We, on Earth, are exceptionally lucky. While we are very rarely conscious of it, we exist
and are alive today because Earth has an oxygen rich atmosphere that we are breathing right
now. And not only that, but the atmosphere provides safety from the cruel environment
of space, it is visually beautiful, and gives us a place to grow, play and develop. I’m
Alex McColgan and you’re watching Astrum, and today I will unravel the different layers
of Earth’s atmosphere and explain how they are so important to our existence. Let me
start by showing you some of these remarkable videos. If I were to ask you where Earth’s
atmosphere is, I think most people would say quite confidently it was all in this area
here. Surprisingly, this is not actually the case. This video was taken on the ISS, the
International Space Station, which orbits the Earth at a distance of 340km. Earth’s
Exosphere, which depending on who you talk to is the final frontier of Earth’s atmosphere,
starts at roughly 500km and extends up to halfway to the moon!
The layers you are looking at in this video are in fact only the troposphere, stratosphere
and mesosphere. Beyond the mesosphere, but only barely visible are the thermosphere and
then the exosphere. But what is the difference between all these spheres? What do they actually
do? Let’s start right at the base layer, and the layer we are the most familiar with
because we live in it – the troposphere. At only 20km high at the equator, it is the
thinnest of all the atmospheric layers. However, the troposphere is not a consistent height across the globe.
The further towards the poles you go, the lower it reaches. At the mid-latitudes of
the planet it rises to 17km, and by the poles it is only 7km high during winter.
Most commercial airlines in fact try to fly above the troposphere where possible,
as they can void most weather disturbances in the stratosphere, but we’ll get back to that point later. The defining
The further towards the poles you go, the lower it reaches. At the mid-latitudes of
experience of living in Switzerland, this can especially be seen when its 20c at the
base of a mountain but at the top it’s still snow-capped. This disparity in temperatures
is due to the way heat energy is transferred, with conduction, convection and radiation.
Let’s understand how this works. Firstly, heat is emitted from the Sun in all forms
of electromagnetic radiation; most noticeable to us is light. This includes not just visible
light, but infrared light and ultraviolet. Light is simple energy, and this energy turns
to heat when it impacts particles. Dark, non-glossy materials absorb this energy the best, which
is why asphalt of a sunny day can reach temperatures of over 65c. On the other hand, Earth’s
atmosphere is not as good at absorbing this energy, and most of the light passes straight
through. But interestingly, the atmosphere does interact with some wavelengths of light. Particularly
blue wavelengths. This is a process called Rayleigh scattering. Gas molecules
are smaller than a wave of light. So when a light wave hits one, it can only absorb
some of it. All the different colours can be absorbed, but the higher frequency blue
through. But interestingly, the atmosphere does interact with some wavelengths of light. Particularly
blue wavelengths. This is a process called Rayleigh scattering. Gas molecules
seeing is the blue from sunlight being scattered by a gas particle exactly in your direction. This
is why the sky appears blue! Due to the atmosphere’s poor absorption of solar energy, the atmosphere
is not heated very much by the Sun itself, but rather through the two other processes
of heat transfer, conduction and convection. Solids are much more densely packed than gases,
seeing is the blue from sunlight being scattered by a gas particle exactly in your direction. This
easily by the Sun. Light colours and glossy materials are not good absorbing light and
will often reflect it back, snow for instance does this. Dark, matte materials absorb a
lot more of this solar energy and then release the heat back into the atmosphere through
conduction. Conduction is the process of a particle being heated, and releasing this
heat energy by exciting the particles next to it. An easy way to imagine this is a pot
of water on a stove. The hot pot conducts the heat to the water particles around the
lot more of this solar energy and then release the heat back into the atmosphere through
of conduction is why cities are often hotter than rural locations. My capital city of London
is often a few degrees warmer than the UK average on a consistent basis because the
tarmac and concrete from the streets and buildings absorb and release the Sun’s heat easily
into the nearby atmospheric gas particles. But this happens to some degree or another
across the whole world. A surface absorbs the heat of the Sun, and releases it through
conduction into the surface atmosphere. Gases are free moving particles, moving as fast
tarmac and concrete from the streets and buildings absorb and release the Sun’s heat easily
expanding and increasing in volume. This is where convection comes into play. Because
the volume of the gas is increasing, but the amount of particles stays the same, its density
decreases and so the gas rises. As is rises, air pressure decreases and the gas particles
spread out further. Individual particles begin to lose their heat energy through radiation.
