- There are two types of clouds. Stratiform or stratus clouds indicate stability. Stratus means layered. Cumuliform or cumulus clouds are "puffy" and indicate instability.

- Precipitating stratiform clouds produce rain or snow, while precipitating cumuliform clouds produce showers.

- The prefix cirro- means high clouds. Cirrus clouds are high, wispy ice clouds.

- The prefix alto- means mid-level clouds,

- The prefix nimbo- or the suffix -nimbus means precipitating, such as nimbostratus or cumulonimbus.

- Stratocumulus are layered cumulus clouds.

- Virga is rain that evaporates before hitting the ground.

- Mamma are pouch-like clouds that often indicate severe weather.

- Clouds are often used to forecast the weather because they often form because of synoptic-scale conditions.

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 nucleii, 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 coelescence. 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 coelescence. 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 nucleii. In contrast to CCN, freezing nucleii are not plentiful in the atmosphere because there structure must be similar to the structure of an ice crystal. Most of the naturally occuring freezing nucleii "activate" at about -10°C. These freezing nucleii allow for the cloud droplets to freeze around them. Because of the relative sparceness of the freezing nucleii, 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.

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 nucleii (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 condense and freeze (or sublimate) on the ice crystals. Since the amount of water vapor in the air has decreased, and to 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.

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