Review Weather versus Climate

The atmosphere is the central component of the complex, connected, and interactive global environmental system upon which all life depends.

Climate may be broadly defined as the long-term behavior of this entire environmental system, which includes the sun, oceans, ice sheets, solid earth, and all forms of life. Oftentimes, when people talk about climate, they are referring only to the atmospheric component of the climate system. This is the component that we will focus on in this course, but keep in mind that to understand fully and to predict changes in the atmospheric component of the climate system, one must also understand the sun, oceans, ice sheets, solid earth, and all forms of life as well as their influences on the atmosphere.

Weather is the moment-by-moment and place-by-place description of the state of the atmosphere, with events taking place on a small scale in both space and time.

Climate, on the other hand, encompasses the totality of weather effects, accumulated over an extended period of time. Climate is usually described in terms of normals (or averages), standard deviations (or average variability), and extremes of a variety of weather elements including temperature, precipitation, and wind. By international convention, climate norms are computed from averages of weather elements compiled over a 30-year period. Because the Earth contains so many different climate regions, we generally speak about the climate of a particular region as opposed to the climate of the entire planet Earth.

In order to describe the climate of a region, it is necessary to know the extremes as well as the averages. It is one thing to know the mean annual temperature and rainfall of a place, but it is much more meaningful to know how hot it gets in the summer and how cold it gets in the winter; whether the rains tend to fall in thunderstorms or in extended drizzles; whether there are wet and dry seasons; and whether the place is subject to occasional high winds, tornadoes, or hurricanes. In some sense, it is the extremes of weather that determine what types of plants and animals are able to live in a particular climate. Examples of important climate statistics include:

• Average number of days per year with below freezing temperatures
• Average date of last frost
• Frequency of flooding rains
• Frequency of droughts
• Average number of days per year with high temperatures above 100°F
• Timing of precipitation during the year, e.g., (rainy summers, dry winters) or (dry summers, rainy winters) or (even distribution of rain during whole year)

Perhaps the biggest misunderstanding by the general public concerning weather vs. climate is to cite extreme weather events as evidence that something is up with the climate. For example: the debate about global warming seems to get more attention during a prolonged summer heat wave; or after a disasterous severe weather event such as a tornado outbreak or a strong hurricane, you get people saying that the climate is becoming more hostile (possibly due to the evil actions of people). We need to realize that a particular extreme weather event by itself tells us nothing about climate change. Extreme weather events have been noted all through human history and will continue to occur into the future. Changes in climate take place over many years, thus for climate change, there would have to be a measurable change in the average conditions, the frequency of extreme events, or both over a prolonged period.