Extreme temperature variations in the deserts and very low on some islands. physical explanations.
Deserts such as the Sahara, the Kalahari, or the great Australian desert are the place of very strong thermal amplitudes over a day.
While the temperature can reach around fifty degrees under a blazing sun of the Sahara in a city like Djanet in Algeria, it can drop by around thirty degrees the following night.
How is it that in some places, such as islands in the middle of the ocean, the temperature hardly varies between day and night, while in others, such as deserts, the temperature variation is so strong ?
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 fluid and radiation, which corresponds to a transfer of electromagnetic waves between heated bodies . To take heat exchange into account, it is also necessary to take into account the thermal inertia, which designates the capacity of a material to store heat.
The heat balance in deserts is strongly dominated by radiative exchanges (linked 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 point on the planet's surface, the desert floor receives and absorbs thermal radiation from the sun. Like any body heated to ordinary temperatures, the heated ground emits mainly in the infrared range and radiates towards the Earth's atmosphere and the sky. However, if the Earth's atmosphere is quite transparent for visible radiation, such 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 emission of the ground through the atmosphere, so that in the end the ground heats up to a small thickness of ten centimeters. At night, the absorption of solar radiation no longer takes place and the ground cools towards 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 vapour), its thickness and the cloud cover. Thus, in the infrared, the drier the atmosphere and the thinner the atmosphere (for example at altitude) the more transparent it will be. On the contrary, an atmosphere containing clouds is practically opaque to infrared radiation. In a desert, the 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 ground exchanges directly with space whose temperature is only a few degrees above absolute zero. The conditions are then met for a very significant radiative cooling.
Other parameters can further increase this phenomenon. First of all, the thermal emission of the ground depends on its composition. Sand and snow thus emit much more than vegetation, in particular that constituting plains and forests. On the other hand, the presence of wind increases the exchanges between the air and the ground.
We are then in the presence of a heat transfer by convection. If during the night the air becomes warmer than the ground, the latter will warm the ground and therefore reduce its cooling. On the contrary, the absence of wind will reduce transfers by convection and lead to very favorable conditions for cooling.
In addition, the sand constituting 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 earth, damp, concrete or water. For a given cooling, the lower the quantity of stored heat, the faster the evacuation of heat will take place, leading to a sudden drop in temperature during the night. Thus, the low humidity of the deserts, their absence of cloud cover and their constitution of a highly emissive material with low thermal inertia are responsible for the strong nocturnal cooling and the large thermal amplitude observed in these spaces.
The preceding considerations also make it possible to explain other situations where the daily thermal amplitude can be very significant. Thus the village of Mouthe (Doubs) in the Jura massif holds the record for cold (-36,7°C) and daily thermal amplitude in France (37,8°C). Its location in winter on a snowy plateau makes it a site that emits strongly into space when the humidity is very low. This plateau configuration is also more favorable to the absence of wind, unlike that of mountain towns where temperature differences at variable altitudes trigger convection currents that attenuate the cooling of the ground.
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 ocean, whose temperature is uniform over a large thickness, has a high thermal inertia. Its temperature hardly varies throughout 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 these elements that limit radiative cooling mean that the temperature on the islands and more generally at the seaside varies much less throughout the day than inland.
Karl Joulain, Professor of Physics and Energetics, University of Poitiers
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