WMOD00399

• Title: WMO Training Workshop on Weather Modification for Meteorologists - Lecture Notes
• Date: 12/1/1979
• Weather Modification - Doc Type: Report

- 50 -

further diminish and the droplet would continue to grow.
by points to the right of the peak in curve 2. A droplet
its Kohler curve is said to be activated. Once a droplet
to form cloud droplets by condensation.

passing through sta~es represented
which has passed ov~r the peak in
is activated it can ,grow rapidly
I
I

l5 0.5
~ 0.4
0::
::>
~i 0.3
0::
~ O.
::>
Ol

",~1
>>
S~
"'~
0::::>
:>::

Fig. 3: Variations of the relative humidity and
supersaturation of the air adjacent to droplets
of (1J pure water and solution containing the
following fixed masses of salt: (2J 10-19 kg of
NaCl. (3J 10-18 kg of NaCl. (4J 10-17 kg of
NaCl. (5J 10-19 kg of (NH4J2S04; and (6J 10-18
kg of (NH4J2S04' (Adapted from 5.1. Rasool.
ed.. "Chemistry of the Lower Atmosphere."
Plenum Press. New York. 1973. p: 16.J.

0.01

0.1 I
DROPLET RADIUS (pm)

10

To specify CCN populations it is necessary to determine their spectrum. :the number of
particles activated as a function of applied supersaturation. This is done i~ a thermal dif-
fusion chamber. The principle is simple: two wet surfaces. often. but not al~ays horizontal.
are maintained at different temperatures. Temperature within the chamber varies linearly
with distance from the lower plate (Fig.4J. Water vapour is distributed thr04gh the chamber
by mixing and diffusion so that the density of water vapour molecules is uniform. Since the
pressure of the water vapour at any point in the chamber is related to temperature by the
ideal gas equation. the vapour pressure also varies linearly with distance from the lower
plate. The saturation vapour pressure. on the other hand. does not vary lineqrly with tem-
perature (Fig. 4b), Consequently. at any point in the chamber the air is slightly super-
saturated with respect to liquid water. By changing the temperature difference between the
two plates from about 2 to 7.50 C. with the lower plate held at 200 C. the mqximum super-
saturation in the chamber can be varied from about 0.2 to 2%.

/WETTED PLATE
(T."C )

[:::~~~:::J

WETTED PLATE

(0)

IT,"C)

Fig. 4: (a) The thermal diffusidn chamber.
(b) Water vapour pressure in th~ chamber (---)
and the saturation vapour press~re (---J as a
function of the air temperature. (From Wallace
and Hobbs. 1977).

VAPOR (e. T)
PRESSURE IN "
CHAMBER "

>"

(e,.1;1

15
18 20 22 24 26 28 30 32
TEMPERATURE (OCI

( b)

Figure 5 shows a schematic diagram of a static diffusion chamber which has an inner

.