Earth’s atmosphere is a protective gaseous layer that sustains life by shielding against harmful solar radiation and regulating Earth’s temperature, making it habitable. The atmosphere is structured into distinct layers, each with unique characteristics and temperature profiles. Understanding these regions helps explain our planet’s climate and atmospheric phenomena.
Earth’s Atmospheric Layers
The atmosphere is organized into five primary layers, distinguished by how temperature changes with increasing altitude. Moving upward from the surface, the first layer is the troposphere, which extends up to about 11 to 20 kilometers (7 to 12 miles). In this layer, where almost all weather occurs, temperature generally decreases with height, from an average of around 17°C (62°F) near the surface to approximately -51°C (-60°F) at its top.
Above the troposphere lies the stratosphere, extending up to 50 kilometers (31 miles). Unlike the troposphere, the temperature in the stratosphere increases with altitude, reaching about -15°C (5°F) at its highest point. This warming is primarily due to the ozone layer within the stratosphere, which absorbs high-energy ultraviolet (UV) radiation from the sun.
Following the stratosphere is the mesosphere, which spans from about 50 to 85 kilometers (31 to 53 miles) above Earth’s surface. In this layer, temperature again decreases with increasing height. Beyond the mesosphere, the thermosphere begins, extending from approximately 85 kilometers (53 miles) to between 500 and 1,000 kilometers (311 to 621 miles). Here, temperatures rise sharply due to the absorption of energetic solar radiation, reaching up to 2,000°C (3,632°F). The outermost layer is the exosphere, starting from around 500-1,000 kilometers and gradually merging with outer space. While temperatures in the exosphere can also be very high, the air density is extremely low, meaning few molecules transfer that heat.
The Mesosphere: Our Planet’s Coldest Layer
Among Earth’s atmospheric layers, the mesosphere stands out as the coldest, with temperatures plummeting significantly with increasing altitude. At its uppermost boundary, known as the mesopause, temperatures can drop to extreme lows, averaging around -90°C (-130°F), making it the coldest region.
Several factors contribute to the mesosphere’s frigid conditions. One reason is the diminishing absorption of solar radiation. While the stratosphere below is warmed by the ozone layer, the mesosphere contains very few gas molecules like ozone to efficiently absorb incoming sunlight. Furthermore, the mesosphere experiences efficient radiative cooling. The sparse molecules present in this layer effectively radiate heat away into space. Low atmospheric pressure and density mean fewer molecules are available to retain any absorbed heat, further contributing to the extreme cold.
What Happens in the Mesosphere?
Despite its extreme cold and thin air, the mesosphere is a region of notable atmospheric phenomena. It serves as a protective shield for Earth, as most meteors that enter our atmosphere burn up within this layer. As these space rocks hurtle through the mesosphere, they encounter enough gas molecules to create significant friction, generating intense heat that causes them to glow and disintegrate.
Another fascinating occurrence in the mesosphere is the formation of noctilucent clouds, also known as polar mesospheric clouds. These rare clouds are the highest in Earth’s atmosphere, forming at altitudes around 76 to 85 kilometers (47 to 53 miles). Composed of tiny ice crystals that form on minute dust particles, possibly from micrometeors, they become visible during twilight hours when the sun illuminates them from below the horizon while the ground is already in darkness. The extremely cold temperatures within the mesosphere are conducive to the formation of these shimmering, blue or silvery clouds.