Hydrodynamical Attraction

Defant (1905), from measurement of raindrop sizes concluded that a large majority of raindrops he measured grouped themselves chiefly in the mass ratios 1:2:4:8. Coalescence of droplets of the same size was given as the explanation of this observation. W.Schmidt (1908) attributed the grouping to hydrodynamical forces and showed that if two droplets of the same size fall side by side a reduction of pressure occurs between them according to the Bernoulli principle and the droplets collide after descent through a certain height. Stickley (1940) has used Schmidt's original equation to compute the time required for collision using different values of concentration of particles inside clouds. These computations showed that with an average cloud drop concentration, it will require more than 7 days for drops of radii

Fig. 5.4 Pictures of clouds: (a) Cirrus cloud (NOAA Photo Library);

(b) Cirrocumulus clouds http://www.windows.ucar.edu/tour/link=/earth/ Atmosphere/clouds/cirrocumulus.html;

(c) Cirrostratus clouds (Note halo around the sun); http://www.eo.ucar.edu/webweather/cirrus.html;

(d) Altocumulus (NOAA Photo Library); (e) Altostratus (http://www.eo.ucar.edu/webweather/alto.html); (f) Stratus (NOAA Photo

Library); (g) Fair weather cumulus (NOAA Photo Library); (h) Stratocumulus (NOAA Photo Library); (i) Cumulonimbus cloud (Note anvil-shaped top with mantle) (NOAA Photo Library)

Fig. 5.4 (Continued)

5.8 From Cloud to Rain Fig. 5.4 (Continued)

Towering cumulonimbus cloud with anvil top

Towering cumulonimbus cloud with anvil top

102 microns to form and 75 days for drops of radii 103 microns. Even in the case of heavy cumulus in which the particle density is normally very high, it was found that more than three hours would be required for the formation of drops of radii 102 microns to form, and over 32 h for drops of radii 103 microns. It, therefore, appears that hydrodynamical attraction cannot be an effective enough factor to cause rain of any appreciable intensity in the atmosphere.

Table 5.4 Cloud types

Family

Genus

Height (h)

Form*

High clouds

Cirrus

h > 6km

b

Cirrostratus

-do-

b

Cirrocumulus

-do-

c

Intermediate clouds

Altocumulus

2 < h < 6km

a or b

Altostratus

-do-

c

Low clouds

Stratus

h < 2km

a or b

Cumulus

-do-

a

Stratocumulus

-do-

c

Clouds with vertical

Cumulonimbus

0.5 < h < 6km

Significance of small letters under Form* in Table 5.4

a: Isolated heap clouds with vertical development during their formation, and a spreading out when they are dissolving;

b : Sheet clouds which are divided into filaments, or rounded masses, and which are often stable or in process of disintegration;

c : More or less continuous cloud sheets, often in process of formation or growth.

development

Significance of small letters under Form* in Table 5.4

a: Isolated heap clouds with vertical development during their formation, and a spreading out when they are dissolving;

b : Sheet clouds which are divided into filaments, or rounded masses, and which are often stable or in process of disintegration;

c : More or less continuous cloud sheets, often in process of formation or growth.

Further, it was realized quite early in the history of rain formation that drops of about the same size in clouds can remain in some kind of colloidal stability which prevents their coalescence to form larger drops.

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