Potential Evapotranspiration Precipitation

Figure 4.7 The average annual moisture budget for stations in western, central and eastern Britain determined by Thornthwaite's method. When potential evaporation exceeds precipitation soil moisture is used; at Berkhamsted in central England and Southend on the east coast, this is depleted by July to August. Autumn precipitation excess over potential evaporation goes into replenishing the soil moisture until field capacity is reached.

Source: From Howe (1956). Reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

is estimated from mean temperature. Figure 4.7 illustrates this for stations in western, central and eastern Britain (compare Figure 10.22). In the winter months there is an excess of precipitation over evaporation; this goes to recharging the soil moisture, and further surplus runs off. In summer, when evaporation exceeds precipitation, soil moisture is used initially to maintain evaporation at the potential value, but when this store is depleted there is a water deficiency, as shown in Figure 4.7 for Southend.

In the United States, monthly moisture conditions are commonly evaluated on the basis of the Palmer Drought Severity Index (PDSI). This is determined from accumulated weighted differences between actual precipitation and the calculated amount required for evapotranspiration, soil recharge and runoff. Accordingly, it takes account of the persistence effects of drought situations. The PDSI ranges from >4 (extremely moist) to <-4 (extreme drought). Figure 4.8 indicates an oscillation between drought and unusually moist conditions in the continental USA during the period October 1992 to August 1993.

D CONDENSATION

Condensation is the direct cause of all the various forms of precipitation. It occurs as a result of changes in air volume, temperature, pressure or humidity. Four mechanisms may lead to condensation: (1) the air is cooled to dew-point but its volume remains constant; (2) the volume of the air is increased without addition of heat; this cooling occurs because adiabatic expansion causes energy to be consumed through work (see Chapter 5); (3) a joint change of temperature and volume reduces the moisture-holding capacity of the air below its existing moisture content; or (4) evaporation adds moisture to the air. The key to understanding condensation lies in the fine balance that exists between these variables. Whenever the balance between one or more of these variables is disturbed beyond a certain limit, condensation may result.

The most common circumstances favouring condensation are those producing a drop in air temperature; namely contact cooling, radiative cooling, mixing of airmasses of different temperatures and dynamic cooling of the atmosphere. Contact cooling occurs

U 30

U 30

RAINFALL

_ EXTREMELY MOIST (4 or more)

EXTREME DROUGHT (-4.0 or less)

DROUGHT

octInovIdecI

1992

jan feb i mar i apr 1 may i jun i jul 'aug 1993

octInovIdecI

1992

jan feb i mar i apr 1 may i jun i jul 'aug 1993

RAINFALL

_ EXTREMELY MOIST (4 or more)

EXTREME DROUGHT (-4.0 or less)

DROUGHT

Figure 4.8 Percentage of the continental USA affected by wet spells or drought, based on the Palmer Index (see scale on right), during the period October 1992 to August 1993.

Sources: US Climate Analysis Center and Lott (1994). Reprinted from Weather, by permission of the Royal Meteorological Society. Crown copyright ©.

within warm, moist air passing over a cold land surface. On a clear winter's night, strong radiation will cool the surface very quickly. This surface cooling extends gradually to the moist lower air, reducing the temperature to a point where condensation occurs in the form of dew, fog or frost, depending on the amount of moisture involved, the thickness of the cooling air layer and the dew-point value. When the latter is below 0°C, it is referred to as the hoar-frost point if the air is saturated with respect to ice.

The mixing of contrasting layers within a single airmass, or of two different airmasses, can also produce condensation. Figure 4.9 indicates how the horizontal mixing of two airmasses (A and B), of given temperature and moisture characteristics, may produce an airmass (C) that is supersaturated at the intermediate temperature and consequently forms cloud. Vertical mixing of an air layer, discussed in Chapter 5 (see Figure 5.7), can have the same effect. Fog, or low stratus, with drizzle - known as 'crachin' - is common along the coasts of south China and the Gulf of Tonkin between February and April. It develops either through airmass mixing or warm advection over a colder surface.

The addition of moisture into the air near the surface by evaporation occurs when cold air moves out over a warm water surface. This can produce steam fog, which is common in arctic regions. Attempts at fog dispersal are one area where some progress has been made in local weather modification. Cold fogs can be dissipated

Global Temperature Semi Log Plot
Figure 4.9 The effect of airmass mixing. The horizontal mixing of two unsaturated airmasses (A) and (B) will produce one supersaturated airmass (C). The saturation vapour pressure curve is shown (cf. Figure 2.14B, which is a semi-logarithmic plot).

Source: After Petterssen (1969).

locally by the use of dry ice (frozen CO2) or the release of propane gas through expansion nozzles to produce freezing and the subsequent fall-out of ice crystals (cf. p. 101). Warm fogs (i.e. having drops above freezing temperatures) present bigger problems, but attempts at dissipation have shown some limited success in evaporating droplets by artificial heating, the use of large fans to draw down dry air from above, the sweeping out of fog particles by jets of water, and the injection of electrical charges into the fog to produce coagulation.

The most effective cause of condensation is undoubtedly the dynamic process of adiabatic cooling associated with instability. This is discussed in Chapter 5.

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