Eventually, the surrounding temperature is equal to the gas particle itself, and it can
no longer rise. Instead the particles start dispersing sideways. As the heat is further
expended, the gas becomes cooler and denser, and starts to sink again. This process happens
on a grand scale near the equator, and explains why the troposphere is much higher near there.
The equator is exposed to the most sunlight and so when the air is heated near the surface,
it rises very high. Following the process of convection, it reaches the point of no
longer being hotter than the surrounding air, moves sideways, cools and drops again. Once
at the surface, it fills the gaps created by more rising air near the equator, creating
this circle. This is called the Hadley Cell, and similar cells can be found around the
mid-latitudes and the Polar Regions, just to smaller extents. Because of these cells,
gases from the stratosphere and troposphere do not often mix. These cells are also the
cause of trade winds, tropical rain-belts and hurricanes, subtropical deserts and the
jet streams. Interestingly, because the cooled air is descending around the 30 degree latitudes,
and at this point it’s lost much of its heavier and denser humidity, precipitation
will be much less common in these parts of the world where the dry air descends. Looking
at an average rainfall throughout the year, it becomes very apparent that the Hadley Cells
have a big impact on the weather of our planet.
75% of the total atmospheric
mass of Earth can be found in the troposphere. Its composition is mostly uniform throughout
the planet, being a mix of 78% nitrogen, 20% oxygen, and small amounts of other gases.
have a big impact on the weather of our planet.
75% of the total atmospheric
form of humidity, we can’t see the water vapour, but rather can feel it.
Water vapour we can see comes in the form of clouds. In the atmosphere are very small
particles of dust and salt, known as aerosols. They are much larger than water vapour particles,
and provide a surface for the water vapour to condense on to when the vapour rises and
form of humidity, we can’t see the water vapour, but rather can feel it.
there are enough droplets, a cloud becomes visible. As the cloud becomes saturated with
the cloud droplets, they can merge and eventually fall from the sky in the form of a rain drop.
There are many different types of clouds, and they are formed in slightly different
ways, but this basic explanation will suffice for this video.
As I mentioned, temperature in the troposphere decreases with
height, at a rate of about 6.5c every 1km. Also decreasing with height is air pressure.
Dividing the troposphere from the next atmospheric layer, the stratosphere, is the tropopause,
defined as the point air temperature stops decreasing the higher you go. In fact, just
above the tropopause and going into the stratosphere, it starts to get hotter again. This is because
of a layer in the stratosphere that is better at absorbing solar radiation thus warming
ways, but this basic explanation will suffice for this video.
As I mentioned, temperature in the troposphere decreases with
or O2.
It is very beneficial in the stratosphere,
as it absorbs a lot of dangerous UV light before it reaches us on the ground. The concentration
of Ozone in the Ozone layer is actually only about 10 parts in a million, but that is enough
to protect us. Ozone is formed by UV light splitting a normal O2 molecule into two oxygen
atoms. One of those atoms then combines with some unbroken O2 to form O3. UV light then
breaks up the O3 molecule back into O2 and an oxygen atom. This cycle continues constantly.
or O2.
It is very beneficial in the stratosphere,
to the Earth, the Ozone increasing the temperature the higher you go. At the equator, the stratosphere
starts at 20km and ends at 50km. There is not much in the way of convection in the stratosphere,
the only temporary exceptions being caused by huge storms or volcanic eruption pillars.
When air moves in the stratosphere, it is more often horizontally than vertically.
This is why commercial airliners prefer to fly in the stratosphere where possible,
there are rarely any clouds or weather, and the air isn’t moving about as much. Interestingly,
you may have heard of the world record skydiving jump performed by Felix Baumgartner, he jumped
from a height of roughly 39km, still well within the limits of the stratosphere.
Clouds very rarely form in the stratosphere as the air is so dry. Very occasionally however,
around the poles, nacreous clouds can form. Sometimes these clouds are not even made of
When air moves in the stratosphere, it is more often horizontally than vertically.
after dusk, these clouds are beautiful as they glow brightly with vivid iridescent colours.
