The Earth’s atmosphere is a complex, multi-layered system that acts as a protective shield and regulates the planet’s temperature. These layers are defined primarily by how temperature changes with altitude, creating distinct regions. The mesosphere, derived from the Greek word for “middle,” occupies the third position up, sitting high above the familiar troposphere and the stable stratosphere.
Defining the Mesosphere’s Boundaries
The mesosphere is defined by its vertical location, sandwiched between the stratopause below and the mesopause above. It begins at an altitude of approximately 50 kilometers (31 miles) above the Earth’s surface, which marks the top boundary of the stratosphere. This starting point is remarkably consistent across different latitudes and seasons.
The layer extends upward to the mesopause, which typically lies around 85 kilometers (53 miles) high. This means the mesosphere has a total thickness of roughly 35 kilometers (22 miles). While the exact upper limit can fluctuate slightly with seasonal changes, the range of 80 to 90 kilometers (50 to 56 miles) is generally accepted as the transition point into the next layer, the thermosphere.
Air pressure within the mesosphere decreases dramatically as altitude increases due to the decreasing density of gas molecules. Even at the bottom boundary, the air is already extremely thin compared to sea level. By the time the mesopause is reached, the atmospheric density is so low that the pressure is only a tiny fraction of what it is at the surface.
The Defining Feature: Temperature Profile
The thermal structure is the most distinctive feature of the mesosphere, characterized by a sharp drop in temperature with increasing altitude. This cooling trend is the opposite of the layer immediately below it, the stratosphere, where temperatures rise due to ozone absorption of solar radiation. The temperature begins around the stratopause at approximately -15°C (5°F), and plummets as one ascends.
This rapid decrease in temperature is mainly caused by the decreasing absorption of solar radiation and the increasing distance from the Earth’s surface, a source of thermal radiation. Because the gas density is already very low, there are fewer molecules to absorb the solar energy that penetrates this high.
The mesopause, the top boundary, is consequently the coldest region in the entire Earth’s atmosphere. Temperatures here can reach as low as -100°C (-148°F). This extreme cold marks the point where the temperature profile reverses again, beginning to warm up in the thermosphere above. The low density and the cooling effect make the upper mesosphere an environment unlike any other layer.
Key Phenomena Within the Mesosphere
The extreme conditions within the mesosphere are responsible for several unique, observable atmospheric events. Most notably, this is the layer where incoming space debris, such as meteors, burn up due to friction with the rarefied but still present gases. The resulting fiery streaks, commonly known as shooting stars, are a direct visualization of the mesosphere acting as a protective shield for the planet.
The intense cold at the mesopause also creates the conditions for the highest clouds in the atmosphere, known as Noctilucent Clouds (NLCs). These “night-shining” clouds are composed of tiny ice crystals that form on dust particles, potentially including the “meteoric smoke” left behind by ablating meteors. They are most often seen at high latitudes during twilight because their high altitude allows them to reflect sunlight after the sun has set below the horizon.