Deserts often get very cold at night, primarily because they do not have the two main elements that usually help in moderating temperature: water and dense vegetation. These elements act like a protective blanket, retaining the heat that is absorbed during the day. Without these elements, the stored heat is rapidly radiated into the atmosphere after sunset, causing temperatures to drop dramatically. The comparison can be made to a thin, uninsulated house versus one with thick walls and a good heating system – the former loses heat much faster. To grasp why deserts get cold at night, we must first consider why they get so scorchingly hot during the day. It’s not merely the sun; there are specific reasons why the heat builds up so intensely.
A major factor is intense solar radiation. Deserts are usually situated in regions with clear skies and very little atmospheric moisture. The lack of clouds & humidity allows sunlight, which is full of energy, to directly hit the Earth’s surface without much interference.
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The desert floor experiences a similar, though broader, treatment compared to the magnifying effect of sunlight. There’s nothing to diffuse or absorb a significant portion of the incoming solar radiation before it hits the ground. Darker surfaces, which have a low albedo, also play a role. Although many envision deserts as large areas of light-colored sand, a significant portion of desert surfaces consist of darker rocks, gravel, & compacted soil. Materials with a low albedo, such as darker ones, absorb a significant amount of incoming solar radiation instead of reflecting it.
On a sunny day, wearing a black shirt versus a white one, the black shirt gets much hotter. The absorbed energy turns into heat, which then warms the ground & the air immediately above it. Minimal Evaporative Cooling
Evaporation is a highly effective way for the Earth and its atmosphere to cool down. Water evaporation takes with it a significant amount of heat energy – this is why sweating helps cool us down. By definition, deserts have very little water. This means that almost no evaporative cooling occurs from the land surface or from plants.
In humid environments, the sun’s energy is largely used for evaporating water, which keeps the surface temperature from rising too high. Without this process, all the solar energy directly contributes to heating the ground. When the sun sets, the desert landscape switches from an efficient heat absorber to an equally efficient heat radiator. The rapid release of stored energy is the primary reason for the dramatic temperature drop.
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Terrestrial Radiation Clarified
Just like any warm object, the Earth’s surface constantly emits thermal energy in the form of infrared radiation. This phenomenon is called terrestrial radiation. During the day, the ground absorbs more solar radiation than it emits, leading to a rise in its temperature. In the absence of solar radiation, the ground keeps radiating its infrared energy into the atmosphere during nighttime. The key difference in deserts is the rapid and unobstructed escape of this emitted heat.
A Dry Atmosphere’s Role
A natural greenhouse gas in the atmosphere is water vapor. It’s really good at absorbing outgoing longwave (infrared) radiation from the Earth’s surface, which it then re-emits back to the ground. The process effectively traps heat, slowing the rate at which the surface cools.
Deserts possess extremely low atmospheric humidity by their very nature. There’s very little water vapor available to absorb the outgoing infrared radiation. It’s similar to having a thin, leaky blanket over the ground.
Heat escapes nearly directly into space, with very little being absorbed and re-radiated back. The absence of a ‘thermal blanket’ is a significant factor in the rapid nighttime cooling. Cloud-Free Conditions
Like water vapor, clouds also act as a significant barrier to outgoing terrestrial radiation. The infrared radiation emitted by the Earth is absorbed by clouds, which then radiate some of it back down, similar to a thick blanket.
This explains why cloudy nights are typically warmer than clear nights in many parts of the world. Deserts are characterized by clear skies and little cloud cover. Outgoing heat lacks a cloud layer to intercept it. With little interference, heat can radiate directly from the hot desert floor to the upper atmosphere & space. The clear path is a major factor in the swift and drastic temperature drops after sunset. The absence of water and dense plant life plays an indirect, critical role in the cold desert nights.
Both daytime & nighttime, these elements typically regulate temperature. Water’s High Thermal Capacity
The specific heat capacity of water is remarkably high. A lot of energy is needed to heat water, but it releases a significant amount of energy as it cools down. Imagine the difference in time it takes for a pot of water to boil or cool down versus a metal pan. Large bodies of water, including oceans and big lakes, soak up a lot of solar energy during the day without extreme temperature increases. Nighttime sees these large water bodies slowly releasing the stored heat, which moderates the surrounding air temperature.
By definition, deserts have very little water available in a standing state. There are no significant water reservoirs in deserts to absorb & store heat during the day and release it at night. Rock and sand, dominating the land surface, have a much lower heat capacity compared to water.
The rapid heating & cooling of the land lead to much greater temperature swings. Both evaporative cooling and latent heat play important roles. While we discussed evaporative cooling earlier in the context of daytime heating, reiterating its role in temperature moderation is essential. Evaporation of water absorbs latent heat from the surrounding area.
