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Concept
Diurnal air temperature variation
Résumé
In meteorology, diurnal temperature variation is the variation between a high air temperature and a low temperature that occurs during the same day.
Temperature lag, also known as thermal inertia, is an important factor in diurnal temperature variation. Peak daily temperature generally occurs after noon, as air keeps absorbing net heat for some period of time after noon. Similarly minimum daily temperature generally occurs substantially after midnight, indeed occurring during early morning in the hour around dawn, since heat is lost all night long. The analogous annual phenomenon is seasonal lag.
As the solar energy strikes the Earth's surface each morning, a shallow layer of air directly above the ground is heated by conduction. Heat exchange between this shallow layer of warm air and the cooler air above is very inefficient. On a warm summer's day, for example, air temperatures may vary by from just above the ground to chest height. Incoming solar radiation exceeds outgoing heat energy for many hours after noon and equilibrium is usually reached from 3–5 p.m. but this may be affected by a variety of different things such as large bodies of water, soil type and cover, wind, cloud cover/water vapor, and moisture on the ground.
Diurnal temperature variations are greatest very near Earth's surface. The Tibetan and Andean Plateaus present one of the largest differences in daily temperature on the planet.
High desert regions typically have the greatest diurnal-temperature variations, while low-lying humid areas typically have the least. This explains why an area like the Pinnacles National Park can have high temperatures of during a summer day, and then have lows of . At the same time, Washington D.C., which is much more humid, has temperature variations of only ; urban Hong Kong has a diurnal temperature range of little more than .
While the National Park Service claimed that the world single-day record is a variation of (from to ) in Browning, Montana in 1916, the Montana Department of Environmental Quality claimed that Loma, Montana also had a variation of (from to ) in 1972.
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