The earth's tilt, rotation and land/sea distribution affect the global weather patterns we observe. While the weather varies from day-to-day at any particular location, over the years, the same type of weather will reoccur. The reoccurring "average weather" found in any particular place is called climate.

German climatologist and amateur botanist Wladimir Köppen (1846-1940) divided the world's climates into several major categories based upon general temperature profile related to latitude. These categories are as follows:

A - Tropical Climates
Tropical moist climates extend north and south from the equator to about 15° to 25° latitude. In these climates all months have average temperatures greater than 64°F (18°C) and annual precipitation greater than 59".

B - Dry Climates
The most obvious climatic feature of this climate is that potential evaporation and transpiration exceed precipitation. These climates extend from 20°-35° North and South of the equator and in large continental regions of the mid-latitudes often surrounded by mountains.

C - Moist Subtropical Mid-Latitude Climates
This climate generally has warm and humid summers with mild winters. Its extent is from 30°50° of latitude mainly on the eastern and western borders of most continents. During the winter, the main weather feature is the mid-latitude cyclone. Convective thunderstorms dominate summer months.

D - Moist Continental Mid-latitude Climates
Moist continental mid-latitude climates have warm to cool summers and cold winters. The location of these climates is poleward of the C climates. The average temperature of the warmest month is greater than 50°F (10°C), while the coldest month is less than -22°F (-30°C). Winters are severe with snowstorms, strong winds, and bitter cold from Continental Polar or Arctic air masses.

E - Polar Climates
Polar climates have year-round cold temperatures with the warmest month less than 50°F (10°C). Polar climates are found on the northern coastal areas of North America, Europe, Asia, and on the land masses of Greenland and Antarctica.

H - Highlands
Unique climates based on their elevation. Highland climates occur in mountainous terrain where rapid elevation changes cause rapid climatic changes over short distances.


The map (below) shows where these major categories occur in the mainland United States.

The major Köppen zones in the U.S.

Take it to the MAX! Learn about further sub-divisions of these climate zones.

The classical length of record to determine the climate for any particular place is 30 years, as defined by the World Meteorological Organization (WMO). The quantities most often observed are temperature, precipitation, and wind.

The "normals" are computed once every 10 years which helps to smooth out year-to-year variations. For example, the current 30-year normals were calculated from the actual weather data that occurred during the 30 years of 1971-2000. In 2011, a new set of 30-year normals will be calculated using the data from the period of 1981-2010. So, when you hear what the normal high and low temperature for your location, for example, they come from these 30-year averages.

Next: Precipitation Types