Which layer of the atmosphere is the coldest? The mesosphere, which lies between about 50 and 85 kilometers (31–53 miles) above Earth’s surface, is the coldest. Temperatures in the mesosphere can fall to approximately −90°C (−130°F) near its upper boundary. This cooling occurs because gases absorb very little solar energy at these altitudes, while infrared radiation from warmer lower layers escapes to space. Below, we define the main atmospheric layers, compare their temperatures, explain why the mesosphere is cold, and explore practical effects on satellite motion, orbital decay, and meteors that enter the atmosphere.
Atmospheric Layers and Temperature Trends
Earth’s atmosphere is divided into several layers, each with distinct temperature patterns driven by different heating and cooling mechanisms.
Troposphere: Weather and Cooling Aloft
The troposphere extends from the surface to about 8–15 km (5–9 miles), depending on latitude and season. Temperature generally decreases with altitude in the troposphere, because the air is heated from the ground up by absorbed infrared radiation and latent heat released during water vapor condensation. This layer contains most weather phenomena and roughly three-quarters of the atmosphere’s mass.
Stratosphere: Warming by UV Absorption
Above the tropopause, the stratosphere extends to about 50 km (31 miles). Here, temperature increases with altitude due to ozone absorbing high-energy ultraviolet (UV) radiation. The stratosphere is stable and layered, which suppresses vertical mixing and supports commercial-flight cruise altitudes.
Mesosphere: The Coldest Layer
From the stratopause at about 50 km up to the mesopause near 85 km (53 miles), the mesosphere shows decreasing temperature with increasing altitude. At the mesopause, temperatures reach their lowest values in the atmosphere—about −90°C (−130°F)—making the mesosphere the coldest atmospheric layer. Above this boundary lies the thermosphere.
Thermosphere: Hot but Thin
The thermosphere extends from the mesopause to about 600 km (370 miles) or higher. Despite very high measured temperatures (hundreds to thousands of degrees Celsius), the air density is so low that a human would not feel heat; a thermometer would register a low reading because it lacks enough molecular collisions to transfer energy. Temperatures rise with altitude in the thermosphere due to absorption of extreme UV and X‑ray solar radiation.
Exosphere: The Atmosphere’s Gradual End
Above the thermosphere, the exosphere thins into space, with atoms and light gases gradually escaping into the vacuum of outer space. This layer has essentially no measurable temperature in the conventional sense, as collisions between particles are exceedingly rare.
Why the Mesosphere Is the Coldest Layer
The mesosphere is cold primarily because there is little solar radiation to heat it and because it sits above the stratosphere, which blocks much of the UV that could warm higher air. Notably:
- Ozone resides mainly in the stratosphere, so the mesosphere lacks a major UV‑driven heating mechanism.
- Carbon dioxide and other gases emit infrared radiation to space more efficiently than they absorb weak solar energy at these altitudes.
- Atmospheric tides and planetary waves can transport heat horizontally, but they do not prevent the overall cold at the mesopause.
As a result, energy loss by emitted infrared radiation exceeds absorption of solar radiation, producing a persistent cold trap near the mesopause. While short-term warming or cooling can occur due to atmospheric waves and solar-cycle variability, the long-term climatology keeps the mesosphere as the coldest layer of the atmosphere.
Practical Effects of Mesospheric Cold
The cold mesopause has measurable impacts on high-altitude phenomena and human activities.
Meteors and Meteor Trails
Most meteors burn up in the mesosphere due to friction with air, producing bright visible streaks. The cold ambient temperature does not stop ablation but influences the rate at which meteoroids vaporize and the formation of persistent meteor smoke particles that can affect polar clouds.
Noctilucent Clouds
In summer at high latitudes, water vapor lifted to the mesosphere can freeze onto meteoritic dust, forming noctilucent clouds near the mesopause. They are the highest clouds on Earth and become visible when illuminated by sunlight while lower layers are in darkness.
Satellites and Orbital Decay
Even though the mesosphere is extremely thin, residual air density at the upper edge of this layer is sufficient to cause gradual orbital decay for low Earth orbit spacecraft. Cold temperatures and low pressure do not prevent drag; rather, the combination of modest atmospheric density and satellite speed produces friction that must be countered with occasional reboosts.
| Atmospheric Layer | Altitude Range (km) | Typical Temperature Range (°C) | Key Heating/Cooling Mechanism |
|---|---|---|---|
| Troposphere | 0–8 (mid‑latitudes) | ≈15 to −60 | Ground heating, adiabatic cooling |
| Stratosphere | 8–50 | −60 to −0 | Ozone UV absorption |
| Mesosphere | 50–85 | −0 to −90 (coldest layer) | Limited solar heating, infrared cooling |
| Thermosphere | 85–600+ | 0 to >1,000 (hot but tenuous) | Absorption of extreme UV/X‑ray radiation |
| Exosphere | 600+ | Not well defined in everyday terms | Particle escape into space |
How Temperature Changes with Altitude: A Summary
Temperature in the atmosphere does not decline monotonically with height. Instead, each layer responds to different physical processes:
- Troposphere: Temperature decreases with altitude due to surface-driven heating.
- Stratosphere: Temperature increases with altitude as ozone absorbs UV energy.
- Mesosphere: Temperature decreases with altitude, reaching its minimum at the mesopause.
- Thermosphere: Temperature increases sharply with altitude despite the cold feel, because sparse molecules carry less energy.
- Exosphere: Gradual transition to interplanetary space.
Key Takeaways
- The mesosphere (about 50–85 km altitude) is the coldest layer of Earth’s atmosphere, with temperatures around −90°C (−130°F) at the mesopause.
- This cold occurs because little solar radiation reaches the mesosphere and because infrared radiation escapes efficiently to space.
- The mesosphere plays a key role in meteor ablation and in hosting noctilucent clouds that reveal atmospheric dynamics.
- Low Earth orbit satellites experience atmospheric drag even in the tenuous mesosphere, requiring periodic reboosts to maintain altitude.
- Temperature in the atmosphere varies in a non-monotonic way: warming in the stratosphere and thermosphere due to absorption of UV and extreme radiation, cooling in the troposphere and mesosphere.
FAQ
Reader questions
Does the mesosphere protect Earth in any way?
Yes. By burning up most incoming meteors, the mesosphere (together with the lower atmosphere) prevents larger fragments from reaching the surface. It also hosts noctilucent clouds that provide clues about atmospheric circulation and climate at high altitudes.
Can temperatures in the mesosphere fall below −90°C?
Measurements indicate that the mesopause can reach about −90°C to −100°C (−130°F to −148°F) at high latitudes during summer. During winter, the polar mesopause is generally warmer due to dynamical effects, so −90°C represents a typical extreme rather than a universal minimum.
Why do satellites in low Earth orbit eventually fall if the mesosphere is so thin?
Even a very small atmospheric density produces drag over time. The cold temperatures do not eliminate collisions; they only affect how air molecules behave. Over weeks to months, this drag removes orbital energy, causing altitude loss that must be corrected by spacecraft propulsion.
How do scientists measure temperatures in the mesosphere and mesopause?
Researchers use satellite remote sensing (infrared and microwave instruments), lidar, radar, and balloon-borne probes to infer temperatures. Meteors burning up and the altitude of noctilucent clouds also provide indirect evidence for mesospheric conditions.