The stratopause is the boundary layer in Earth’s atmosphere that separates the stratosphere from the mesosphere. It marks the altitude where the temperature profile reaches its maximum value within the stratosphere, before the air begins to cool rapidly again. This feature is important for understanding the planet’s vertical thermal structure.
Defining the Stratopause’s Altitude
The stratopause is found at an approximate altitude of 50 kilometers (31 miles) above the Earth’s surface. This height is not uniform and can vary slightly depending on geographical location and the time of year. For instance, it may be found at a slightly lower altitude over the winter pole compared to the summer pole. At this altitude, the air pressure is extremely low, measuring only about one-thousandth of the pressure experienced at sea level.
Temperature Maximum
The temperature at the stratopause represents the maximum point within the atmosphere’s vertical structure up to that altitude. Temperatures here typically hover near the freezing point of water, often recorded around 0°C (32°F). This temperature is remarkably warmer than the air just below it in the stratosphere, which can be as cold as -55°C (-67°F). This warm band of air is a reversal of the typical atmospheric pattern, where air usually cools with increasing altitude. This temperature inversion is a defining characteristic of the entire stratosphere, peaking at the stratopause.
The Role of Ozone in Warming
The stratopause’s temperature maximum is directly attributed to the heating effect caused by ozone molecules in the stratosphere. Ozone is concentrated in this layer and is efficient at absorbing high-energy solar radiation. Specifically, it absorbs most of the Sun’s ultraviolet (UV) radiation, which is energy that would otherwise reach the Earth’s surface.
This absorption process is a photochemical reaction where the UV energy converts ozone molecules into an oxygen molecule and a free oxygen atom. When the oxygen atom then recombines with an oxygen molecule to reform ozone, or when the absorbed energy is simply converted, it releases thermal energy into the surrounding atmosphere. This continuous cycle of formation and destruction converts short-wavelength UV light into heat. The highest concentration of this heat release occurs near the top of the stratosphere, causing the temperature to rise steadily with height until it peaks at the stratopause.
Boundary Between Stratosphere and Mesosphere
The stratopause serves as the definitive upper limit of the stratosphere, marking the transition into the mesosphere above it. Once past this boundary layer, the temperature gradient immediately shifts direction. This change signifies the end of the temperature inversion that characterizes the stratosphere.
In the mesosphere, the air temperature begins to decrease rapidly with increasing altitude, returning to the more typical atmospheric temperature lapse rate. The air above the stratopause cools because there is a significantly lower concentration of ozone to absorb the solar UV radiation and generate heat. This shift makes the mesosphere the coldest layer in the atmosphere, with temperatures plummeting toward the mesopause.