EDITOR’S NOTE: Part 3 of a series
In Part 2 of our journey through Earth’s atmosphere, we explored the troposphere, the layer we call home. It is where clouds form, rain falls, thunderstorms grow, snowstorms develop, and nearly all of the weather we experience takes place. We also discovered that as we climb through the troposphere, temperatures generally become colder until we eventually reach a boundary known as the tropopause.
Today, we are going higher.
Once we cross the tropopause, we enter an atmospheric world that behaves very differently from the one below. Clouds become scarce. The violent vertical motions associated with thunderstorms largely disappear. The air becomes increasingly thin and dry. Perhaps most interesting of all, the normal temperature pattern we learned about in the troposphere begins to reverse.
The stratosphere begins at the tropopause, generally around 5 to 11 miles above Earth’s surface depending on latitude and season and extends upward to approximately 31 miles. Its lower boundary is higher near the equator and lower toward the poles, just as we discussed when exploring the troposphere.
While the troposphere contains most of our familiar weather, the stratosphere is comparatively calm and stable. In fact, the word stratosphere comes from the idea of atmospheric gases being arranged in relatively stable layers, or strata.
But there is something inside this layer that makes the stratosphere incredibly important to every person, animal, and plant living on Earth.
The Ozone Layer
Most people have heard of the ozone layer, but it is sometimes misunderstood as a separate layer of Earth’s atmosphere. It isn’t. The ozone layer is a region within the stratosphere where there is a relatively high concentration of ozone, a molecule made of three oxygen atoms.
Although ozone represents only a small portion of the atmosphere, its role is enormous.
The Sun provides the energy necessary for life on Earth, but it also produces ultraviolet radiation. Too much ultraviolet radiation can damage living tissue. Stratospheric ozone absorbs much of the Sun’s harmful ultraviolet radiation before it reaches Earth’s surface.
In a sense, the stratosphere contains part of Earth’s natural sunscreen.
Without this protection, life on the surface would face much greater exposure to damaging ultraviolet radiation. So, while we may rarely think about something happening 10, 15, or 20 miles above our heads, processes taking place there directly affect life down here.
Remember what we learned about the troposphere. As altitude increases through most of that layer, temperature generally decreases. You might naturally assume that if we kept climbing, temperatures would simply continue getting colder.
The atmosphere has other plans.
Within the stratosphere, temperature generally increases with altitude. This is known as a temperature inversion. Why do you ask? Ozone absorbs ultraviolet radiation from the Sun. That absorbed energy is converted into heat, warming portions of the stratosphere. As a result, the upper stratosphere is warmer than the lower stratosphere.
That temperature structure is also one of the reasons the stratosphere is so stable. Warmer air sitting above colder air discourages the vigorous vertical mixing that is common in the troposphere. Think about a pot of water being heated from underneath. The warmer water near the bottom rises, while cooler water sinks, creating circulation. Our troposphere behaves somewhat similarly because Earth’s surface helps heat the atmosphere from below.
The stratosphere is different. Its temperature structure puts a lid on much of that vertical movement.
That is one reason you will not normally find towering thunderstorms developing throughout the stratosphere. When thunderstorms in the troposphere encounter the stable air near the tropopause, their rising air usually begins spreading horizontally. That creates the familiar anvil shape we discussed in Part 2.
For the most part, the weather we know remains below it. But the boundary is not an impenetrable wall. Extremely powerful thunderstorms can produce overshooting tops that temporarily push above the tropopause and into the lower stratosphere. Large volcanic eruptions can also inject ash and gases high into the atmosphere, sometimes reaching the stratosphere.
When volcanic material reaches this high, its effects can become much more significant than material remaining in the troposphere. Rain and ordinary weather systems can remove particles from the lower atmosphere relatively quickly. Material reaching the dry, stable stratosphere can remain there much longer and, in some cases, influence climate by changing how much solar energy reaches Earth’s surface.
Even though our everyday weather happens below, the stratosphere and troposphere are not completely disconnected. Changes high in the atmosphere can sometimes influence circulation patterns farther down, making the relationship between atmospheric layers another fascinating piece of meteorology.
By the time we reach the stratosphere, atmospheric pressure has dropped dramatically compared with what we experience on the surface. There are simply fewer air molecules as we climb higher.
Commercial passenger jets commonly cruise near the upper troposphere and sometimes around the lower stratosphere, depending on altitude and atmospheric conditions. Flying high allows aircraft to operate above much of the turbulent weather occurring below, but it also creates an environment humans could not tolerate without a pressurized cabin and supplemental life-support systems.
Look out the window of an aircraft at cruising altitude and you may see thunderstorms below you, their brilliant white tops spreading outward beneath the deep blue sky. You are looking down upon much of the weather-producing atmosphere that seemed so enormous when you were standing on the ground.
Perspective changes quickly when you start climbing through the atmosphere. Eventually, at roughly 31 miles above Earth, we reach another boundary called the stratopause.
This marks the transition between the stratosphere and our next atmospheric layer. By this point, we are far above airplanes, ordinary clouds, and nearly everything we associate with day-to-day weather. But something interesting is about to happen again. Remember our temperature pattern: temperatures decreased through the troposphere, then generally increased through the stratosphere.
Once we cross the stratopause, the pattern reverses again. We are about to enter one of the coldest regions anywhere in Earth’s atmosphere.
When I look out my window, I usually see only the lowest few miles of an atmosphere that extends far beyond anything visible from my backyard. The clouds may grab our attention, especially when a thunderstorm is building on the horizon, but there is an incredible world above those clouds that is quietly helping make life on Earth possible.
The stratosphere does not give us the thunderstorms that shake our windows or the snow that covers our roads. It does something less noticeable but incredibly important. Its ozone absorbs much of the harmful ultraviolet radiation coming from the Sun, while its unusual temperature structure creates a stable atmospheric layer above our weather.
It is a good reminder that some of the most important parts of our atmosphere are the ones we rarely see or think about.
We started this series standing on the ground. We traveled through the troposphere, climbed past the highest thunderstorms, crossed the tropopause, and have now traveled roughly 31 miles above Earth’s surface.
And we are not even close to finished.
In Part 4 of Discovering the Atmosphere, we will cross the stratopause and enter the mesosphere, where temperatures plunge, the air becomes incredibly thin, and those streaks of light we call “shooting stars” meet their fiery end.
Until then, keep looking out the window. Sometimes understanding the sky means discovering what lies far beyond what we can see.
