Seasons and Climate


Outline

  1. The Reasons for the Seasons
  2. Circulation of the Atmosphere
  3. Continents and Oceans
  4. Koeppen Climate Classification System
  5. References
  6. Review Questions

Reasons for Seasons

The Earth rotates along its axis, which is tilted about 23.5 degrees. Throughout the year (or throughout its orbit), the earth maintains the same angle of inclination, parallel to itself at all points in its orbit. This means that when the Earth rotates to different locations on its orbit around the sun, there are different parts of Earth that are tilted toward the sun.

Alone, this does not explain why we have seasons. The sun emits rays, which hit the Earth's surface at different angles. When these rays strike the earth at a perpendicular angle (90 degrees), a high level of energy is transferred. Hence, the locations where these so-inclined sun rays hit tend to be the hottest places on the planet. Other locations, where the sun's rays hit at lesser angles, tend to be cooler. When it is summer in North America, it is because the North Pole and the rest of the Northern Hemisphere are pointing toward the Sun. Because we are pointed toward the Sun longer each day, it seems like the Sun appears more in the sky longer. When we spend more time in the sunshine and less in the shadow of night, our part of the Earth gets warmer, and we experience summer.

There are four major lines of latitude that are related to the inclined sun’s rays and the changing of the seasons. The first line of latitude is at 23.5 degrees North of the equator, in which the Tropic of Cancer exists. This latitude is the farthest north on the Earth that the sun’s rays will be directly overhead. Reciprocally, at 23.5 degrees South latitude, lays the Tropic of Capricorn. Then, there are two “circles” to recall are the Arctic Circle, which lies at 66.5 degrees North latitude. There is also the Antarctic Circle, which lies at 66.5 degrees South latitude. Two important dates concerning the Tropics are June 21st and December 21st. On June 21st, the sun's rays hit the earth perpendicular at 90 degrees upon the Tropic of Cancer. This day is known as the Summer Solstice. The same happens on December 21st at the Tropic of Capricorn.

  

Circulation of Atmosphere

Equalization of temperature is air that circulates through the atmosphere within cold and warm fronts. With the help of these fronts, precipitation reaches areas further away from water masses.

The Hadley-Ferrell Model was a particular atmospheric model that incorporates three counter-rotating, compensating cells for the north and south hemispheres. These cells consist of moving air masses, which eventually create rainfall and other forms of weather. From north to south, they turn as follows: counterclockwise, clockwise, counterclockwise, clockwise, counterclockwise, and clockwise. As you can tell, starting at the equator is the Hadley cell in which warm air converges and rises, which formulates a low pressure. Then there is the Ferrel cell, which consists of high pressure and descending air. Finally is the Polar cells that correspond to the North and South Poles, which are very cold with high-pressure, hence the Polar front.

This is the Hadley-Ferrel Model.

Continents and Oceans

Land/Water Differences--were the world to consist of one all-encompassing, continuous land mass, precipitation/dryness patterns would run true to lines of latitude instead of just partially conforming to these lines as they actually do. Why? Land heats up and cools down faster than water. This explains why temperatures are so extreme here in Nebraska (landlocked, with no ocean to absorb heat.) and so moderate in places like San Diego (with the Pacific to absorb heat).

Ocean Currents--as it is with air in the atmosphere, water circulates across the planet; these movements are called currents. The Gulf Stream, an ocean current consisting of largely warm water, contributes to the mild climate of Western Europe because it flows past the western part of this continent.

Circulations of ocean currents are much like the circulation of air. Warm ocean currents are warm water from the tropics releasing energy as it reaches other area. Cold ocean currents are cold water moving from higher latitudes to the equator. They absorb energy received in the tropics thus cooling the air above. A distinct correlation between the pattern of ocean currents and the air circulation above them can me made.

Koeppen Climate Classification System

The Koeppen Climate Classification System was developed by Wladimir Koeppen and Rudolf Geiger. The Koeppen Climate Classification System is the most widely used for classifying the world's climates. Most classification systems used today are based on the one introduced in 1900 Koeppen. Koeppen divided the Earth's surface into climatic regions that generally coincided with world patterns of vegetation and soils. The Koeppen system recognizes five major climate types based on the annual and monthly averages of temperature and precipitation. Each type is designated by a capital letter.

The first class of climates, Humid Tropics (A), is known for its high temperatures year-round and for their large amounts of perennial precipitation. These regions are found near the equator. As seen on the map below, the Humid Tropics stretch between the Northern parts of South America all the way to South Asia.

 

The second class, Desert climates (B), is characterized by little precipitation and huge daily temperature range. These climates are dry and can be harsh to tolerate. The Sahara is part of this arid climate and can be seen on this map below. Along with the Sahara are also parts of Southern Africa, Eastern North America, Southern South America, parts of Asia, and most of Australia.

 

In the case of the third class, the Maritime Climate (C), the aforementioned land/water differences come into play. The Eastern seaboard of the U.S. would be an example of this class, with cool, wet winters and warm, dry summers. Below more Humid Middle Latitudes or C climate can be see across many of the B climates.

