Diurnal cycle

A diurnal cycle (or diel cycle) is any pattern that recurs every 24 hours as a result of one full rotation of the planet Earth around its axis.^[1] Earth's rotation causes surface temperature fluctuations throughout the day and night, as well as weather changes throughout the year. The diurnal cycle depends mainly on incoming solar radiation.^[2]

Earth's rotation relative to the Sun causes the 24-hour day/night cycle.

Climate and atmosphere

Diurnal variation of air temperature (blue) lag by 3 to 4 hours behind insolation at solar noon (red).

In climatology, the diurnal cycle is one of the most basic forms of climate patterns, including variations in diurnal temperature and rainfall.^[1] Diurnal cycles may be approximately sinusoidal or include components of a truncated sinusoid (due to the Sun's rising and setting) and thermal relaxation (Newton cooling) at night.^[1] The diurnal cycle also has a great impact on carbon dioxide levels in the atmosphere, due to processes such as photosynthesis and cellular respiration.^[3]

Biological effects

Diurnal cycles of light and temperature can result in similar cycles in biological processes, such as photosynthesis in plants^[3] and clinical depression in humans.^[4] Plant responses to environmental cycles may even induce indirect cycles in rhizosphere microbial activities, including nitrogen fixation.^[5]

Semi-diurnal cycle

There are typically three different types of tides: diurnal, which has one high tide and one low tide each day; semi-diurnal, which has two high tides and one low tide each day; and mixed, which has two high tides and two low tides each day with varying heights.

Map of areas with the different tidal cycles.

A semi-diurnal cycle refers to a pattern that occurs about every twelve hours or about twice a day. Often these can be related to lunar tides, in which case the interval is closer to 12 hours and 25 minutes.

See also

• Chronotype
• Circadian rhythm
• Diel vertical migration
• Diurnality
• Diurnal Climate [de]