How much warmer is the day compared to the night? Or, in other words, how large is the diurnal air temperature range (DTR)? This might seem like a simple question, but the DTR actually varies in surprisingly complex ways across regions and periods. What shapes these variations? What happens to DTR on cloudy days and under clear skies? How does it respond to how wet or dry the land is? And what happens to DTR as the planet warms? We answer these questions in our new study led by Sarosh, published in Geophysical Research Letters. Our goal was to understand the physics behind the DTR using an approach that links the short and long-term variations in DTR to things we can observe and measure, like clouds, sunlight, and surface dryness.
Estimating DTR from changes in atmospheric heat storage
Air temperatures are typically measured at 2 meters above the ground, essentially reflecting the balance between heating and cooling near the surface. This is shaped by how energy is exchanged with the atmosphere just above the surface. To estimate this change between day and night, we modelled the lower atmosphere as a “box” and looked at the rate at which this box is being heated and cooled over the course of a day. We then used a combination of satellite data and flux tower observations to test our approach.
During the day, the sun heats the Earth’s surface, which in turn heats the lower atmosphere – by emitting radiation, but also by the turbulent fluxes of sensible and latent heat. The latter, we derive from the limit of maximum power (see blogpost here). But not all of the solar energy goes into warming the air, some is used for evaporating water (or is stored in the ground, but this flux is quite small). So we would expect that the DTR depends primarily on solar radiation, but is enhanced in the absence of water (Figure 1). By tracking how energy accumulates in the lower atmosphere and is lost over the course of day and night, we estimated the total change in temperature in the lower atmosphere.
This approach results in a simple expression of DTR that requires solely on observed incoming solar energy and water limitation of the surface.
DTR response to clouds and surface dryness
Our approach explains and reproduces the day-to-day variations of DTR to changes in cloud-cover and surface dryness very well. DTR reduces with more clouds. This can be explained by decrease in the atmospheric heat input on a cloudy day due to decrease in solar radiation.
On the other hand, DTR is enhanced by surface dryness. When the surface is wet, a significant portion of incoming energy is used for evaporation, which cools the surface, less heat is added to the lower atmosphere, and thereby limits daytime warming. But when the land is dry, evaporation is suppressed. More of the energy goes directly into heating the lower atmosphere, increasing daytime temperatures and resulting in a higher DTR. As a result, the highest DTR usually occurs on days with clear-skies and dry surfaces (Figure 2).
Explaining Long-Term decline in DTR
One of the most notable climate trends in recent decades is that night-time temperatures have been rising faster than daytime temperatures, leading to a global decline in DTR. Our model helps explain why.
As greenhouse gas concentrations increase, the atmosphere emits more downward longwave radiation, which warms the surface – especially at night when there is no cooling from convection. During the day, however, this additional energy leads to enhanced evaporation, which uses up heat that would otherwise warm the air. This results in a stronger rise in night-time temperatures and a muted rise in daytime temperatures, causing DTR to decrease.
Our theory yields a mean reduction of 0.23 K in DTR per 1 K rise in temperatures, in response to changes in greenhouse gas forcings. This estimate matches well with the long-term decline in DTR in global observations and climate model projections (Figure 3).
So what?
Historically, DTR has been used as a proxy indicator of atmospheric transmittance and solar radiation. Our work offers a physical explanation for this relationship and additionally shows that DTR also holds important information about surface water stress.
This means:
1) the framework we developed can be inverted: instead of using surface conditions to predict DTR, we can now use DTR observations to infer surface dryness. This supports recent advances that estimate evaporation rates using only near-surface meteorological data.
2) It also means one should be cautious when using DTR as a proxy for solar radiation in dry conditions, as the DTR may be large due to reduced evaporation rather than increased sunlight – leading to a potential overestimation of solar input.
To summarize, the diurnal air temperature range (DTR) is more than just a measure of how much warmer the day is than the night. It reflects how energy moves between the land and the lower atmosphere. By linking DTR to changes in energy, our study helps explain how DTR changes across different atmospheric and land-surface conditions.
This also shows that despite the many complexities of weather and climate, much of the complexity in observed DTR patterns can be explained by simple physics.
References
Ghausi, Sarosh Alam, Kaighin McColl, Erwin Zehe, and Axel Kleidon. “Explaining observed daily variations and decadal trends in the diurnal air temperature range.” Geophysical Research Letters 52, no. 14 (2025): e2024GL113595
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