Before we get started, I'd like to address the Comcast High Speed Internet customer support division. I'm going to address this entity as a single person, but know that my comments apply to every fucking useless one of you. Sir, you are the single most spectacularly incompetent and heinously dishonest, loathsome sack of shit I have ever had the misfortune of dealing with. Had your father pulled out early and spilled his seed on a half decomposed pile of dog shit, the resulting mixture would be eons ahead of you in intelligence, compassion and worth as a physical entity. In my short time on this planet, I have been subjected to a number of craptastic customer service experiences, but you, by far, blow them all away, in your complete and utter lack of caring in regards to your customer's experience. Were you in front of me, it would take all of my energy to not cave your fucking head in with whatever blunt instrument was at hand. Now, is it wrong of me to lump all of your workers into one entity upon which to unload my hatred? Probably, however since last week I have dealt with a half dozen of them and not a goddamn one, not one fucking person, has done anything above the bare fucking minimum to resolve the problems I'm having with your service. Service, might I add, I pay almost 50 bucks a month for. I'd threaten to leave, but you don't care, and if I'm going to use my breath to inform you of something, I'd rather inform you of how much better the world would be if your presence were erased completely from it.
Now that we've gotten that out of our way, we can move on with today's question. It comes from my mom, who, as part of her job, traverses this great land of ours with a frequency that would fell your's truly. The last time she was here, she asked me the following (paraphrased due to my horrible memory):
Why can it be raining when you take off and raining when you land, but not
raining when the plane is at it's cruising altitude?
At the time I saw her, I told her the answer, which is that planes fly above the clouds, so even though it's raining on the ground upon taking off or landing, when the plane is at its cruising altitude, it's above all of that. However my curious mind wasn't satisfied with that answer and I have since probed further to find out why clouds only form at certain altitudes.In order to talk about this, we have to talk about the levels of the atmosphere we're most interested in, namely the troposphere and the stratosphere. The troposphere is the first "level" of the Earth's atmosphere and it extends from the surface of the planet to about 7km high at the poles and 17km high at the equator. This is the layer of the atmosphere that contains our friends the clouds. It also includes a great deal of moving air, due to the heating of air in the troposphere from surface radiation. Basically, as the sun's rays hit the surface of the planet, the surface heats up. This radiation then heats the air in the troposphere which in turn rises, expands and subsequently drops in temperature. As a result, air in the troposphere is warmer at lower altitudes and colder at higher altitudes.
The troposphere is the atmosphere's chief repository of water vapor which explains why pretty much every cloud you see in the sky is hanging out in the troposphere. There are some exceptions to this rule, which we'll talk about in a minute, however if, in the course of your normal day, you see a cloud, most likely, it's in the troposphere. If the troposphere contains all of the atmosphere's water vapor, and clouds consist of water vapor and/or ice crystals, then it makes sense that clouds exist mostly in the troposphere.
So, in answering why clouds exist in the troposphere, we can derive why they do not exist in the stratosphere, namely because there's no water vapor in the stratosphere, or at least a very, very small amount. There are clouds called polar stratospheric clouds that exist in the stratosphere, over the poles during polar winters. As I'm assuming you don't spend a heck of a lot of time at the poles in the winter, you can see why I was pretty confident in saying that pretty much all of the clouds you see exist in the troposphere. These stratospheric clouds consist of mixtures of water vapor, nitric acid and sulfuric acid. It has to be pretty damn cold to cause the acidic components of these clouds to crystallize, allowing the clouds to form, hence the clouds occuring over the poles during polar winters. Those two together make some pretty nippy conditions.
There are also notilucent clouds that exist in the mesosphere, which is above the stratosphere. Folks aren't quite sure how clouds can exist in the mesosphere due to the lack of water, but despite this, the general feeling is that they consist of water ice. How the ice got all the way up there is another question altogether.
The movement of air in the troposphere that we mentioned before is the chief reason your pilot looks to achieve a comfortable cruising altitude of about 10km. All that moving air tends to slam planes around leading to some nasty turbulence and possible vomitous outcomes. This is why, usually, when turbulence occurs during a flight, the pilot will attempt to go a little higher, unless of course, you're already on your way back down. The stratosphere is not subject to all of this moving air, and, as a result, is a fairly calm place to hang out. An interesting note about the stratosphere is that it gets warmer as you get higher up in it, as the outer regions of the stratosphere are warmed by solar radiation and the lower regions are cooled by the higher, cooler regions of the troposphere. The top level of the stratosphere is around -3 Celsius, or 27 degrees Farenheit. Not balmy by any measure, but certainly warmer than I'd expect it to be 50km above the planet's surface.
So, lets put it all together. You take off in your plane and head up through the troposphere, which includes a ton of water vapor, some in cloud form, some falling from the clouds, some just hanging out, and a ton of moving air. This moving air makes for a bumpy flight, so your pilot heads up to the stratosphere where there's no moving air, no water vapor, and subsequently, no clouds. From then on out, it's smooth sailing until you have to descend and do it all in reverse. Soon you're on the ground at your dismal, rainy destination wondering how it's possible that not 30 minutes ago you were sailing through blue skies and sunshine. Science baby, that's how.
As always, we welcome questions at suburbanjoe at gmail dot com or in the comments section of these here posts. Keep 'em comin' folks. We love to hear them.
Wikipedia - Clouds
Wikipedia - Earth's Atmosphere
Wikipedia - Troposphere
Wikipedia - Stratosphere
Wikipedia - Polar stratospheric clouds
Wikipedia - Noctilucent Clouds