Saturated

So far we have assumed that the rising parcel does not become saturated as it cools. If it does become saturated, then any further adiabatic cooling is partly offset by the resulting condensation, which releases latent heat (Note 7.C). The consequent lapse rate of the saturated parcel is referred to as the saturated adiabatic lapse rate (SALR). This is the second of the two theoretical lapse rates mentioned above for comparison with the measured lapse rate, the ELR.

Clearly, offsetting some of the cooling makes the SALR less than the dry adiabatic lapse rate DALR. The SALR also differs from the DALR in being variable, not fixed, since the amount of latent heat released depends on how much condensation occurs, which in turn is governed by the air's temperature. In practice, values are usually around 6 K/km, which is not far from the lapse rate of the standard atmosphere (Section 1.6), though the SALR is less in warm climates than in cold (Note 7.C). In view of the importance of the DALR and the SALR, they are both represented by appropriate lines on aerological diagrams (Note 7.D).

The difference between the DALR and SALR can be used to explain the foehn effect (Note 7.E). (The German word is pronounced 'fern', and comes from the Latin name of a wind in Rome coming from the warm south.) The effect is the drying and warming observed downwind of mountains, after moist air has been lifted by the mountains to the extent that temperatures fall to dewpoint, cloud forms and there is precipitation (Figure 7.2). The downwind air is relatively dry because rain has been removed from it, and it is warmer because of the latent heat from the condensation of the water which became the rain. The resulting warm dry wind is known as a 'foehn wind' in the European alps, a 'chinook' (i.e. 'snow-eater') east of the Rockies in North America, and a 'zonda' downwind of the Andes, in Argentina. They have also been observed near the Ballany Islands at 67°S in Antarctica. The foehn effect partly explains why temperatures at Bega (on the New South Wales south coast) can be 10 K higher in winter than upwind at Albury, on the other side of the Dividing Range.

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