The Earth’s atmosphere is a complex structure of distinct layers, defined by how temperature changes with increasing altitude. These layers include the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The mesosphere, meaning “middle sphere,” occupies a position directly between the stratosphere and the thermosphere.
Defining the Mesosphere’s Altitude and Boundaries
The mesosphere is the third layer up from the surface and its definition is related to a specific temperature profile. It begins at the stratopause, the upper boundary of the stratosphere, which is typically found at an altitude of approximately 50 kilometers (31 miles) above the Earth’s surface. The layer extends upward to a height of about 85 to 90 kilometers (53 to 56 miles), where the mesopause is located.
This means the mesosphere itself has a vertical thickness of roughly 35 to 40 kilometers (22 to 25 miles). The boundaries are not perfectly fixed, varying slightly based on latitude and season, but the layer is consistently defined by a temperature decrease with increasing height. The stratopause marks the point where the warming trend of the stratosphere comes to an end. Conversely, the mesopause marks the altitude where the atmosphere transitions into the thermosphere, a region where temperatures begin to rise again due to the absorption of solar radiation.
The Extremely Cold Mesopause
The mesosphere cools steadily with altitude, unlike the layer below it. This cooling occurs because there is a diminishing concentration of ozone to absorb solar ultraviolet radiation as altitude increases. Additionally, the air density is significantly lower, meaning there are fewer molecules to absorb and retain heat.
This temperature drop culminates at the mesopause, the upper edge of the mesosphere, which is the coldest naturally occurring place on Earth. Temperatures here can plummet to approximately \(-90^\circ\) Celsius (\(-130^\circ\) Fahrenheit). This extreme cold is partially driven by carbon dioxide in the rarefied atmosphere, which acts to radiate heat efficiently out into space.
The mesopause is sensitive to seasonal changes, with the summer mesopause actually being colder than the winter one due to large-scale atmospheric circulation patterns. Air rising at the summer pole expands and cools significantly, creating the lowest observed temperatures. This boundary layer is sensitive to changes in greenhouse gas concentrations, which can enhance the radiative cooling process.
Key Phenomena of the Mesosphere
The mesosphere plays a substantial role in protecting the planet from incoming space debris. This layer is where most meteors, often referred to as “shooting stars,” burn up due to intense friction with the thinning atmospheric gases. As these fast-moving celestial objects encounter the increasing density of the mesosphere, the compression of air in front of them causes rapid heating and vaporization.
This process transforms meteoroids into visible streaks of light, which is why the mesosphere is sometimes called the “shield of Earth”. Another remarkable phenomenon of this region is the formation of noctilucent clouds (NLCs), also known as polar mesospheric clouds. These are the highest clouds in the atmosphere, forming near the mesopause at altitudes of around 80 kilometers (50 miles).
NLCs are composed of tiny ice crystals that form in the extremely cold environment of the upper mesosphere. They are typically observed during summer months at high latitudes, appearing to glow electric blue or silver just after sunset or before sunrise. This luminosity occurs because the clouds reflect sunlight from below the horizon, illuminating them against the dark sky.