Diurnal cycles refer to processes or events 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 temperature, diurnal cycles, diurnal tidal, and solar cycles affect global processes.
A diurnal temperature cycle consists of daily increases and decreases in temperature. The daily rotation of Earth causes the progression of daytime and nighttime, and the amount of sunlight falling on a given area (known as solar insolation). Insolation fluctuations give rise to both air and surface temperature changes. Except in unusual terrain, the daily maximum temperature generally occurs between the hours of 2 PM and 5 PM, and then 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 PM, 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 consist of one low tide and one high tide within a 24-hour period. Oceans and the solid earth both respond to tides, although ocean tides are much larger than earth tides.
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 received at any location on Earth.
Diurnal cycles are of interest to biologists and physicians because 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.