Head in the Clouds: 101: The Atmosphere

As seasons begin to change, so does the weather. Discussion of this weather often uses much more “science talk” than what I’ve covered so far. Therefore, I’ll be making some informational posts along the way (such as this one) to make sure things stay clear. Each of these posts will be part of what I’m calling the “101 series”, so the title of each post of this type will begin with “101”.

THE ATMOSPHERE

Perhaps the best way to begin is with the largest scale. Many of the terms used here are used so frequently by atmospheric scientists that it can be easy to forget that others might not have clue as to what they are talking about. I plan on alleviating that issue, so without further ado, here is The Atmosphere.

The atmosphere is divided up in to several layers; the exact number varies slightly depending on the source. At the top of each layer is a boundary called a pause (essentially meaning “the limit of”). Each layer of the atmosphere is more-or-less defined by its temperature profile, that is, how temperature changes with height, also known as the lapse rate. The character of the lapse rate often results in. or results from, other defining characteristics which are quite important. To begin this journey through the atmosphere, let’s start at the bottom:

-The Troposphere

This layer is home to the weather. Virtually all the clouds you see in the sky are contained in this one, relatively thin layer of the atmosphere. At the very bottom of layer, adjacent to earth’s surface, is the Planetary Boundary Layer (PBL). The air in the PBL is strongly influenced by the drag on the air caused by friction with the ground, which causes the air to be well mixed and rather chaotic. For example, it is because of this chaos that wind gusts occur. The rest of the troposphere is, for the most part, poorly mixed, relatively speaking. In the troposphere temperature generally decreases with height, it is because of this property that pockets of air may rise (being warmer than their environment) which leads to clouds and ultimately helps produce the weather. At the top of the troposphere is the tropopause, which is where the temperature profile suddenly begins warming with height, preventing air pockets to rise past this point, which is why virtually all clouds exist in the troposphere. The height of the tropopause varies widely but generally decreases from about 17km in the tropics to around 11km near the poles.

-The Stratosphere

As the name suggests, this layer is highly stratified, meaning there is almost no mixing in this layer thanks to the increasing temperature with height profile. The jet streams that direct weather in the troposphere often exist at the very bottom of this layer. Higher up is the ozone layer, which is by far the most defining feature of the stratosphere. Ozone is highly important because it absorbs ultraviolet radiation that is deadly to life on the surface. In absorbing the radiation, ozone molecules dissociate and emit some heat in the process. It is this heat that is responsible for the layer’s temperature profile.

-The Mesosphere

The “middle” layer of the atmosphere is also the coldest. The temperature profile here decreases rapidly from its peak at the stratopause, reaching a minimum of nearly -100 degrees Celsius (-135 Fahrenheit) at the mesopause. It is in this layer that most meteors burn up, or at least begin to burn up, since above the mesosphere the air is just too thin to produce much friction. Due to its high altitude, from roughly 50km to 80km above the surface, not much is known about the mesosphere as it is too high for planes to reach, yet too low for satellites to orbit in. It is for this reason that this layer is sometimes called the “ignore-o-sphere”.

-The Thermosphere

By the mesopause the air has become very thin and is virtually unprotected from intense ultraviolet and x-ray solar radiation. This bombardment breaks apart molecules and strips atoms of electrons, creating a highly ionized environment. These processes produce a lot of heat causing the temperature profile to increase significantly with height, however because the air is so thin, you would likely freeze almost instantly in what is essentially outer space. In fact, many satellites orbit within the upper part of the thermosphere.

From there the atmosphere fades into the void of space. Some sources call this the exosphere, but it basically doesn’t have any effect on weather down near the surface. Below is a diagram from NOAA depicting these layers and the temperature profile.