Diurnal Cycles | Research & Encyclopedia Articles

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Diurnal Cycles

Diurnal cycles refer to patterns within about a 24-hour period that typically reoccur each day. Most daily cycles are caused by the rotation of the earth, which spins once around its axis about every 24 hours. The term diurnal comes from the Latin word diurnus, meaning daily. Diurnal cycles such as temperature diurnal cycles, diurnal tides, and solar diurnal cycles affect global processes.

A temperature diurnal cycle is composed of the daily rise and fall of temperatures, called the daily march of temperature. The daily rotation of the earth causes the progression of daytime and nighttime, thus controlling the air temperature. The daily maximum temperature occurs between the hours of 2 P.M. and 5 P.M. and then continually decreases until sunrise the next day. The angle of the Sun to the surface of the earth increases until around noon when the angle is the largest. The intensity of the Sun increases with the Sun's angle, so that the Sun is most intense around noon. However, there is a time difference between the daily maximum temperature and the maximum intensity of the Sun, called the lag of the maximum. This discrepancy occurs because air is heated predominantly by reradiating energy from Earth's surface. Although the Sun's intensity decreases after 12 P.M., the energy trapped within the earth's surface continues to increase into the afternoon and supplies heat to the earth's atmosphere. The reradiating energy lost from the earth must surpass the incoming solar energy in order for the air temperature to cool.

Diurnal tides are the product of one low tide and one high tide occurring roughly within a 24-hour period. Tides in general result from the relationship of the earth's gravitational attraction to the Moon and Sun. Additionally, the motion of the earth and Moon as well as the geometric relationship of the earth's location to both the Moon and Sun affects the tides. More specifically, the tilted axis of the earth in relation to the plane of the Moon plays an important role in creating diurnal tides. Typically, diurnal tides are weak tides that occur when the Moon is furthest from the equator. Diurnal tides occur in the northern part of the Gulf of Mexico and Southeast Asia.

The earth experiences varying hours of daylight due to the solar diurnal cycle. Solar diurnal cycles occur because the earth's axis is tilted 23.5 degrees and is always pointed towards the North Star, Polaris. The tilt of the earth in conjunction with the earth's rotation around the Sun affects the amount of sunlight the Earth receives at any location on Earth.

On March 21 or 22, the vernal equinox occurs where every location on Earth experiences exactly 12 hours of daylight. On this first day of spring, the Sun rises in the east and sets in the west. After the vernal equinox occurs, the Sun shifts north with each day until the day of the summer solstice on June 21 or 22. On this first day of summer, the Sun is as far north as possible and the Northern Hemisphere experiences its longest day of the year, while the Southern Hemisphere experiences its shortest day of the year. Over the next three months, the Sun moves progressively south and the days begin to get shorter in the Northern Hemisphere. On the day of the autumnal equinox, for the second time a year, the Sun rises due east and sets due west. On this first day of autumn, September 21 or 22, there are exactly 12 hours of daylight everywhere on Earth. After the autumnal equinox, the Sun continues to move south with each day and there continues to be less daylight with each passing day in the Northern Hemisphere until the winter solstice. On this first day of winter, December 21 or 22, the Sun has shifted as far south as possible and the Northern Hemisphere experiences its shortest day of the year while the Southern Hemisphere experiences its longest day of the year.