The air lost from regions of high pressure is restored as an influx of air converges
into the region and descends. As we have seen, descending air is associated with clear
skies. It also becomes warmer by compression as it descends, so regions of high pressure
are associated with high surface temperatures. In winter, the cold surface air may
prevent the complete fall of air, and result in a temperature inversion, with a layer of
warm air over a layer of cold air. Geographical conditions may also trap cool air, as in
Los Angeles, and the photochemical pollutants we know as smog may be trapped
under the warm layer.