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 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. The daily rotation of Earth causes the progression of daytime and nighttime, and the amount of solar insolation (i.e., the amount of sunlight falling on a given area). Insolation fluctuations result in changes in both air and surface temperatures. Except in unusual terrain, the daily maximum temperature generally 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 Earth increases until around noon when the angle is the largest (i.e., the sunlight most direct). 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 Earth's surface continues to increase into the afternoon and supplies heat to Earth's atmosphere. The reradiating energy lost from 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.
Earth experiences varying hours of daylight due to the solar diurnal cycle. Solar diurnal cycles occur because Earth's axis is tilted 23.5 degrees and is always pointed towards the North Star, Polaris. The tilt of Earth in conjunction with the Earth's rotation around the Sun affects the amount of sunlight Earth receives at any location on Earth.
Diurnal cycles are of increasing interest to biologists and physicians. A number of physiological and behavioral functions are correlated to diurnal cycles. For example, the release of the cortical hormones is controlled by adrenocorticotropic (ACTH) from the anterior pituitary gland. The level of ACTH has a diurnal periodicity, that is, it undergoes a regular, periodic change during the 24-hour time period. ACTH concentration in the blood rises in the early morning, peaks just before awaking, and reaches its lowest level shortly before sleep.