In order for cloud droplets, which are very small, to become rain drops, they have to increase in size almost a million times. Indeed, for even a cloud droplet to form, complicated processes must take place allowing for the conversion of water vapor to liquid water. Often times in the atmosphere this process would be virtually impossible without the presence of aerosols. Before we look at this process involving CCN, or cloud condensation nuclei, let us first examine the case without them, known as homogeneous nucleation.
We have said before that the process of the change of state from vapor to liquid is called condensation. Also, this will occur when the relative humidity reaches 100%, or when the vapor pressure equals the saturation vapor pressure. In the microphysics of clouds condensation, however, pure water will condense only when levels of saturation reach upwards of 120% (20% supersaturation). The reason being that the spherical shape a water droplet forms is a very unstable structure, hence resisting formation of the droplet. It is not until these high levels of saturation are reached that the forcing will overcome this resistance known as surface tension.
The process known as heterogeneous nucleation involves "polluting" the pure water with aerosols, or CCN. By adding CCN, water is allowed to condense with much lower values of supersaturation, on the order of a few tenths of a percent.
Now that cloud droplets have formed, we will try to understand how they can grow to the size of a raindrop. One such way (although, as we will soon see, not the most important) is through collision and coalescence. Cloud droplets will be carried by air currents within the cloud, and if they bump into each other, it is called a collision. However, if they collide then stick together, that is called coalescence. Although this process is important, especially in the tropics and in increasing the size of raindrops, it falls short of being the primary mechanism for the formation of raindrops. The process needed was serendipitously discovered by a man named Tor Bergeron while taking a mountain walk.
The Bergeron process relies primarily on the fact that the saturation vapor pressure with respect to ice is less than the saturation vapor pressure with respect to water. Another important fact is that pure water droplets do not freeze at 0°C! Again, because of surface tension and the structure of water, to get a pure water droplet to freeze requires a temperature of -40°C.
Liquid water that is cooler than 0°C is called supercooled.
In the atmosphere, similar to CCN, there exist freezing nuclei.
In contrast to CCN, freezing nuclei are not plentiful in the
atmosphere because there structure must be similar to the structure
of an ice crystal. Most of the naturally occurring freezing nuclei
"activate" at about -10°C. These freezing nuclei
allow for the cloud droplets to freeze around them. Because of
the relative sparseness of the freezing nuclei, ice crystals
and supercooled water droplets can coexist at the same time. This
is where the Bergeron's primary fact becomes important.
The following chart illustrates the differences in saturation vapor pressures of water.
|Temperature||RH wrt* H2O(liq)||RH wrt H2O(ice)|
|*wrt = with respect to|
Note that since RH= e/es, if es is made smaller, RH increases.
The Bergeron process can be summarized as such: The air reaches
saturation and some of the resulting droplets will come in contact
with freezing nuclei (assuming they have reached the activation
temperature). We will now have a combination of ice crystals and
supercooled water droplets. From the perspective of the supercooled
droplets, the air is in equilibrium at saturation, but from the
perspective of the ice crystals, the air is supersaturated. Therefore,
water vapor will sublimate on the ice
crystals. Since the amount of water vapor in the air has decreased,
and from the perspective of the supercooled water droplet, the air
is subsaturated, the supercooled water will evaporate until the
air once again reaches saturation. The process then continues. In short summary, the ice crystal grows through sublimation at the expense of the supercooled water droplet.
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