It not only stops surface temperatures from peaking excessively during the day but also helps to stabilize the overall thermal environment. Even in humid regions without large bodies of water, moisture in the soil and atmosphere causes some sun energy to be used for evaporation, limiting temperature peaks. The ‘thermostat’ effect is largely absent in deserts, resulting in extremely hot surface temperatures. The Heat-Trapping Role of Vegetation
Dense vegetation behaves like an insulating blanket. While absorbing some solar radiation during the day, plants also create shade, which reduces direct sunlight reaching the ground. A thick canopy of leaves and branches is essential for trapping air beneath it during nighttime.
This trapped air, if it contains moisture from plant transpiration, acts as an insulator, slowing the rate of heat radiating from the ground. Deserts typically have very sparse vegetation. The ground is mostly uncovered. A canopy is missing to provide shade or trap warm air.
This means the surface is directly exposed to the clear night sky, letting heat radiate freely into space. The disappearance of this insulating layer after sunset is a major factor in the quick and severe temperature decrease. The rate of temperature changes in deserts is largely influenced by the surface composition. While sand often comes to mind, desert soils are varied, and their specific properties play a role in heat transfer.
The Low Thermal Conductivity of Dry Sand
Dry sand surprisingly has low thermal conductivity. This implies it does not conduct heat efficiently. While the top layer of sand heats up intensely during the day, the heat does not penetrate deeply into the sand.
On a hot beach, the top inch of sand is scorching, but a few inches down, it’s surprisingly cool. The low thermal conductivity of sand works in reverse at night. Radiating away swiftly, the heat in the shallow top layer of sand dissipates. Since heat isn’t efficiently conducted up from deeper layers, the surface cools off rapidly without a steady supply of heat from below.
Even if deeper sand layers retain some warmth from the day, the surface temperature drops sharply. Radiation from Exposed Gravel & Rocks
Deserts feature not only sand but also large expanses of exposed rock, gravel, and compacted soil. Compared to loose, dry sand, these materials often have higher thermal conductivity. These materials absorb and conduct heat more efficiently during the day, heating up to very high temperatures. After sunset, these rocks and gravel, having absorbed substantial heat and being good conductors, begin to radiate it effectively.
They quickly emit their stored heat into the clear, dry night air. Rocks, unlike sand which traps heat deeper, quickly release all their stored thermal energy, contributing to the desert landscape’s cooling. With a low albedo, their dark color means they absorb more heat during the day and have more to release at night. Absence of Organic Matter
Typically, desert soils have very little organic matter. Decaying plant and animal material, which constitutes organic matter, helps to retain moisture in the soil & moderates soil temperature.
It can hold onto some heat and absorb it, slightly changing the soil’s thermal properties. Due to the lack of significant organic matter, desert soils are largely mineral-based, dry, and often loosely packed or rocky. These characteristics, including their mineral-based, dry nature, lead to relatively low heat capacity compared to water-rich soils & a tendency to heat up and cool down rapidly, reinforcing extreme temperature swings. All these factors combine to create what’s known as a high diurnal temperature range – the difference between the daytime high & nighttime low.
Desert environments are defined by this extreme temperature swing. Daily Temperature Variation
Temperatures in many deserts often swing by 20 to 30 degrees Celsius (36 to 54 degrees Fahrenheit) or more over the course of a single day. Particularly in high-altitude deserts, temperature differences can be much more pronounced.
For instance, the Sahara Desert experiences temperatures as high as 40°C (104°F) during the day, which can plummet to near freezing at night. This significant daily temperature range occurs because solar radiation is efficiently absorbed during the day and heat is efficiently radiated back into space at night, without the obstruction of water vapor, clouds, or vegetation. Such a dramatic display of how fundamental heat transfer principles operate in a unique environment is quite striking.
Consequences for Life
The extreme temperature swings have significant impacts on the plants & animals that live in the desert. Living organisms need specialized adaptations to cope with both the scorching daytime heat and the freezing nighttime temperatures. Avoiding the intense daytime heat, many desert animals are nocturnal, becoming active when nighttime temperatures are more tolerable, albeit still cold. In order to withstand the extreme desert environment, plants often develop small leaves, waxy coatings, or deep root systems to minimize water loss.
As temperatures drop quickly, it influences the physical landscape, contributing to thermal weathering where rocks expand and contract due to temperature changes, eventually breaking down. Variability Compared to Humid Environments
A clearer understanding of why deserts are so cold at night comes from contrasting them with more humid regions. In tropical rainforests, temperature fluctuations are generally small, often only 5-10°C (9-18°F). The presence of abundant atmospheric moisture, a dense tree canopy, and the higher heat capacity of water-rich soil all help absorb and retain heat, preventing extreme temperature changes.
The air remains saturated with water vapor, acting as a thick insulating blanket that allows heat to escape slowly. The desert, on the other hand, is the complete opposite. With its dry air, clear skies, and sparse vegetation, there’s nothing to slow down the heat from escaping the surface once the sun goes down. It highlights the powerful moderating influence that water, in all its forms, has on Earth’s climate.
