Earth’s atmosphere is a unique reservoir that forms a protective boundary between outer space and the biosphere, which is where all life exists on Earth’s surface.

Air is really a mixture of gases that are odorless, tasteless, colorless, formless, and blend together so well they tend to act like a single gas.  The recipe close to the Earth's surface is 99% nitrogen and oxygen, slightly less than 1% argon, and a tiny percent of volume comes from minor gases, like carbon dioxide.  But this mostly consistent mixture starts to deviate as we get towards the outer edge of the atmosphere. 

To better study the progression, we can break up the atmosphere into vertical layers or “spheres” using several different characteristics.  The four layers we hear most about are the troposphere, stratosphere, mesosphere, and thermosphere, and they come from studying the atmosphere based on its temperature structure. Each layer has a different starting temperature that decreases or increases as we move towards outer space. Temperature even affects the thickness of the layers. 

The layer closest to the Earth where all weather and most of the air molecules exist -- the troposphere -- can extend out anywhere from 8 to 16 kilometers above the surface, depending on the season or latitude of where you are on the globe.  That sounds kind of arbitrary, but it’s really just physics: when the air molecules are cold, they huddle together, making the air denser and more compact. So in winter or near the poles, the troposphere is thinnest. And it’s thickest where the air molecules spread out, like in warm places at the equator. 

When we move to the stratosphere, the next layer, temperatures tend to be layered and get progressively warmer. Here we have the ozone layer, which is the section of the atmosphere with the highest concentration of ozone, one of the minor gases in our recipe. The ozone layer lets the wavelengths of light conducive to life to pass through, while filtering out those that are harmful, like most of the ultraviolet waves. Absorbing UV rays is what causes temperatures to increase in the stratosphere. 

Then temperatures drop in the mesosphere and increase again in the incredibly hot thermosphere where the few air molecules floating around out there can get to be 1,100 degrees Celsius!  Altogether, the atmosphere extends 480 kilometers above Earth’s surface.

Which sounds like a lot, but the diameter of the Earth is 12,756 kilometers. Compared to that, the atmosphere seems like...the peel of an orange.  This thin layer of gas is so critical for life to exist, which is why it’s so important to talk about early on in our journey into physical geography.  Without the atmosphere, none of the processes we’ll study in the hydrosphere, lithosphere, and biosphere would function.

Energy from the Sun is constantly passing through the different layers of the atmosphere as waves, landing on and being absorbed into the Earth’s surface to provide the heat and warmth for life. 
Really, that “energy” is electromagnetic radiation, or different wavelengths of energy that travel away from the surface of an object. All objects -- the Sun, the Earth, our skin -- are constantly emitting waves of electromagnetic radiation.
Very hot and high energy objects, like the Sun, emit vast amounts of energy as short wavelengths, or solar radiation, in the form of light. Cooler objects, like the Earth, emit much longer heat waves, or terrestrial radiation.  Since there's constant sunlight passing through the atmosphere, we might expect temperatures to keep increasing, like when you're sitting under a blanket and get so hot that you want to throw it off.  Fortunately, the Earth doesn't get swelteringly hot because the Earth and the atmosphere naturally balance the shortwave solar energy that arrives with the longwave energy sent back to space. This is the atmospheric energy budget, which is achieved with three common types of energy transfers.

The first type of energy transfer is that radiation we’ve been talking about, which is more generally the transfer of energy through waves. Like when we warm our hands over a campfire. Insolation, or incoming solar radiation, reaches us by -- you guessed it! -- radiation.  Imagine we’re riding a sunbeam of solar radiation, trying to navigate the atmosphere as it hurtles towards the surface.

The atmosphere protects the Earth by filtering sunlight, so there are quite a few obstacles to make it through.  If our sunbeam has 100 units of radiant energy, most of those units will be intercepted before we make it to the surface. Let’s go to the Thought Bubble.

The outer layer of the Sun is extremely hot thanks to its intense energy, so our sunbeam radiates at short wavelengths.  This radiant energy can pass easily through oxygen and nitrogen molecules because they're basically little tiny windows letting short-wave energy in and out.  But other gases throughout the atmosphere absorb short-wave energy like a sponge filling up with water.  In the stratosphere, ozone is a major obstacle.  And in the troposphere, water vapor in the clouds is the enemy.  Overall, 19 units of radiant energy from our sunbeam are intercepted by absorption.  Other particles in the air are pesky, too!  Dust, smoke, and volcanic emissions scatter radiation and change the direction of the light’s movement without altering its wavelength.  8 units of energy from our sunbeam will get returned to space, while 20 units will get scattered as diffuse radiation but persevere to reach Earth’s surface.

