Above the warm stratosphere lies another atmospheric layer that is almost the opposite in character — the mesosphere. Stretching from approximately 50 to 85 kilometres altitude, the mesosphere is the third major layer of the atmosphere counted upward from the surface. It is the least-studied and least-known of the major atmospheric layers, partly because it is too high for aircraft and balloons to easily reach but too low for satellites to orbit through. Its temperature drops dramatically with altitude — going from about 0°C at the bottom to as cold as -100°C at the top, making the mesopause (the boundary at its top) the coldest point in the Earth's entire atmosphere. The mesosphere is where most meteors entering the atmosphere burn up, producing the meteors we see streaking across the night sky. It hosts the highest clouds on Earth, the rare noctilucent clouds visible only in summer at high latitudes. And it is the layer that has been changing most dramatically under climate change — though in counter-intuitive directions.
The Name and Position
"Mesosphere" comes from the Greek "mesos" (middle) and "sphaira" (sphere), reflecting its position in the middle of the atmosphere. It sits between the warming stratosphere below and the warming thermosphere above, but the mesosphere itself cools strongly with altitude. The boundary at the bottom (the stratopause, at about 50 km) is the warmest point in the middle atmosphere. The boundary at the top (the mesopause, at about 85 km) is the coldest point in the entire atmosphere. The mesosphere is therefore a cold sandwich between two warmer layers. The temperature inversion at the stratopause-mesosphere transition reflects the diminishing influence of ozone heating (which dominates the stratosphere); the warming above the mesopause reflects the beginning of EUV absorption by oxygen and nitrogen that heats the thermosphere.
The Temperature Profile
Temperature in the mesosphere decreases at a rate of about 3°C per kilometre — gradually but continuously cooling as you go up. The cause is the lack of significant absorbers of solar radiation at these altitudes. Ozone, which dominated heating in the stratosphere, is too sparse in the mesosphere to absorb much UV. The atmospheric density is too low for substantial heating by collisions. The result is a layer that cools by infrared radiation to space faster than it can be heated by anything else, producing the strong negative temperature gradient. The very low temperatures at the mesopause (-100°C in summer at the polar regions) are among the most extreme low temperatures found anywhere in the atmosphere — even colder than the stratosphere over the winter polar vortex.
Density and Composition
Atmospheric density at the bottom of the mesosphere is about 0.001 of the surface density. At the top, it is about 0.0001. Despite this very low density, the mesosphere still has essentially the same gas composition as the lower atmosphere — primarily nitrogen and oxygen — though some lighter gases (hydrogen, helium) are beginning to become relatively more important. The mean molecular weight starts to decrease slightly with altitude in the upper mesosphere, reflecting the gradual transition toward the "heterosphere" above, where lighter gases dominate the upper layers due to differential diffusion.
Meteors
The mesosphere is where most meteors burn up. Meteoroids — fragments of asteroids or comets ranging in size from grains to boulders — enter the Earth's atmosphere at speeds of 11-72 kilometres per second. Friction with the atmosphere heats them to several thousand degrees, vaporising them and producing the visible glow we call a meteor or shooting star. Most of this happens between 75 and 100 kilometres altitude, which is mostly in the upper mesosphere and lower thermosphere. Approximately 25 million visible meteors enter the Earth's atmosphere each day; most are too small to be seen by the naked eye, but the brighter ones are the source of the meteor showers we observe and the occasional fireballs. About 30,000 tonnes of meteoritic material enters the atmosphere each year, most of which is vaporised in the mesosphere. The dust that remains gradually settles down through the atmosphere over weeks and months.
Noctilucent Clouds
One of the most remarkable features of the mesosphere is noctilucent clouds (NLCs), also called polar mesospheric clouds. These are the highest clouds in Earth's atmosphere, forming at altitudes of about 76-85 kilometres — near the mesopause. They consist of ice crystals that condense onto meteoric dust particles in the extreme cold of the summer polar mesosphere. They appear in summer because that's when the mesopause is coldest (due to atmospheric circulation patterns that bring cold air to the summer pole). They are visible only at high latitudes during summer twilight, when the lower atmosphere is in shadow but the noctilucent clouds high above are still illuminated by sunlight. They appear as wispy, bluish or silvery streaks high in the night sky. Noctilucent clouds were first observed in 1885, two years after the Krakatoa eruption, and may have been initiated by the dust and water vapour the eruption injected. They have become more frequent and more brightly visible over the past few decades — possibly a sign of changing mesospheric conditions driven by climate change.
