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Here side by side we see all of life...and life in its smallest package: a single cell.

Like an intricate fractal, when we zoom in and out, the structures of life don't look

that much different. 

What allows both these structures to survive is a thin layer of protection. The cell's

membrane, or outer barrier, selectively filters what goes in and out of the cell. 

For Earth, we have our atmosphere, or the envelope of gases held around the planet by

gravity, that acts as “the world's biggest membrane.” 

The physician and writer Lewis Thomas most famously made this comparison in 1973. Like

a cell membrane, the atmosphere filters what's allowed in and out, like different wavelengths

of light. 

Thomas marveled at how photographs like the famous “Earthrise” showed the sterile

surface of the moon with the “exuberant” Earth rising in the background. It seemed

miraculous that the Earth was so alive while everything around it seemed so...not.

Most of us take the atmosphere for granted and rarely think about what it's made of

or how it moves or changes. We notice it when bad weather inconveniences us or appreciate

it when enjoying the outdoors. But it's been 5 billion years in the making and it's

how we and every other cell of life are able to survive. 

I'm Alizé Carrère and this is Crash Course Geography.

Intro

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. 

Thanks for watching this episode of Crash Course Geography which was made with the help

of all these nice people. If you want to help keep Crash Course free for everyone, forever,

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