The Seasons

The Seasons

Principle

All of the earth's weather results from the fact that the atmosphere is unequally heated by the sun. The most significant cause of unequal heating of the atmosphere is because the earth is a sphere and, hence, sunlight does not strike the earth at the same angle at every latitude.

Figure 1 - The Earth's Orbit Around the Sun

The Earth-Sun System

The earth spins counterclockwise when viewed from above the North Pole.
The earth makes one revolution every 24 hours.
The earth averages 150 kilometers (93 million miles) from the sun.
The earth is closest to the sun in January and most distant in June.
The earth tilts 23.5 degrees off vertical.
The Northern Hemisphere tilts toward the sun in summer and away from the sun in winter.
The seasons are marked by the movement of the earth around the sun and, hence, which way the earth is tilted with respect to the sun.

June 22 = Summer solstice = sun is directly above 23.5 degrees North
Sept. 23 = Autumnal equinox = sun is above equator (length of day = length of night)
Dec. 22 = Winter solstice = sun above 23.5 degrees South
Mar. 21 = Vernal equinox = sun again above equator (days = nights)

Figure 2 - Sunlight Reaching Earth at Equinox

Equinoxes

During the equinoxes (March 21 and September 21), a day lasts 12 hours and a night lasts 12 hours at all latitudes.

Sunlight strikes the earth most directly at the equator.

Figure 3 - Sunlight Reaching Earth at the Winter Solstice

Solstices

During the winter solstice (pictured above), the Northern Hemisphere day lasts fewer than 12 hours and the Southern Hemisphere day lasts more than 12 hours.
During the winter solstice, the North Pole has a 24-hour night and the South Pole has a 24-hour day.
During the winter solstice, sunlight strikes the earth most directly at 23.5 degrees South (the Tropic of Capricorn).

During the summer solstice (not pictured), the Northern Hemisphere day lasts more than 12 hours and the Southern Hemisphere day lasts fewer than 12 hours.
During the summer solstice, the North Pole has a 24-hour day and the South Pole has a 24-hour night.
During the summer solstice, sunlight strikes the earth most directly at 23.5 degrees North (the Tropic of Cancer).

End

OK-FIRST Project, Oklahoma Climatological Survey, 100 East Boyd Street, Suite 1210, Norman, OK 73019.
Copyright © 1996-2005 Oklahoma Climatological Survey. All Rights Reserved.
Send comments or questions concerning OK-FIRST to okfirst@mesonet.org

	The Seasons

	Principle	All of the earth's weather results from the fact that the atmosphere is unequally heated by the sun. The most significant cause of unequal heating of the atmosphere is because the earth is a sphere and, hence, sunlight does not strike the earth at the same angle at every latitude.

	Figure 1 - The Earth's Orbit Around the Sun

	The Earth-Sun System	The earth spins counterclockwise when viewed from above the North Pole. The earth makes one revolution every 24 hours. The earth averages 150 kilometers (93 million miles) from the sun. The earth is closest to the sun in January and most distant in June. The earth tilts 23.5 degrees off vertical. The Northern Hemisphere tilts toward the sun in summer and away from the sun in winter. The seasons are marked by the movement of the earth around the sun and, hence, which way the earth is tilted with respect to the sun. June 22 = Summer solstice = sun is directly above 23.5 degrees North Sept. 23 = Autumnal equinox = sun is above equator (length of day = length of night) Dec. 22 = Winter solstice = sun above 23.5 degrees South Mar. 21 = Vernal equinox = sun again above equator (days = nights)

	Figure 2 - Sunlight Reaching Earth at Equinox

	Equinoxes	During the equinoxes (March 21 and September 21), a day lasts 12 hours and a night lasts 12 hours at all latitudes. Sunlight strikes the earth most directly at the equator.

	Figure 3 - Sunlight Reaching Earth at the Winter Solstice

	Solstices	During the winter solstice (pictured above), the Northern Hemisphere day lasts fewer than 12 hours and the Southern Hemisphere day lasts more than 12 hours. During the winter solstice, the North Pole has a 24-hour night and the South Pole has a 24-hour day. During the winter solstice, sunlight strikes the earth most directly at 23.5 degrees South (the Tropic of Capricorn). During the summer solstice (not pictured), the Northern Hemisphere day lasts more than 12 hours and the Southern Hemisphere day lasts fewer than 12 hours. During the summer solstice, the North Pole has a 24-hour day and the South Pole has a 24-hour night. During the summer solstice, sunlight strikes the earth most directly at 23.5 degrees North (the Tropic of Cancer).

	End


	OK-FIRST Project, Oklahoma Climatological Survey, 100 East Boyd Street, Suite 1210, Norman, OK 73019. Copyright © 1996-2005 Oklahoma Climatological Survey. All Rights Reserved. Send comments or questions concerning OK-FIRST to okfirst@mesonet.org