Climate Change in the Mesosphere
Unlike the warming troposphere, the mesosphere is cooling under climate change. The same greenhouse gases — particularly CO₂ — that warm the lower atmosphere actually cool the upper atmosphere, including the mesosphere. The mechanism: in the dense lower atmosphere, CO₂ absorbs and re-emits infrared radiation, trapping heat. In the thin upper atmosphere, the same CO₂ molecules more efficiently radiate heat to space because they are less crowded by other molecules and their emitted infrared can escape. The net effect is upper-atmospheric cooling of several degrees per decade in the upper mesosphere. The mesosphere is contracting (becoming denser at given altitudes), and the mesopause is dropping. Noctilucent clouds may be increasing in frequency partly because of this cooling, partly because increased methane in the upper atmosphere (oxidised to water vapour) is increasing the water content. The mesosphere is therefore one of the most directly observable indicators of human impact on the atmosphere, showing changes that are larger than those in the troposphere when measured in terms of relative magnitude.
Sounding Rockets
Studying the mesosphere is challenging because it is between the altitudes accessible to aircraft and balloons (which can reach about 30-40 kilometres at the highest) and the altitudes accessible to satellites in stable orbits (which need to be at least 200-300 kilometres up to avoid atmospheric drag). The main tool for direct mesospheric measurement is sounding rockets — small unmanned rockets that fly briefly into the mesosphere on parabolic trajectories, taking measurements during their few minutes of flight. The Norwegian Andoya Space Center and the Swedish Esrange Space Center are particularly active in mesospheric sounding rocket science. Other techniques include lidar (laser radar) from the ground, which can probe the mesosphere by measuring backscatter from atmospheric molecules; satellite limb sounders that look horizontally through the mesosphere from orbit; and radio occultation techniques.
Mesospheric Dynamics
The mesosphere has its own circulation patterns distinct from the troposphere and stratosphere. Atmospheric tides (large-scale daily oscillations driven by solar heating) become important in the mesosphere because there is less mass and damping at these altitudes. Gravity waves — wave-like disturbances generated by mountains, thunderstorms, and other tropospheric features — propagate upward into the mesosphere and break, depositing momentum and driving large-scale circulation. The breaking of gravity waves is one of the major drivers of mesospheric meridional circulation, including the summer-pole-cold/winter-pole-warm temperature pattern that produces the cold mesopause in summer. The dynamics are complex and not fully captured in current models.
Sprites, Elves, and Halos
The upper mesosphere is the site of some of the most spectacular but ephemeral lightning-related phenomena: sprites, elves, and halos. These are large electrical discharges that occur high above thunderstorms, triggered by very large lightning strikes in the troposphere below. Sprites are red, jellyfish-shaped discharges that briefly illuminate the upper mesosphere. Elves are expanding rings of light at the mesopause. Halos are doughnut-shaped flashes above thunderstorms. These phenomena were only documented in the 1990s — they happen briefly (milliseconds) and are usually invisible from the ground because of intervening clouds. They are now routinely captured by high-altitude cameras and aircraft. The energy involved is substantial — sprites in particular can extend tens of kilometres in size — but the phenomena are still poorly understood.
The Mesosphere's Connections
The mesosphere connects the lower and upper atmosphere. Vertical motions through the mesosphere transport material between the well-mixed lower atmosphere and the more variable upper atmosphere. Gravity waves and tides propagating through the mesosphere couple weather systems at the surface with phenomena in the thermosphere and ionosphere above. Even though the mesosphere itself is not directly important to human activity, understanding it is essential for understanding the atmosphere as a whole — particularly the increasingly complex interactions between the atmospheric layers under climate change. The mesosphere is one of the more remote frontiers of atmospheric science, and one where major discoveries continue to be made decades after its basic structure was outlined.