I usually like travelling by plane … that is when I manage not to think about all the things that could go wrong 😛 Luckily I’ve recently read a really cool book on air travel, Cockpit confidential by Patrick Smith, which has helped reduce some of my worries. Apart from the images from TV with planes that break in half and people flying around, there are also lots of questions that pop into my head while high up in the clouds. Like how does the temperature vary in the atmosphere? This question popped into my head when I was watching on the screen that map that shows the route and a tiny plane that slooowly moves closer to the destination. The screen also showed the outside temperature of -50 degrees Celsius. After landing safely, instead of properly enjoying my holiday, I looked for a bit online and found an image with the way the temperature of the atmosphere varies depending on height. It looked something like this:
Wait… Whaaat? The image puzzled me even more … I had no idea the temperature varied so much! I knew that it gets colder as you go up and that eventually it has to get hot from the sun, but other than that… So I had to do a proper investigation, after the holiday of course 🙂 Here it is! (Hopefully I got it right! If you do spot some inaccuracies, please let me know! 
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First things first, what is the atmosphere?
The atmosphere is the layer of gases surrounding the Earth and retained by the Earth’s gravity. It absorbs the ultraviolet solar radiation, warms the surface of the earth by heat retention and reduces the temperature extremes between day and night. Air suitable for the survival of plants and animals on Earth (for breathing and photosynthesis) is only known to be found in the troposphere and artificial atmospheres.
The gases in Earth’s atmosphere include nitrogen (78 percent), oxygen (21 percent), argon (0.93 percent), carbon dioxide (0.038 percent). Water vapor (on average around 1% at sea level, and 0.4% over the entire atmosphere) and other gases exist in small amounts as well.
The atmosphere has a mass of about 5.15×1018 kg, three quarters of which is within about 11 km of the surface.
Which are the layers of the atmosphere?
Earth’s atmosphere is divided into five main layers, starting from the surface of the Earth: the troposphere, the stratosphere, the mesosphere, the thermosphere and the exosphere. The are many differences between them in terms of temperature, chemical composition, movement, and density. Between them there are some “pauses” where the greatest changes in thermal characteristics, chemical composition, movement and density occur.
The atmosphere thins out in each higher layer until the gases dissipate in space. There is no distinct boundary between the atmosphere and space, but an imaginary line about 110 kilometers from the surface, called the Karman line, is usually where scientists say atmosphere meets outer space.
image source: http://ete.cet.edu/gcc/?/volcanoes_layers/
The troposphere is the layer closest to Earth’s surface. It is 7 to 20 km thick and contains half of Earth’s atmosphere. The height of the troposphere varies from the equator to the poles, it is higher at the equator (18-20 km) and lower at the poles (around 7 km). As the density of the gases in this layer decrease with height, the air becomes thinner and this causes the temperature to decreases with height. As one climbs higher, the temperature drops from an average around 62°F (17°C) to -60°F (-51°C) at the tropopause. Nearly all of the water vapor and dust in the atmosphere are in this layer and that is why clouds are found here. Almost all weather occurs in this region.
The stratosphere is the second layer. It starts above the troposphere and ends about 50 km above ground. This layer holds 19 percent of the atmosphere’s gases but very little water vapor. Ozone is abundant here and it heats the atmosphere while also absorbing harmful radiation from the sun. In this region the temperature increases with height, from an average -60°F (-51°C) at tropopause to a maximum of about 5°F (-15°C) at the top of the stratosphere. The heat is produced in the process of the formation of ozone. The air here is very dry and it is about a thousand times thinner here than it is at sea level. Because of that, this is where jet aircraft and weather balloons fly.
The mesosphere starts at 50 km and extends to 85 km high. In this layer, the temperature decreases with height. This is due to decreasing solar heating and increasing cooling by CO2 radiative emission. The top of the mesosphere, called the mesopause, is the coldest part of Earth’s atmosphere with temperatures averaging about minus 130 degrees F (minus 90 C). The gases in the mesosphere are now thick enough to slow down meteors falling into the atmosphere, where they burn up. This layer is hard to study because jets and balloons don’t go high enough and satellites and space shuttles orbit too high.
The thermosphere extends from about 90 km to between 500 and 1,000 km. Temperatures can get up to 2,700 degrees F (1,500 C) at this altitude. While still extremely thin, the gases of the thermosphere become increasingly more dense as one descends toward the earth. As such, incoming high energy ultraviolet and x-ray radiation from the sun begins to be absorbed by the molecules in this layer and causes a large temperature increase. Because of this absorption, the temperature increases with height. From as low as -184°F (-120°C) at the bottom of this layer, temperatures can reach as high as 3,600°F (2,000°C) near the top. However, despite the high temperature, this layer of the atmosphere would still feel very cold to our skin due to the very thin atmosphere. The high temperature indicates the amount of the energy absorbed by the molecules but with so few in this layer, the total number of molecules is not enough to heat our skin. The thermosphere is considered part of Earth’s atmosphere, but air density is so low that most of this layer is what is normally thought of as outer space. In fact, this is where the space shuttles flew and where the International Space Station orbits Earth. This is also the layer where the auroras occur.
The exosphere, the highest layer, is extremely thin and is where the atmosphere merges into outer space. It is composed of very widely dispersed particles of hydrogen and helium. It extends from the top of the thermosphere to 10,000 km above the earth. In this layer, atoms and molecules escape into space and satellites orbit the earth.
Recap: temperature variation in the atmosphere
The temperature varies much more than I expected due to many factors, including incoming solar radiation, humidity and altitude.
The coldest temperatures lie near the mesopause, an area approximately 85 km (53 mi) to 100 km (62 mi) above the surface. In contrast, some of the warmest temperatures can be found in the thermosphere, which receives strong radiation.
Lets see the image with the temperature profile of the atmosphere:
image source: http://www.srh.noaa.gov/jetstream/atmos/atmprofile.htm
The reason it is warmer at the surface is simple. The air is warmed by heat given off by the Earth! The farther away from the surface the air moves, the less heat there is to absorb. The air also becomes thinner with height. From 10 to 20 kilometers the atmosphere is stable. This region is called the tropopause. From 20 to about 50 kilometers is the stratosphere. In this region the air actually warms with height! Ozone is concentrated in this part of the atmosphere and it absorbs ultraviolet light from the Sun. More light is absorbed at higher altitudes compared to the lower stratosphere, so the temperature increases. But at 50 kilometers, the temperature levels out again in a region called the stratopause. At about 55 km, the mesosphere begins. In the mesosphere, the temperature decreases with height again, because there is very little ozone to warm up the air. Also, the CO2 radiative emission cause the air to cool. Finally, the mesopause divides the mesosphere from the thermosphere, which is the section of the atmosphere higher than 90 km. In this region, the temperature increases again! This time, it is molecular oxygen (O2) that causes the temperature increase. The oxygen absorbs light from the Sun, and since there is very little air in the thermosphere, just a little absorption can go a long way!
And that the short version of how the temperature varies in the atmosphere! I hope everything is a bit clearer now. 🙂 I also hope I got this right! If you find some inaccuracies, please let me know! 🙂
Info sources:
- http://en.wikipedia.org/wiki/Atmosphere_of_Earth
- http://www.space.com/17683-earth-atmosphere.html
- http://www.srh.noaa.gov/jetstream/atmos/layers.htm
- http://www.windows2universe.org/kids_space/temp_profile.html
- http://www.cwb.gov.tw/V7e/knowledge/encyclopedia/me006.htm
- http://en.wikipedia.org/wiki/Atmospheric_temperature
- http://en.wikipedia.org/wiki/Mesosphere