Defining Climate

Annual Averages and Variations

Thousands of years ago the United States' climate was a lot different than it is today. To understand this climate change uses the study of climatology. Climatology is the study of Earth's climate and the factors that cause past, present, and future climate changes. Climate describes the long term weather patterns an area.


Data uses to describe an area's climate are compiled from meteorology records, gained from many parts of the world. The data is averaged on a monthly or annual basis for at least 30 years to determine the normals, which then are set as standard values for a location.

Limitations of normals

Weather conditions on any given day might differ completely from normals. Normals are not used to describe usual weather conditions; they are the average values over a long period of time. Normals apply only to the specific place where the meteorological data were collected. Most of this data is collected at airports, which cannot operate without up-to-date, accurate weather.

Causes of climate

There are several reasons for such climatic variations, including differences in latitude, topography, closeness of lakes and oceans, availability of moisture, global wind patters, ocean currents, and air masses.


Different parts of Earth receive different amounts of sunlight due to the being tilted on its axis. The area between 23.5º S and 23.5º N of the equator is known as the tropics. The temperate zones lie between 23.5º and 66.5º north and south of the equator. The polar zones are located from 66.5 º north and south of the equator to the poles.

•Topographic effects

Water heats up and cools down more slowly than land causing large bodies of water affect the climates of coastal areas. Also, temperatures in the lower atmosphere generally decrease with altitude. Mountain climates are usually cooler than those at sea level. There are two climates on both sides of a mountain, the windward side-wet and cool, and the leeward side-dry and warm.

•Air masses

Two main causes of weather are the movement and interaction of air masses. Air masses also affect climate. Air masses have distinct regions of origin, caused by differences in the amount of solar radiation. The priorities of air masses also depend on whether they formed over land or water.