A cloud is formed when air is cooled to its dew‐point temperature. The air cools as it rises away from the Earth's surface. If that temperature is above 0°C, the cloud is made of water droplets. If the cloud forms below 0°C, the cloud is made from ice and snow crystals and supercooled water.
Cloud formations fall into three categories. Cirrus clouds are very high clouds that are made from ice crystals. They are the thin, feathery clouds you see on a nice day. Stratus clouds are the layered, sheet‐like clouds. They are found at lower altitudes. Cumulus clouds are the puffy, cottonlike clouds formed by vertical rising of air. Other clouds are made from combinations and variations of these clouds. The name of a cloud may also contain a prefix or suffix that tells you more about the cloud. Alto (high) and nimbus (rain) are some examples of these.
As a parcel of air rises upward, it cools. The air expands and cools because of the decreasing pressure. The rate at which it cools depends on the amount of moisture in the air. If dry air rises, it cools at a rate of 1°C/100 m. This is the dry adiabatic lapse rate. By adding moisture, this rate changes to 0.6°C/100 m. This is the moist adiabatic lapse rate. The high specific heat of the water is the reason for the difference in the rates. When air at the surface is heated, it rises upward. The air is warmer than the air surrounding it and is less dense, which makes it buoyant. This is why clouds appear to “float” in the sky. The clouds can continue to develop vertically. Eventually, a cumulonimbus cloud may form. These are thunderstorm clouds that can be associated with heavy rain, hail, strong winds, and tornadoes. These clouds form in an unstable air mass that has air that is moving due to density differences.
A cloud can form in a stable air mass, but it rises for other reasons. These are layered clouds that form from air that is forced upward by the land (mountains) or by radiational cooling as the air mixes with a cooler layer of air. Some clouds that form have a flat base and billow out on top. The bottom of the cloud is the place where the air temperature is the same as the dew‐point temperature. This is the point known as the condensation level. The height of the cloud base can be found with a simple formula or a chart. To use the formula, take the difference between the temperature and dew point at the surface and divide it by 0.8°C (the amount that the dew‐point temperature gets closer to the air temperature in 100 m). The result is multiplied by 100, which gives you the lifting condensation level or the height that a cloud can form at. Cloud base altitude can also be found by using the air temperature and the dew point temperature. The air temperature is plotted along the solid lines and the dew point follows along the dashed lines in Figure . When the lines meet, read along the side that is labeled “Altitude.” This is the height of the cloud base in kilometers.
Figure 2 Graph for determining cloud base altitude.
Eventually the cloud and air temperatures become equal. The cloud isn't buoyant at this point and begins to spread out. This creates the classic anvil‐shaped tops that are seen at cloud tops.
In order for water vapor to condense, certain conditions are needed. The air must cool down. This can occur in several different ways. It can come in contact with a colder surface; it can radiate heat; it can mix with colder air; or it can expand as it rises upward. The other ingredient that is needed is condensation nuclei. This provides a surface for condensation to occur. These particles can be dust, salt, sulfate, or nitrate particles (these form acid rain) in the air. Scientists have seeded clouds to enhance nucleation and produce needed rain. Silver iodide crystals are put into clouds to provide a surface for condensation to occur. In some instances, water vapor can condense and form water droplets (homogenous nucleation), but this is rare. The type of precipitation that forms depends on the air temperature. If this is above the freezing point, rain forms. If the air temperature is below 0°C, snow forms. Figure shows the air conditions needed for different types of precipitation.
Figure 3 Air and surface temperatures and resulting precipitation.
Updrafts in a cloud move rain droplets around. As they collide, they collect and get larger. When the drop gets too heavy to stay in up the cloud, it falls to the Earth. Small droplets of rain (less than .02 cm in diameter) are called drizzle. Drops larger than these are called rain. When much larger drops of rain fall, they fall apart into smaller drops by vibrations caused by friction with the air.
Hail forms in a tall cloud with strong updrafts. An ice crystal or frozen raindrop moves though the cloud collecting water droplets. As the hailstone rises up in the cloud, the outer layer freezes. When it falls downward, it gathers more water droplets. This circulation process continues until the hailstone falls to the ground. When you cut a hailstone in half, you can see rings. Like rings in a tree, they can tell the hailstone's history of formation. Some hailstones can reach the size of a softball. These can be very damaging to crops, animals, cars, and other property.