Evaporation and condensation affect the temperature of an air mass. When cool rainfalls through a warmer air mass, evaporation takes place, taking heat from the air. This often affects the maximum temperature on a summer day on which afternoon thundershowers occur. The temperature may be affected at the surface by condensation to a small extent during fog formation, raising the temperature a degree or so because of the latent heat of condensation.

The surface and aloft situations that are indicative of the onset of cold waves and heat waves are discussed in the following text.

A forecast of a cold wave gives warning of an impending severe change to much colder temperatures. In the United States, it is defined as a net temperature drop of 20F or more in 24 hours to a prescribed minimum that varies with geographical location and time of the year. Some of the prerequisites for a cold wave over the United States are continental Polar, or Arctic air with temperatures below average over west central Canada, movement of a low eastward from the Continental Divide that ushers in the cold wave, and large pressure tendencies on the order of 3 to 4 hPa occurring behind the cold front. Aloft, a ridge of high pressure develops over the western portion of the United States or just off the coast. An increase in intensity of the southwesterly flow over the eastern Pacific frequently precedes the intensifying of the ridge. Frequently, retrogression of the long waves takes place. In any case, strong northerly to northwesterly flow is established aloft and sets the continental Polar or Arctic air mass in motion. When two polar outbreaks rapidly follow one another, the second outbreak usually moves faster and overspreads the Central States. It also penetrates farther southward than the first cold wave. In such cases, the resistance of the southerly winds ahead of the second front is shallow. At middle and upper levels, winds remain west to northwest, and the long wave trough is situated near 80 west. 

Most cold waves do not persist. Temperatures moderate after about 48 hours. Sometimes, however, the upper ridge over the western portion of the United States and the trough over the eastern portion are quasi-stationary, and a large supply of very cold air remains in Canada. Then, we experience successive outbreaks with northwest steering that hold temperatures well below normal for as long as 2 weeks.

In the summer months, heat wave forecasts furnish a warning that very unpleasant conditions are impending. The definition of a heat wave varies from place to place. For example, in the Chicago area, a heat wave is said to exist when the temperature rise above 90F on 3 successive days. In addition, there are many summer days that do not quite reach this requirement, but are highly unpleasant because of humidity. 

Heat waves develop over the Midwestern and eastern portions of the United States when along wave trough stagnates over the Rockies or the Plains states, and along wave ridge lies over or just off the east coast. The belt of westerlies are centered far to the north in Canada. At the surface we observe a sluggish and poorly organized low-pressure system over the Great Plains or Rocky Mountains. Pressure usually is above normal over the South Atlantic, and frequently the Middle Atlantic states. An exception occurs when the amplitude of the long wave pattern aloft becomes very great. Then, several anticyclonic centers may develop in the eastern ridge, both at upper levels and at the surface. Frequently, they are seen first at 500 hPa. Between these highs we see formation of east-west shear lines situated in the vicinity of 38 to 40N. North of this line winds blow from the northeast and bring cool air from the Hudson Bay into the northern part of the United States. A general heat wave continues until the long wave train begins to move.

In this chapter we discussed condensation and precipitation producing processes. Following a discussion on condensation and precipitation producing processes, we then covered condensation and precipitation dissipation processes. Forecasting of frontal clouds and weather was then discussed, including the topics of frontal cloudiness and precipitation, air mass cloudiness and precipitation, vertical motion and weather, vorticity and precipitation, and middle clouds in relation to the jetstream. We then covered short-range extrapolation techniques, which included use of the nephanalysis, frontal precipitation, lowering of ceilings in continuous rain areas, the trend chart as an aid, and the time-liner as an aid. A discussion of cloud layer analysis and forecasting was then presented along with the importance of RAOB use in cloud analysis and identification, the humidity field, a 500-hPa level analysis of the dewpoint depression, a three-dimensional analysis of the moist layer, precipitation and clouds, and cirrus indications. A discussion of the prediction of snow versus rain followed. Topics presented were geographical and seasonal considerations, the physical nature of the problem, general synoptic considerations, forecasting techniques, and areas of maximum snowfall. The last topics of discussion were factors affecting temperature, and the forecasting of temperatures during special situations.

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