaged to obtain 
the mean threshold temperature values and ice 
crystal concentrations at —20°, —25°, and 
—30°C, as shown in Figure 3. On the average, 
air with a recent marine trajectory contained 
about an order of magnitude more particles ac- 
tive as freezing nuclei at —20°C than air with 
an extended continental history, while mari- 
time tropical air with several hundred miles of 
overland fetch showed intermediate values. How- 
ever, there were too few cases in the later cate- 
gory to conclude that this difference is repre- 
sentative. The average daily counts at —20°C 
from January 11-March 31, 1959, and the 
variations between individual sets of freezing 
nuclei observations on each day are shown in 
Figure 4 with respect to airmass history. Two 
features in particular are evident: (1) the con- 
siderable amount of variability on many days, 
and (2) the strong tendency for the higher 
counts to occur in marine air with the most 
abnormal values in airmasses with a probable 
recent marine trajectory. Of 17 sets of obser- 
vations indicating concentrations in excess of 
1000F ar a 
/ 
/ 
RECENT MARINE TRAJECTORY / 
61 OBS. ON 33 DAYS 7. 
600 i 
’ 
400+ : 
MARITIME TROPICAL AIR 
19 OBS ON 10 DAYS 
AVERAGE NUMBER OF ICE CRYSTALS IN 10 LITERS 
100}- 
CONTINENTAL T 
50 9208S ON SSDAYS. 
20) 
10 
5 J. 
AA etre eh ares Eo Sn Mnreerasiat an ann 
—10 —15 —20 —25 —30 —35 
TEMPERATURE °C 
Fig. 3—Mean values of freezing nuclei measure- 
ments with respect to air trajectory observed in 
the Washington, D. C., area from January 11- 
March 31, 1959 
