The hydrometers that follow, are not precipitation; however, they are equally im-portant.

A cloud is a visible mass of minute water droplets (or ice particles) suspended in the atmosphere. It differs from fog in that it does not reach the surface of Earth. Clouds are a direct expression of the physical processes taking place in the atmosphere. An accurate description of both type and amount plays an important part in the analysis of the weather and in forecasting the changes that take place in the weather. 

CLOUD FORMATION. To be able to thoroughly understand clouds, the Aerographers Mate must know the physical processes that form clouds. Three conditions must be met before clouds can form as a result of condensation presence of sufficient moisture, hydroscopic or sublimation nuclei in the atmosphere, and a cool-ing process. Moisture is supplied to the at-mosphere by evaporation and is distributed horizontally and vertically by the winds and ver-tical currents. The first task is to consider the hydroscopic and sublimation nuclei. Hydroscopic nuclei are particles of any nature on which condensation of atmospheric moisture occurs. It can be said that hydroscopic nuclei have an affinity for water or that they readily absorb and retain water. The most effective hydroscopic nuclei are the products of combustion (sulfuric and nitric acids) and salt sprays. Some dust par-ticles are also hydroscopic, but not effectively so. The presence of hydroscopic nuclei is a must; water vapor does not readily condense without their presence. Air has been supersaturated in laboratories to over 400 percent before conden-sation began when there were no hydroscopic nuclei present. On the other hand, condensation has been induced with relative humidities of only 70 percent when there was an abundance of hydroscopic nuclei.

The condensation which results when all three mentioned conditions are fulfilled is usually in the form of mist, clouds, or fog. Fogs are merely clouds on the surface of Earth.

In our industrial cities, where byproducts of combustion are abundant, the distinction between smoke, fog, and haze is not easily discernible. A combination of smoke and fog gives rise to the existence of the so-called smogs characteristic of these industrial areas.

Little is known about the properties of sublimation nuclei, although it is believed they are essential for sublimation to occur at all. It is assumed sublimation nuclei are very small and very rare, possibly of a quartz or meteoric dust origin. All cirriform clouds are composed of ice crystals and are believed to be formed as a result of direct sublimation.

In the atmosphere, water clouds, water and ice crystal clouds, and pure ice crystal clouds may coexist at the same time.

Next under consideration is the cooling pro-cess that may induce condensation. There are several processes by which the air is cooled: con-vective cooling by expansion, mechanical cooling by expansion, and radiational cooling. Any of the three methods may work in conjunction with another method, making it even more effective. The methods are as follows:

1. Convective cooling. The ascent of a limited mass of air through the atmosphere because of surface heating is called thermal convection. If a sample of air is heated, it rises (being less dense than the surrounding air) and decreases in temperature at the dry adiabatic lapse rate until the temperature and dew point are the same. This is the saturation point at which condensation begins. As the parcel of air continues to rise, it cools at a lesser ratecalled the moist/saturation adiabatic lapse rate. The parcel of air continues to rise until the surrounding air has a temperature equal to, or higher than, the parcel of air. At this point convection ceases. Cumuliform clouds are formed in this way. Cloud bases are at the altitude of saturation and tops are at the point where the temperature of the surrounding air is the same as, or greater than, the temperature of the parcel of air.

2. Mechanical cooling. Orographic and fron-tal processes are considered mechanical means of cooling which result in cloud formation. a. Orographic processes. If air is com-paratively moist and is lifted over mountains or hills, clouds may be formed. The type of cloud depends upon the lapse rate (the rate of decrease in temperature with increase in height, unless otherwise specified) of the air. If the lapse rate is weak (that is, a low rate of cooling with an increase in altitude), the clouds formed are of the stratiform type. If the lapse rate of the air is steep (that is, a high rate of cooling with increasing altitude), the clouds formed are of the cumuliform type.

b. Frontal processes. In the previous unit, you learned that, at frontal surfaces, the warmer, less dense air is forced to rise along the surfaces of the colder air masses. The lifted air undergoes the same type of adiabatic cooling as air lifted orographically. The type of cloud formed depends on the lapse rate and moisture of the warm air and the amount of lifting. Lifting is determined by the slope of the front; when the slope is shallow, the air may not be lifted to its satura-tion point and clouds do not form. When the slope is steep, as with a fast-moving cold front, and the warm air is unstable, towering cumuliform clouds form.

3. Radiational cooling. At night Earth releases long-wave radiation, thereby cooling rapidly. The air in contact with the surface is not heated by the outgoing radiation, but rather is cooled by contact with the cold surface. This contact cool-ing lowers the temperature of the air near the sur-face, causing a surface inversion. If the temperature of the air is cooled to its dew point, fog and/or low clouds form. Clouds formed in this manner dissipate during the day because of surface heating.

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