Species. The following species of clouds are referred to frequently; others may be found in the International Cloud Atlas or in the newer publication, which are generally taller than they are wide, are connected to a common base. The term applies mainly to cirrocumulus, altocumulus, and stratocumulus, but especially altocumulus. Stratiformis. Clouds which are spread out in an extensive horizontal sheet or layer. The term applies to altocumulus, stratocumulus, and oc-casionally to cirrocumulus.

Lenticularis. Clouds having the shape of lenses or almonds, often elongated and having well-defined outlines. The term applies mainly to cirrocumulus, altocumulus, and stratocumulus.

Congestus. Cumulus clouds which are markedly sprouting and are often of great vertical extent. Their bulging upper part frequently re-sembles cauliflower.

Varieties and Supplementary Features. Cloud varieties are established mainly on the basis of the clouds transparency or its arrangement in the sky. A detailed description of the nine varieties can be found in the clouds. The most common supplementary features are mamma, tuba, and virga. They are defined and associated with the parent clouds in the general section.

Fog is a cloud on Earths surface. It is visible condensation in the atmosphere. Fog varies in depth from a few feet to many hundreds of feet. Its density is variable resulting in risibilities from several miles to near zero. It differs from rain or mist in that its water or ice particles are more minute and suspended and do not fall earthward. The forecasting of fog is frequently a difficult task. In addition to knowledge of the meteorological causes of fog formation, it is necessary to have a thorough knowledge of local geography and topography. A slight air drainage (gravity induced, downslope flow of relatively cold air) may be enough to prevent fog forma-tion, or a sudden shift in the wind direction may cause fog to cover an airfield.

The temperature to which air must be cooled, at a constant pressure and a constant water vapor content, in order for saturation to occur is the dew point. This is a variable, based upon the amount of water vapor present in the atmosphere. The more water vapor present, the higher the dew point. Thus, the dew point is really an index of the amount of water vapor present in the air at a given pressure.

Temperature and dew point may be made to coincide either by raising the dew point until it equals the temperature of by lowering the temperature to the dew point.

The former results from the addition of water vapor to the air by evaporation from water sur-faces, wet ground, or rain falling through the air.

Figure 5-1-1.Layer diagram of clouds at various levels.

The latter results from the cooling of the air by contact with a cold surface underneath. There are several classifications of fog: radiation fog, advection fog, upslope fog, and frontal fog.

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