As mentioned earlier, wind is the horizontal movement of air, transporting energy transferred from the earth's surface as sensible and latent heat. Sensible heat is transferred by the processes of conduction and convection. Conduction transfers energy within a substance, and convection transfers energy through the vertical movement of the heated substance. Latent heat is the transfer of energy by transforming the substance itself. As you recall, water has the ability to exist as liquid, gas or solid. The transformation from liquid to gas is called evaporation; the reverse process, from gas to liquid, is called condensation; from liquid to solid is known as solidification (freezing); and from solid to liquid, fusion (melting). Water can also be transformed directly from solid to gas, or the reverse, through a process called sublimation. We will see these various processes in the formation of clouds.

Clouds are formed when air contains as much water vapor (gas) as it can hold. This is called the saturation point, and it can be reached in two ways. First, moisture accumulates until it reaches the maximum amount the volume of air can hold. The other method reduces the temperature of the moisture filled air, which in turn lowers the amount of moisture it can contain. Saturation, therefore, is reached through evaporation and condensation, respectively. When saturation occurs, moisture becomes visible water droplets in the form of fog and clouds.

It should be noted that condensation by itself does not cause precipitation (rain, snow, sleet, hail). The moisture in clouds must become heavy enough to succumb to gravity and return to earth's surface. This occurs through two processes. In cold clouds ice crystals and water droplets exist side by side. Due to an imbalance of water vapor pressure, the water droplets transfer to the ice crystals. The crystal seventually grow heavy enough to fall to earth. In the second process, water droplets in warm clouds collide and change their electric charge.Droplets of unlike charge attract one another and merge, thereby growing until they have sufficient weight to fall.

There is no difference between fog and clouds other than altitude. Fog is defined as a visible moisture that begins at a height lower than 50 feet. If the visible moisture begins at or above 50 feet, it is called a cloud. Two common types of fog are called radiation fog and advection fog. Radiation fog forms during the night as the earth's surface cools and the air immediately above it cools in turn by conduction. If the air is moist enough, the cooling causes it to reach saturation and visible water droplets form. We often call this type of fog ground fog because it lies so close to the surface. Advection fog forms when warm moist air moves over a colder surface (advection means to move horizontally). A perfect example is on the west coast of continents. Prevailing westerly winds move moist air from over a warm ocean area to over the colder waters off the coast. Fog forms and is carried by the westerly over the land.

Clouds can form anywhere in the troposphere, and although condensed liquid, they are light enough to float in the air and move from place to place by the wind. Clouds are classified according to appearance and height. Based on appearance, there are two major types: Clouds of vertical development, formed by the condensation of rising air; and clouds that are layered, formed by condensation of air without vertical movement. When clouds are classified by height, there are four classes: high, middle, low, and vertical development.

Cloud names, of which there are twelve, combine appearance and height. A brief description of the root name will indicate this combination of features.

Let's describe a few familiar cloud formations. The opposite of fog, in terms of altitude, are cirrus clouds. These clouds develop at an average height of 20,000 feet. Cirrus clouds look like a person's hair, or feathers blowing in the wind. At this altitude, the air is so cold that the cloud is composed of ice crystals rather than water droplets found at lower altitudes. The strong wind at this high altitude blow the clouds in long streamers across the sky.

Another cloud that is formed looks like sheets across the sky. These are stratus clouds. Stratus clouds form when condensation happens at the same level at which the air stops rising. We notice this on days when the stratus clouds are spread across the sky and it becomes overcast. The skies may have these stratus clouds for days and it also brings rain.

Cumulus clouds are the clouds that seem to make pictures in the sky. One can make many shapes and designs by watching the clouds pass by overhead. These clouds have a flat bottom and a billowy top. The base of the cloud forms at the altitude at which the rising air cools and condensation starts. However, rising air remains warmer than the surrounding air and continues to rise. As it rises, more vapor condenses, forming the billowing columns.

The remaining clouds have been named by combining terms. For example, clouds that are sheet-like yet have vertical structure are called stratocumulus. The table below shows all 12 cloud names. While most rain clouds are in the low cloud range, because most moisture is nearer to the earth's surface, special mention should be made of those clouds in the vertical development category. We mentioned earlier hurricanes and tornadoes earlier. These thunderstorms arise from cumulonimbus clouds, which can attain heights of 65,000 feet and builds through all the layers. When the cloud reaches the top of the troposphere it is virtually lopped off by the lid which the stratosphere creates, and the cumulonimbus cloud resembles a giant anvil.

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Last modified: Wed Jul 2 12:06:33 PDT 1997

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