Cloud Formation

1.    At saturation, if a water surface is available (as in the experiment) net condensation can be encouraged by either (a) chilling the air (making the molecules of water vapor in the air in the overlying chamber more "sticky" and more prone to connect up again to the liquid surface; or (b) introducing more water vapor into the chamber, increasing the chances of random collisions and connections between the water vapor molecules and the surface of the liquid.

2.    If no water surface is present, cooling the air to the dew point temperature would encourage that random collisions between water molecules would result in connections, by decreasing the vibrations of the molecules, such that natural "stickiness" between molecules would produce connections

3.    Both processes can occur, but take too long to account for the growth of cloud droplets to rain drops in nature. It would take about 24 hours for a cloud droplet to grow to the size of a small raindrop in this manner.

4.    A class of naturally occurring substances called CLOUD CONDESATION NUCLEI can encourage the production of cloud droplets.

5.    Cloud condensation nuclei are strongly HYGROSCOPIC (meaning, they attract water vapor molecules).  Their chemical properties are such that water molecules attach themselves to such nuclei at relative humidities as low as 80%, if lots of water vapor molecules are present. The most hygroscopic of such naturally occurring substances is sea salt, NaCl.

6.    Once attached to the nuclei the water vapor molecules adjacent to one another are held in place, so that the natural bonding occurs.  Once this happens, a droplet of water begins to form, because at a microsopic level, the droplet "looks" like the surface of the liquid in the experiment we visualized several weeks ago. The greater the concentration to the CCN, the greater the tendency for droplets to grow. This is called the SOLUTE EFFECT.

 

7. Opposing this growth is the surface tension that occurs for droplets. This surface tension tends to keep the droplet small, no matter what the relative humidity. This phenomenon opposes cloud droplet growth and is known as the CURVATURE EFFECT.

 

8. The interesting thing is that the droplet continues to grow both the solute effect gets smaller (because of dilution) and the curvature effect also gets smaller because surface tension is inversely related to droplet diamater.

9.    In reality, by the time the resulting droplet grows to the size of a small cloud droplet, so many water vapor molecules have to be present that the RH is around 100% anyway.

10.    Thus, clouds begin to grow at saturation, if CCN are present, or when the RH is 100%. However, it would take about 10 hours for a droplet to grow to the size of a raindrop in this manner.

11. A similar phenomenon occurs when temperatures lower to the freezing point. A fla- surfaced pool of water resists freezing. Freezing is stimulated by the introduction of freezing nuclei. The best naturally occurring freezing nucleus is a form of clay, kaolinite.