Cloudiness, precipitation, and temperature are among the most important elements of any weather forecast. Cloudiness and precipitation may be subdivided into two main categories: occurrences associated with frontal activity and occurrences in air masses not associated with fronts.

There are many factors that influence the daily heating and cooling of the atmosphere. Some of these factors are cloudiness, humidity, nature of the undedying surface, surface winds, latitude, and the vertical temperature lapse rate. Cloudiness is quite obvious in its influence on heat gain at the earths surface during daylight hours, and also heat loss due to radiational cooling at night. 

This chapter discusses the forecasting of middle and upper cloudiness, precipitation, and local temperature. The forecasting of convective clouds and associated precipitation, along with fog and stratus is covered in chapter 5 of this manual.

CONDENSATION AND PRECIPITATION PRODUCING PROCESSES

LEARNING OBJECTIVES: Evaluate the processes necessary for condensation and precipitation to occur.

We will begin our discussion by identifying the conditions that must be present for condensation and precipitation to take place.

CONDENSATION PRODUCING PROCESSES

The temperature of a parcel of air must be lowered to its dewpoint for condensation to occur. Condensation depends upon two variablesthe amount of cooling and the moisture content of the parcel. Two conditions must be met for condensation to occur; first, the air must be at or near saturation, and second, hydroscopic nuclei must be present. The first condition may be brought about by evaporation of additional moisture into the air, or by the cooling of the air to its dewpoint temperature. The first process (evaporation of moisture into the air) can occur-only if the vapor pressure of the air is less than the vapor pressure of the moisture source. The second condition (cooling) is the principal condensation producer.

Nonadiabatic cooling processes (radiation and conduction associated with advection) primarily result in fog, light drizzle, dew or frost.

The most effective cooling process in the atmosphere is adiabatic lifting of air. It is the only process capable of producing precipitation in appreciable amounts. It is also a principal producer of clouds, fog, and drizzle. The meteorological processes that result in vertical motion of air are discussed in the following texts. None of the cooling processes are capable of producing condensation by themselves; moisture in the form of water vapor must be present.

Precipitation occurs when the products of condensation and/or sublimation coalesce to form hydrometers that are too heavy to be supported by the upward motion of the air. A large and continuously replenished supply of water droplets, ice crystals, or both is necessary if appreciable amounts of precipitation are to occur.

Adiabatic lifting of air is accomplished by orographic lifting, frontal lifting, or vertical stretching (or horizontal convergence). All of these mechanisms are the indirect results of horizontal motion of air.

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