At this point, 53% of our sunbeam is still headed towards the Earth's surface.  Our next obstacles are clouds.  Thick clouds are actually capable of reflecting up to 80 percent of total incoming radiation, like a mirror bouncing the energy back into space.  And even when we make it to the surface, our sunbeam isn't safe -- the ground can reflect short-wave radiant energy too.  Snow and ice have higher albedos and reflect most of the solar energy that hits them, while a black pavement has a low albedo and absorbs all the incoming solar energy. On average 26 units of solar energy are reflected back into space by clouds or albedo on the ground.  If we add it all up, just 27% of our original sunbeam reaches the Earth’s surface without being absorbed, scattered, or reflected!  Thanks Thought Bubble!

So after that rocky journey, 47 units (or so) of radiant energy gets through to the ground as a combination of direct radiation that doesn’t get absorbed, scattered, or reflected, and diffuse radiation that got scattered briefly but still makes it through the atmosphere.   As my personal hero David Attenborough might say, that 47% is just enough for life on our planet. 

Any more and the surface might be too hot for life, like Mercury.  Any less, and it would be too cold for life as we know it.

After being absorbed into the surface, incoming radiation is eventually re-radiated by the. Earth as terrestrial radiation. Here the two other types of heat transfer have a role in moving heat energy away from Earth’s surface, to the atmosphere, and out into space.

Heat is carried upwards from the Earth in convection currents. For example, insolation heats water from the Earth’s surface, which evaporates, becomes water vapor, and condenses into clouds in the troposphere.  As the water vapor condenses and changes from a gas to a liquid, the energy that gets released heats up nearby air molecules.  This is like when water is boiled: convection currents let hot water molecules flow upwards and cool. 

And some heat is actually transferred by conduction, or through actual contact. Like when you go to grab the hot handle of that pot of boiling water.  Heat is transferred to your palm through the physical contact you make with the pot. Conduction is most important in the lowermost layers of air in contact with the ground, but air is actually a pretty poor conductor of heat. So the small amount of heat transferred through conduction ends up being carried further upwards by convection.  So the solar radiation coming in equals the terrestrial radiation, plus convection, plus conduction coming out of the Earth.

It's balanced! The atmosphere actually traps quite a bit of the long-wave terrestrial radiation, re-radiating and reflecting these heat waves back again in a continuous energy exchange. So the atmosphere is actually heated from below.

Certain gases can absorb solar radiation on its way to Earth, but other trace gases like carbon dioxide, methane, water vapor, and nitrous oxide are great at absorbing longwave terrestrial radiation and sending it back to the Earth’s surface. This produces the natural greenhouse effect.  In a greenhouse, the glass lets in insolation but doesn’t allow the warm air inside to escape. These greenhouse gases do the same thing in our atmosphere.  Greenhouse gases get a lot of negative attention, but without the natural greenhouse effect, Earth’s surface would be too cold for human life.

But we're running into problems because human activities -- like burning fossil fuels and massive deforestation -- have increased the concentrations of greenhouse gases. So more heat energy stays in the atmosphere.  This produces a warming trend which upsets the ecological systems of Earth. When the atmosphere energy systems become unbalanced, there’s a cascading effect on other physical and biological processes, from sea levels rising to changing distributions of plants and animals. 

Where we are on the globe plays a big role in how much energy is trapped by the greenhouse effect or allowed to escape. In our previous episode we learned every location on Earth doesn’t get the same amount of solar energy because of how the Earth tilts and moves. The atmosphere ends up emphasizing this imbalance. 

For example, at the equator when the sun is overhead, the incoming radiation only has to get through the vertical thickness of the atmosphere, and a fairly large amount does get through. But at high latitudes, the insolation doesn’t hit head-on and has more atmosphere to make it through. So there’s more opportunity for scattering and reflection.

In theory, if the vertical atmospheric energy budget was all we had, tropical areas would actually get warmer and the Arctic and Antarctic even colder. But this doesn’t happen.  Instead, large horizontal circulation systems -- like ocean currents and wind systems -- move the excess heat the Earth receives at low latitudes to the poles. Soon we’ll see how this energy transfer from the equator to the poles is one of the fundamental driving forces behind the general circulation of the atmosphere around the globe.

In the next several episodes, we’ll explore how the atmosphere and its energy systems are the basic ingredients for weather and climate. Like how the atmosphere ultimately makes it possible for rice to grow in hot and wet places like Vietnam. Or why in cold snowy places like Siberia, houses have steeply pitched roofs.  So much of how humans interact with our environment is shaped by how energy, heat, and water move through the atmosphere.  Far from being a boring blanket of air, the atmosphere is an intelligent, sophisticated shield that performs complex functions to make life viable on our planet. 

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