An atmosphere is the layer of gases surrounding a planet that is held in place by its gravity.
We tend to think of our atmosphere as enormous, however, when seen relative to the size of our planet
(as the image on this screen shows) it is actually surprisingly thin (~100 km compared to the ~6,000 km radius of the earth).
Similarly, the atmosphere seems monolithic with possibly a vague notion of the “air getting thinner” as we go higher.
In reality, the atmosphere is a dynamic and complex structure that varies in temperature, pressure and composition
vertically and horizontally as well as across daily and seasonal time cycles. The variances, however, generally average
out over the span of decades, allowing us to describe both a normal physical structure and normal ranges for conditions (i.e. climate).
Scientists have discovered through a variety of methods, however, that the chemical and temperature composition of the atmosphere
can change dramatically over time, due to both natural causes and, as measured over the past century, human activity.
Humans have often underestimated our ability to influence and change what at the time seemed big enough to withstand our worst efforts.
What we have come to realize, just as we have previously with the oceans and resources such as soil, clean water and forests,
is that something as seemingly vast as the atmosphere can be negatively impacted by humans. Anthropogenic activities such as
the wide scale use of fossil fuels have started to significantly change the composition of this thin life-supporting shell.
To help us understand the role the atmosphere plays in our lives, and vice versa, let us take a look at its structure and composition.
Troposphere: While comprising less than 20% of the total volume of the atmosphere,
this region contains 75% of its mass and 99% of its water vapor. Movement in this region is strongly influenced by the
differential heating of the earth’s surface as well as the rotation of the earth (which leads to a general west to east flow).
The billions of tons CO2, and other greenhouse gases, produced each year enter and primarily stay in this region.
Stratosphere: Containing about 20% of the atmosphere’s mass and with a thickness of
about 30 km, the stratosphere’s most important feature is the “ozone layer” which resides in the lower portion of this region
between 20-30km and protects us from harmful UV radiation.
Mesosphere: Found in a region that is too high for planes and too low for satellites,
this region is relatively unexplored and less understood compared to its higher and lower neighbors.
Thermosphere: This region is most easily characterized by the steadily rising temperatures
that arise through the absorption of solar radiation, particularly in the x-ray and UV portions of the spectrum.
The region is also highly electrically charged which allows radio waves to be refracted and therefore be received beyond the horizon.
Temperature Information:Temperature varies widely (from -100 to 2000C) over the vertical extent of the atmosphere. The specific temperatures in a given region are determined by its composition and its interaction with solar radiation and the earth.
Troposphere: The general trend of decreasing temperature stems primarily from the earth being its major heat source. Because the earth can transfer its heat energy more efficiently to the air closest to its surface, that region is heated more than those farther away.
Stratosphere: The increase in temperature in the stratosphere is due to the interaction between ozone and incoming UV radiation. As the energy of the incoming UV photons is absorbed by the molecules of ozone, the molecules are heated up and release additional infrared radiation that heats the surrounding atmospheric gases.
Mesosphere: The mesosphere gets colder as you go higher driven by the decreasing density which provides less and less mass to absorb energy from incoming solar radiation.
Thermosphere: The very high temperatures found in the thermosphere (up to 2,000C) are due to its location at the top of the atmosphere where it bears the full onslaught of solar radiation. Though the temperature of individual molecules may be very high, the temperature you would feel would be extremely cold at this height due to the thinness of the air greatly limiting the transmission of heat to your body.
Though you do not notice it, there is a considerable force pushing down on you from the 100km of atmospheric gases that sit above you. At sea level, the column of air above a square centimeter has a mass of about 1kg and exerts a force of about 10N (or 14.7psi). Aside from when you fly (until you can equilibrate you ears), you usually do not feel this force as it is equally distributed over your entire body and your internal cavities are pressurized to the same level, so there is no net force (hence no perception). As you move up in the atmosphere there is more of the mass below you and less above you, leading to a decrease in the pressure it exerts. As most of the mass of the atmosphere is in the first 20km, the pressure measured as you increase in elevation decreases quickly until it starts to asymptote to zero near the middle of the stratosphere.
Though the earth’s atmosphere has a total mass of 5 quadrillion tons, that mass is not equally distributed. The atmosphere generally becomes less dense (mass per volume) as altitude increases and the gravitational pull of the earth becomes weaker (the force of gravity decreases by the square of the distance). Other factors, such as temperature and humidity can have smaller influences on density. For example, molecules at higher temperatures move faster and therefore bounce of each other and spread out more (hence lower masses per volume) than colder, slower, less energetic molecules that pack in closer to each other. We see this on hot days when hot air near the ground becomes less dense leading to the mirage effect at small scales, and updrafts and wind at larger spatial scales.
Though when found near the ground ozone is a major constituent of anthropogenically produced air pollution, it is naturally found in a thin layer in the upper atmosphere. This “ozone layer” acts as a life-saving shield against parts of the UV spectrum which could otherwise be absorbed by DNA in our cells, damaging its structure and possibly leading to diseases such as cancer when the damage is incorrectly repaired. Manmade chemicals such as chlorofluorohydrocarbons (CFCs) damaged this layer in the later part of the 20 th century, but a worldwide ban on their use in 1987 has allowed the layer to start recovering.