This beauty image is one of sixteen sunsets seen by the astronauts aboard the International Space Station during one orbit of Earth. It was taken over the Indian Ocean on May 25 and shows several layers of our atmosphere. The limb is the profile or edge of the Earth. Credit: NASA
You get into bed and pull the covers up to stay warm. The atmosphere’s like that. Blankets of air wrap the Earth and keeps us warm. Imagine what life would be like without an atmosphere. On the upside, every night (and day) would be clear so you could go out skywatching any time you’d like. Unfortunately the negatives would outweigh the pluses. After the oceans boiled away for lack of atmospheric pressure, surface temperatures would drop rapidly after every sunset to something close to 300 below zero Fahrenheit, similar to what the moon experiences.
In the moon’s equatorial and mid-latitude regions, daytime temperatures push the mercury up to around 224 degrees, hotter than boiling water. North of 70 degrees latitude in the polar regions, daytime highs aren’t as extreme because the sun is low in the sky there just as it near Earth’s poles. Lunar nighttime lows are around -298 and drop sharply north of 80 degrees latitude. Losing the atmosphere, we’d also lose the ozone layer and have no protection against powerful ultraviolet radiation from the sun. We’d have to go back to living in caves … deep caves.
The atmosphere is divided into five different layers, the most familiar of which is the one closest to the ground called the troposphere. This is the domain of the clouds and where almost all our weather occurs. Its thickness depends upon your latitude. At the equator the troposphere extends to 56,000 feet or just over 10 miles, while at the poles it’s 23,000. Transcontinental planes try to get above the troposphere and into the next layer called the stratosphere if they can. Air resistance is less there plus the ride isn’t nearly as bumpy. In the troposphere air up and down as well as sideways, but in the stratosphere it flows evenly parallel to the Earth’s surface.
This cutaway shows the four main layers of Earth’s atmosphere. Most of the action happens in the troposphere, but some of our favorite sights like aurora and meteors happen at the top of mesophere and lower thermosphere. Credit: NASA
Near the top of the stratosphere lies the ozone layer, which we respect and appreciate even if we can’t see it, because it protects us against otherwise massive doses of harmful ultraviolet light coming from the sun. The stratosphere tops out around 32 miles before giving way to the mesophere. Extending up to about 53 miles, the mesophere is where lots of meteors burn up as well as being host to those eerie night-shining or noctilucent clouds. You’ll find temperatures there over 125 below, the coldest average temperatures on Earth.
Beyond the mesophere, temperatures rise again as we enter the thermosphere which extends from 53 to 480 miles high and is home to the space station and northern lights. The ‘thermo’ part of thermosphere comes from the layer’s extremely high temperature of over 2000 degrees. Don’t worry about the space station burning up — that thermometer reading is mostly a measure of how fast the atoms are moving up there, and since there are so few of them, it doesn’t really feel hot in the sense we know heat.
Some descriptions of the atmosphere include one additional layer, the exosphere. This is the very limit of Earth’s air; beyond it is true outer space. The exosphere is populated by hydrogen and helium atoms many miles apart and all moving quickly on their way into interplanetary space.
Next time you pull the covers over, think about the layers overhead that keep life on this planet humming. Compared to the enormity of the globe, our atmosphere is a thin film, a membrane of sorts, that protects us every day from the brutal reality of outer space.
Tonight and tomorrow night, watch for the game-loving moon to play tag with Mars and Regulus in the western sky starting at the end of evening twilight. There are some nice gatherings in the offing. Wishing you clear skies! Created with Stellarium