This is why commercial airliners prefer to fly in the stratosphere where possible,
effect. After the stratosphere is the middle atmospheric
layer, the Mesosphere. Between these two layers is the stratopause, the region where the air
temperature stops increasing and starts decreasing again. At this point, the air pressure is
only 1/1000th of what you would find on the surface.
The Mesosphere starts at 50km
and ends at roughly 100km. Towards the top of this layer can be found the coldest
places on Earth, where the temperature can drop as low as -143c. Again, there can be
a very rare type of cloud found in the Mesosphere. These are called Noctilucent clouds and they
are again very beautiful. Found between 50° and 70° north and south of the equator, and
around 76 – 85km up, they are the highest type of cloud found on Earth. Like the stratosphere’s
nacreous clouds, they can only be seen while the Sun is below the horizon. They are a type
type of water ice cloud, the difference being they can form with or without collecting on dust particles and are sometimes purely water vapour clouds.
The source of the water and dust this high in the atmosphere are not known with certainty, but could come from micro-meteors
or volcanic eruptions. Water vapour from rockets can also cause Noctilucent clouds, most recently
from the SpaceX rocket, Falcon 9. They really are spectacular, especially when you consider
how high they are. This is a view of them from the perspective of the ISS. Even in this
image, you can visibly see the different atmospheric layers, the troposphere, stratosphere and
mesosphere. Interestingly, the mesosphere is the least understood of all the atmospheric
layers, even referred to as the ‘ignorosphere’, as it is above the flight capabilities of
aircraft, and below the minimum safe orbit of satellites due to atmospheric drag. The
only times measurements can be taken are by rockets and only for only a few minutes as
they ascend to the thermosphere, the next atmospheric level.
As you might have guessed by now, above the Mesosphere at roughly 100 km is the Mesopause.
And as you might have guessed again, the Mesopause is the point where the temperature stops decreasing
and starts increasing again throughout the Thermosphere. In fact, temperature can get
extremely hot in the Thermosphere depending on solar activity, as much as 2,000c. Although
it is so hot here, because of the exceptionally low air pressure at this point, there are
not enough gas particles to transfer much heat so it would still feel well below 0c.
To give you some idea of how little gas there is here, above 160km, molecular interactions
are so low that sound cannot travel anymore. The Thermosphere is the thickest of all the
atmospheric layers, rising from 100km all the way to 600km, but it only contains 0.002%
the Earth’s atmospheric mass. The thermosphere is actually considered by lots of people to
be the start of space. Satellites and even the ISS orbit within the thermosphere. That
is why many satellite’s orbits degrade; eventually burning up in the lower parts of
Earth’s atmosphere, because they were never really in true space and atmospheric drag
was still slowing them down enough that they eventually fell out of orbit. The thermosphere
also serves an important purpose to us on Earth. A lot of the most dangerous and highest
energy electromagnetic waves from the sun are absorbed in the thermosphere, and the
effects of this can be seen as beautiful aurorae in the sky. These high energy waves interact
with particles high in the thermosphere, exciting them and making them glow in many different
colours, the most common on Earth being green. Above the thermosphere is the thermopause,
which can range in height from 500km to 1000km depending on the time of day, amount of solar
activity and a host of other factors. Above the thermopause is the last atmospheric layer,
the exosphere. In the exosphere there are very light gas particles like hydrogen present.
However, due to the distance between these particles, they don’t act as a gas as they
do not collide with other gas particles. They are, however, still gravitationally bound
to the Earth. Near the thermopause is where gas stops behaving like gas due to lack of
collisions, and the top of the exosphere, reaching from 10,000km to half way to the
moon, is where the Sun’s radiation pressure on these gas particles overcomes the gravity
of the Earth. We are now in true interplanetary space. And there we have it! Earth’s atmosphere,
unravelled. I hope you gained some insight and learned something today that you may have
never known before. I love researching these videos, as I always learn something new in
the process. If you liked this video, don’t forget to subscribe, like and share as it
really helps me a lot to make more videos like this in the future. Also, a big thank
you again to those who have donated on Patreon, you can find their names in the description.
Especially Doug Deal, who pledged $50 this episode! So, all the best everyone, and I’ll
see you next time.