Chapter Fourteen CHAPTER FOURTEEN Non-frontal Depressions Look up Convergence and Divergence in the Glossary (Appendix 1) Frontal depressions occur most frequently in temperate latitudes It should not be assumed however that all depressions are associated with fronts Formation of non-frontal depressions (See Figure 14.1) Divergence at upper level reduces the total weight of air within a limited area and barometric pressure falls at the surface Surface air flows in towards the area of low pressure with cyclonic circulation Convergence at surface level causes ascent of moist air with consequent increase in cloud and precipitation For this system to remain active the outflow of air at upper level must exceed the inflow at the surface When the inflow of air at the surface exceeds the outflow at height the depression will till up THERMAL DEPRESSIONS Land surfaces heat up rapidly in the sun's rays, whereas the sea surface will change its temperature very little in the same period of time Thus in some areas the distribution of land and sea surfaces results in unequal heating of the atmosphere in the surface layers Thermal depressions are often caused by strong surface heating over islands and peninsulas in summer, or over inland seas and lakes during winter when the land is very cold relative to the water surface A slack pressure gradient gives very little wind and is therefore also favourable for the formation of thermal lows Take the case of an island in the sun on a day when there is very little wind, the general mechanics of formation are as follows: 1.Surface heating causes the air to expand vertically 2.This, in turn, gives high pressure at upper level (Explanation in Chapter 9) Divergence from the upper high causes reduction in the total weight of the air column and a fall in barometric pressure at the surface 4.Warm air over the island readily ascends to replace the outflow at higher level 127 Chapter Fourteen Moist air from over the sea surface flows into the low pressure area and feeds the ascending air 6.The system is further stimulated by the process of condensation which releases atent heat above the level of the cloud base INSTABILITY DEPRESSIONS (POLAR DEPRESSIONS) Instability or polar depressions often form in the cold air to the rear of a depression They are caused by surface heating where a cold polar airstream crosses the relatively warm ocean The Latent Heat released when water vapour condenses to form the convection cloud produces an area of low pressure Most thermal lows give heavy showers or thunderstorms rather than periods of continuous bad weather Beware of sudden violent squalls with large wind shifts and often complete reversal in wind direction in the vicinity of thunderstorms OROGRAPHIC DEPRESSIONS (LEE DEPRESSIONS) An airstream which is obstructed by a mountain range may sweep around the ends of the range Air, does however, tend to "pile up" on the weather side, producing an increase in surface pressure On the lee side an eddy effect tends to reduce pressure Often this produces only a slight ridge of high pressure on the weather side and a slight trough of low pressure on the lee side, but a shallow depression can develop on the lee side A depression which forms in this way can become very intense if conditions are favourable and may continue to deepen as it moves away Later it may become very similar to a frontal depression Orographic depressions tend to be associated with certain geographical 128 Chapter Fourteen locations They often form to the east of the Rockies and are associated with the region near Genoa in northern Italy, when the wind blows southwards across the Alps QUESTIONS Describe the formation of: (a) a thermal depression (b) an orographic depression Describe the formation and weather associated with an instability depression educe pressure Often this produces only a slight ridge of high pressure on the weather side and a slight trough of low pressure on 129