The Continental climate (D) is next. Land masses in this class of climate can be found in the interior regions of land masses of exceptional size (i.e. Omaha, in the middle of the U.S.A.). Total precipitation is not very high in amount, and seasonal temperatures vary somewhat wildly (i.e. Omaha). 

Finally, we come to the climates which are simply cold. Called Cold Climate (E), these climates are often part of areas where permanent ice and tundra are present. Here, perhaps four months of the year experience average temperatures above freezing.  Most of these areas are in Northern North America and Northern Russia.

References

Blij, H. (2000). Geography: realms regions and concepts 2000(9th). New York: John Wiley & Sons, Inc.

Gabler, R. (1993). Essentials of physical geography(4th). New York: Harcourt Brace College Publishers.

Hobbs, J. (2006). Essentials of world regional geography(5th). Canada: Thomson Books/Cole

Matt Rosenberg. (2007). About: Geography [Online]. http://geography.about.com/od/physicalgeography/a/koppen.htm [2007, April 14]

Review Questions

1. Which of the following is most important in producing the earth's seasons: A. the rotation of the earth on its axis; B. the 23 1/2deg. tilt of the earth's axis; C. the elliptical orbit of the earth; D. night time radiation; E. the distinction between continental and maritime climates.

2. On December 21 or 22, the first day of winter in the northern hemisphere, the sun is vertically overhead at: A. 23 ˝ deg. N; B. the equator; C. 23 1/2deg. S; D. 45deg. S; E. none of the above.

3. On March 21 or 22, the angle of the sun's rays are perpendicular at: A. 23-1/2N; B. 23-1/2S; C. the tropic of cancer; D. the equator; E. none of the above.

4. The Hadley-Ferrel model describes: A. the formation of thunderstorms; B. the general redistribution of heat from the equator to the polar areas; C. hail storms; D. tornado activity.

5. Differences in the heating and cooling characteristics between land and water cause differences in climate. At 40deg. latitude, land surfaces are _________ in the summer and _________ in the winter compared to ocean areas at the same latitude. A. warmer, cooler; B. cooler, warmer; C. warmer, warmer; D. cooler, cooler.

6. Zones of downward moving air in atmospheric circulation are also: A. zones of convergence; B. zones of high precipitation; C. zones of rotation; D. zones of low precipitation.

7. Maritime climates are characterized by: A. large temperature ranges; B. little precipitation; C. relatively small temperature ranges; D. cold temperatures; E. none of the above.

8. 'D' climates are characterized by: A. little precipitation; B. cold winters and mild to hot summers; C. tundra conditions; D. much precipitation; E. none of the above.

9. 'C' climates are: A. continental in location; B. have very hot summers; C. are maritime in location; D. are characterized by cold winters and mild to hot summers; E. none of the above.

10. 'B' climates are characterized by: A. little precipitation; B. maritime locations; C. equatorial locations; D. much precipitation; E. none of the above.

11. 'A' climates are located: A. on the equator; B. in areas of cold winters and mild to hot summers; C. in maritime areas; D. in tundra areas; E. none of the above.

12. The angle of inclination of the earth relative to its axis is: A. 23 1/2deg. B. 45deg. C. 12 1/2deg. D. 66 1/2deg. E. 0deg.

13. Which of the following environmental types is grouped with the B climates: A. desert; B. tundra; C. steppe; D. rainforest.

14. All of the following are major influences on climate, except: A. the general circulation of the atmosphere as described in the Hadley-Ferrel model; B. seasonal variations in the distance between the earth and the sun; C. the differential heating and cooling of land and water; D. the movement of warm air toward the polar areas; E. the angle of inclination of the sun's rays.

15. Continental climates are characterized by: A. small seasonal variation in temperature; B. high humidities throughout the year; C. uniform precipitation throughout the year; D. mild winters and cool summers; E. great ranges in temperature between winter and summer.

16. Seasons are caused by: A. yearly variation in the distance to the sun; B. a 23 1/2 degree tilt of the earth on its axis; C. ocean currents; D. the rotation of the earth; E. variation in precipitation.

17. The angle of inclination of the sun's rays are perpendicular to the earth at the equator, the Tropic of Cancer and the Tropic of Capricorn on the 21st or 22nd of the following months: A. January, March, September, November; B. January, March, June, September; C. April, June, September, December; D. March, June, September, December; E. none of the above.

18. According to the Koeppen climate classification system, Omaha is in category: A. A; B. B; C. C; D. D; E. E.

19. Seattle, Washington and Rome, Italy are both in which category of the Koeppen climate classification: A. A; B. B; C. C; D. D; E. E.

20. Denver, Colorado and Las Vegas, Nevada are both in which category of the Koeppen climate classification: A. A; B. B; C. C; D. D; E. E.


Submitted by Todd Bonkiewicz on January 26, 1996. Revised by Chelsea Mustoe & Danielle Krejci on April 25, 2007.