Chapter 25 Overview

As you learned in chapter 24, the earth's atmosphere can be described by its mass, density, composition, and structure. These terms describe the condition, or state, of the atmosphere at a given time. They describe aspects of the atmosphere that are not expected to change much over a short period of time. Weather, on the other hand, is described by terms that indicate both the present conditions and predicted changes. Some of these terms, in fact, carry with them an expectation of change. Examples of these terms are overcast, rainy, cold front, stormy, and windy. Other weather terms might or might not mean change, for example, clear, cold, hot, humid, dry, and so forth. Sometimes weather changes are slow and gradual, but other times they can be rapid and violent. Usually you listen to or read a weather forecast to find out what types of changes are going to occur (Figure 25.1).

Sometimes you hear weather forecasts with combinations of terms that link together in familiar patterns. For example, during certain times of the year hot and humid weather is often followed by windy and stormy weather, which is then followed by clear and cool weather. When patterns of combinations occur like this it means that there are underlying reasons, that is, cause-and-effect relationships that are producing the observed patterns. These are important relationships that you will want to know, for knowledge about the relationships will help you understand what is happening to the weather at the present time as well as what may happen next.

What causes weather and what causes changes in the weather are the subjects of this chapter. The chapter begins with relationships associated with the weather terms of clear, cloudy, overcast, and rainy. Cloud-forming processes and the origins of precipitation will be explained, with descriptions of why and how clouds and precipitation form. In this section you will learn the type of precipitation that you might or might not expect to receive from the basic types of clouds. You will also learn the atmospheric conditions necessary for clouds to form.

Sometimes the weather changes slowly over several days, but other times rapid changes occur. Some weather terms used to describe predicted rapid changes are windy, rainy, snowstorms, and severe storm warnings. Weather in North America is the most changeable in the world, and the section on weather producers will explain why these changes take place, including the major changes of thunderstorms, tornadoes, and hurricanes.

The section on weather forecasting describes how computers are used to predict the coming weather, problems with making such predictions, and why any prediction beyond three weeks is usually no better than a wild guess. Finally, the average weather for a location, the climate, will be described along with what factors are responsible for producing the climate where you live.