Extreme temperature fluctuations in the deserts and very low on some islands. physical explanations.
Deserts like the Sahara, Kalahari or the great Australian desert are the site of very strong thermal amplitudes over a day.
While the temperature can reach about fifty degrees under a scorching sun in the Sahara in a city like Djanet in Algeria, it can drop by about thirty degrees the following night.
How can it be that the temperature in some places, such as. islands in the middle of the sea, hardly varies between day and night, while the temperature variation is so great in other places, e.g. in deserts?
Conduction, convection and radiation
Heat transfer takes place in three ways. Conduction, which corresponds to a transfer of heat in matter from high to low temperatures, convection, which is a transfer of heat associated with a movement of liquid and radiation, which corresponds to a transfer of electromagnetic waves between heated bodies. In order to take into account heat exchanges, it is also necessary to take into account the thermal inertia, which indicates the capacity of a material to store heat.
The heat balance in deserts is strongly dominated by radiation exchanges (coupled to thermal radiation). On Earth, the main source of energy comes from the sun, which is a body whose surface temperature is 6000 ° C and which emits radiation in the visible range.
During the day, in the absence of cloud cover as at any time on the planet’s surface, the desert floor receives and absorbs thermal radiation from the sun. Like any body that is heated to normal temperatures, the heated earth emits mainly in the infrared range and radiates towards the earth’s atmosphere and sky. However, if the Earth’s atmosphere is quite transparent to visible radiation, this is not the case for infrared radiation. The atmosphere prevents some of this radiation from returning to space. It is the famous greenhouse effect that allows our planet to have an average temperature of 15 ° C, which would be -18 ° C in the absence of this phenomenon. During the day, the absorption of solar radiation is much greater than the earth’s emission through the atmosphere, so that the earth is eventually heated to a thin thickness of ten centimeters. At night, the absorption of solar radiation no longer occurs and the earth cools against the sky through the atmosphere.
A more or less transparent atmosphere
The transparency of the atmosphere depends on several parameters. It depends on the radiation considered (visible, infrared), the concentration of certain greenhouse gases (CO2, Methane, Water vapor), its thickness and cloud cover. In the infrared range, it will thus be more transparent the drier the atmosphere and the thinner the atmosphere (for example in height). On the contrary, an atmosphere containing clouds is virtually opaque to infrared radiation. In a desert, humidity can drop to very low levels in the summer, down to a few percent relative humidity. Under these conditions, the atmosphere is almost transparent, so that the earth exchanges directly with the space, whose temperature is only a few degrees above absolute zero. The conditions are then met for a very significant radiation cooling.
Other parameters may increase this phenomenon further. First of all, the thermal emission of the earth depends on its composition. Sand and snow thus emit much more than vegetation, especially that which forms plains and forests. On the other hand, the presence of wind increases the exchanges between the air and the earth.
We are then in the presence of a heat transfer by convection. If the air during the night becomes warmer than the earth, the latter will heat the earth and therefore reduce its cooling. On the contrary, the absence of wind will reduce convection transmissions and lead to very favorable cooling conditions.
In addition, the sand that makes up the soil of the desert has a relatively low thermal inertia. In other words, the amount of heat stored by sand heated to a certain temperature is lower than for a material such as soil, moisture, concrete or water. For a given cooling, the lower the amount of heat stored, the faster the evacuation of the heat will take place, leading to a sudden drop in temperature during the night. The low humidity of the deserts, their absence of cloud cover and their composition of a very emissive material with low thermal inertia are thus responsible for the strong nocturnal cooling and the large thermal amplitude observed in these spaces.
The foregoing considerations also make it possible to explain other situations where the daily thermal amplitude can be very significant. The village of Mouthe (Doubs) in the Jura massif thus has the record for cold (-36.7 ° C) and daily thermal amplitude in France (37.8 ° C). Its location in winter on a snow-covered plateau makes it a place that emits strongly into space when the humidity is very low. This plateau configuration is also more favorable for the absence of wind, in contrast to mountain towns, where temperature differences at varying altitudes trigger convection currents that attenuate the cooling of the earth.
Conversely, islands like Ouessant, which experience less than one day of frost per day, are known to be places of low thermal amplitude. Ouessant has a very windy, cloudy climate and is located in the middle of the sea. The sea, whose temperature is uniform over a large thickness, has a high thermal inertia. Its temperature hardly varies during the day. In the end, the conditions for strong nocturnal cooling (absence of wind, clear night, low humidity and low thermal inertia) are almost never met. All of these elements that limit radiation cooling mean that the temperature on the islands and more generally by the sea varies much less during the day than inland.
Karl Joulain, Professor of Physics and Energy, University of Poitiers
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