Weather influences our everyday activities, our jobs, and our health and comfort. Many of us pay little attention to the weather unless we are inconvenienced by it or it adds to our enjoyment outdoors. Nevertheless, there are few other aspects of our physical environment that affect our lives more than the phenomena we collectively call “the weather.”

Weather in the United States

The United States occupies an area that stretches from the tropics of Hawaii to beyond the Arctic Circle in Alaska. It has thousands of miles of coastline and extensive regions that are far from the influence of the ocean. Some landscapes are mountainous, and others are dominated by plains. It is a place where Pacific storms strike the West Coast, and the East is sometimes influenced by events in the Atlantic and the Gulf of America (Gulf of Mexico). Those in the center of the country commonly experience weather events triggered when frigid southward-bound Canadian air masses clash with northward-moving ones from the Gulf of America (Gulf of Mexico).

Stories about weather are a routine part of the daily news. Articles and reports about the effects of heat, cold, floods, drought, fog, snow, ice, and strong winds are commonplace. Of course, storms of all kinds are frequently front-page news ▼. Beyond its direct impact on the lives of individuals, the weather has a strong effect on the world economy, influencing agriculture, energy use, water resources, transportation, and industry.

Weather clearly influences our lives a great deal. Yet, it is important to realize that people influence the atmosphere and its behavior as well ▼. Significant political and scientific decisions involve these impacts. Important examples are air pollution control and the effects of human activities on global climate and the atmosphere’s protective ozone layer. So, there is a need for increased awareness and understanding of our atmosphere and its behavior.

Weather and Climate

Acted on by the combined effects of Earth’s motions and energy from the Sun, our planet’s formless and invisible envelope of air reacts by producing an infinite variety of weather, which, in turn, creates the basic pattern of global climates.

Weather is constantly changing, sometimes from day to day and sometimes from hour to hour. It is a term that refers to the state of the atmosphere at a given time and place. Whereas changes in the weather are continuous and sometimes seemingly erratic, it is nevertheless possible to arrive at a generalization of these variations. Such a description of aggregate weather conditions is termed climate. It is based on observations that have been accumulated over many years. Climate is often defined simply as “average weather,” but this is an inadequate definition. To more accurately portray the character of an area, variations and extremes must also be included, as well as the probabilities that such departures will take place. For example, farmers need to know the average rainfall during the growing season, and they also need to know the frequency of extremely wet and extremely dry years. Thus, climate is the sum of all statistical weather information that helps describe a place or region.

Suppose you were planning a vacation in an unfamiliar place. You would probably want to know what kind of weather to expect. Such information would help you select clothes to pack and could influence decisions regarding activities you might engage in during your stay. Unfortunately, weather forecasts that go beyond a few days are not very dependable. Therefore, you might ask someone who is familiar with the area about what kind of weather to expect. “Are thunderstorms common?” “Does it get cold at night?” “Are the afternoons sunny?” What you are seeking is information about the climate, the conditions that are typical for that place. Another useful source of such information is the great variety of climate tables, maps, and graphs that are available. For example, the graph in below ▼ shows average daily high and low temperatures for each month, as well as extremes, for New York City.

Such climate information could no doubt help as you planned your trip. But it is important to realize that climate data cannot predict the weather. Although a place may usually (climatically) be warm, sunny, and dry during the time of your planned vacation, you may actually experience cool, overcast, and rainy weather. There is a well-known saying that summarizes this idea: “Climate is what you expect, but weather is what you get.” The figure below ▼ provides an example.

Weather and climate are expressed in terms of the same basic elements—quantities or properties that are measured regularly. The most important weather elements are (1) air temperature, (2) humidity, (3) type and amount of cloudiness, (4) type and amount of precipitation, (5) air pressure, and (6) the speed and direction of the wind. These elements are the major variables from which weather patterns and climate types are deciphered. Although you will study these elements separately at first, keep in mind that they are very much interrelated. A change in any one of the elements will often bring about changes in the others.

• Weather is the state of the atmosphere at a particular place for a short period of time. Climate, on the other hand, is a generalization of the weather conditions of a place over a long period of time.

• The most important elements—quantities or properties that are measured regularly—of weather and climate are (1) air temperature, (2) humidity, (3) type and amount of cloudiness, (4) type and amount of precipitation, (5) air pressure, and (6) the speed and direction of the wind.

• climate: A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.

• weather: The state of the atmosphere at a given time and place.

• weather elements: Quantities or properties of the atmosphere that are measured regularly and that are used to express the nature of weather and climate.