Geography 110 - Physical Geography

Notes on atmospheric moisture and precipitation; and on air masses, fronts and weather patterns

Having discussed earth's energy balance, global patterns of temperature and their seasonal variations, and the general circulation of the atmosphere and oceans, we now move on to a more comprehensive view of global weather patterns and the processes that control them. Before we can discuss these patterns, however, we must first spend some time examining the role of water in the earth's climate system. Because water has such high specific heat, the oceans serve as an enormous reservoir of heat energy and are often referred to as the flywheel of the earth's climate. In addition, much of the energy absorbed at the surface is used for evaporation and converted to latent heat. Evaporation contributes water vapor to the atmosphere; this water vapor generally remains in the atmosphere for a short time before condensation occurs and precipitation eventually sends the water back down toward the surface. The process of condensation itself liberates the latent heat and makes it available as sensible heat. It is this recycled energy that drives many if not most of our weather patterns. The dynamics of weather patterns and storm systems cannot therefore be properly understood until we understand the dynamics of the atmospheric portion of the hydrologic cycle, and in particular the different kinds of lifting mechanisms and the role of latent heat in condensation and precipitation.

Chapter 7 ends with a discussion of cloud types and cloud formation and fog. Chapter 8 picks up with the discussion of air masses, lifting mechanisms and fronts. The remainder of chapter 8 considers a variety of different weather systems. The focus is really on midlatitude cyclones and on thunderstorms, tornadoes, and tropical cyclones. There are frequent references to advances in understanding that have come about with the help of modern technology. Although the book doesn't make too much of a point about this, virtually all of our major winter storm systems are in fact midlatitude cyclones. Note also that the seasonal shift in location of the polar front brings a change in the dominant types of storm systems; in summer we get convectional storms, thunderstorms triggered by passing cold fronts, and occasional tropical cyclones, as well as long dry (but humid) periods dominated by high-pressure systems; in late fall, winter and spring we tend to see much greater influence from midlatitude cyclones developing in the vicinity of the polar front.

Note also that the material on particular kinds of weather patterns, and especially severe storms and tropical cyclones, can be supplemented with an enormous volume of new and current information that is available over the web. Quite a few of these are referenced in the book. The accompanying list of links (go back to the online syllabus for this) provides a sampling of these sources, but this outline is tied primarily to the topics listed in the textbook.

Chapter 7

Chapter 8