Wednesday, October 30, 2013

ATMOS 301: Composition and Fundamental Physics

Wednesday, October 30, 2013
11:24 a.m.

Greetings everybody. I hope you'll forgive me for my lack of recent posts. You see, there's this thing called college, and, well, it has these things called midterms. And these midterms, well, they take up a fair amount of your time and energy. And by a fair amount, I mean an unfair amount.

Speaking of midterms, I have another one on Friday. So why the dickens am I writing a blog? Well, this midterm is in my atmospheric sciences 301 class! What better way to study than to try and communicate these concepts to the public. Of course, whether you are interested in them remains to be seen. But hopefully you can find my writing engaging and learn a couple things along the way.

There are a lot of things to cover, but let's start with some of the basics. First off, the class website is, and if you want to login to anything, the username is 301 and the password is weather.

Composition of the Atmosphere

The atmosphere is made up of three main gases. These are:
    -  Nitrogen (78 %)
    -  Oxygen  (21 %)
    -  Argon  (1 %)

These fellas make up the so-called "permanent" gases of the atmosphere. But they are by no means the only gases, even though the rounded percentages may suggest otherwise.

Below are the "variable" gases of the atmosphere.

    -  Water Vapor ( 0-4  %)
    -  Methane
    -  Carbon Dioxide
    -  Nitrous Oxide
    -  Ozone
    - CFCs (Chlorofluorocarbons, which are those things which destroy the ozone hole)

Water vapor is by far the most variable. The poles aren't exactly dripping in moisture, while the tropics... well... you get the idea. And after being in Micronesia over the summer, that is something I can verify.

In addition to the gases outlined above (and others that I have not mentioned), there are particulates, or aerosols, as atmospheric scientists like to call them, scattered throughout the atmosphere but primarily in the troposphere. Sea salt, volcanic ash... they're all there, and they play a major role in other atmospheric processes such as condensation and precipitation, which I will talk about in a future blog.

Pressure, Temperature, Humidity, and Wind

I feel as though these things are those types of things that you think that you know, but when somebody asks you to define them, do realize that you don't really know them. We all know what politicians are, but when somebody asks you what a politician is, how are you supposed to respond? Personally, I'd just say something like "A guy who does things." Some dude on Urban Dictionary summed it up well when he said a politician is "a person who practices politics," where " "politics" is derived from the words "poly" meaning "many", and "tics" meaning "blood-sucking parasites." "


Pressure is equal to force/area, and if anybody here knows their physics, they can recall that F = ma, where F = force, m = mass, and a = acceleration. In SI units (or, as the French like to say, Le Système International d’ Unités), force = kg*m/s^2, where kg=kilograms, m=meters, and s^2=seconds squared. So force/area = kg*m/s^2 * 1/m^2 = kg/(m*s^2). Voila. There's your definition of pressure.

While were on the subject of pressure, let's talk about other non-SI units we use. How many of you are familiar with a mercury barometer? For those who aren't, it consists of a dish of mercury that is able to freely flow up a tube with no air inside it due to the force of the atmosphere pushing down on the exposed mercury in the dish until the weight of the mercury balances the weight of the atmosphere above it.

From the UW Atmospheric Sciences' very own website!

We use inches and millimeters of mercury to describe the pressure. Mean sea-level-pressure is 760 millimeters or 29.92 inches of mercury. We also use hectopascals and millibars, where 1 hectopascal = 100 pascals and 1 bar = 1000 millibars. Hectopascals and millibars themselves are equivalent, and mean sea-level-pressure in those units is 1013 mb/hPa.

One more note... atmospheric scientists like to use pressure instead of height as the vertical coordinate. Instead of hearing guys talk about what's happening at 18,000 feet, you'll hear them talk about what's happening at 500mb.


Temperature is manifested for us beings by how hot we feel. But in the world of physics, it is defined as the average kinetic energy of molecules. By kinetic energy, I mean the average amount of which these molecules zip, zap, whip, and snap around. That's it. When two parcels of air of different temperatures are combined, the momentum carried by the different speeds at which molecules are moving around is transferred and the temperature equalizes. It's not magic. It's physics.


There are many different ways to express humidity. Let's go over my two favorite ones.

Relative Humidity:

At a certain temperature and pressure, only a certain maximum of water vapor molecules can exist. The higher the temperature, the higher the maximum. I'll get more into this when I talk about skew-t plots. The relationship is exponential, meaning as the temperature increases, the maximum amount of water vapor increases even more.

Dew Point:

The dew point is the temperature at which are must be cooled at constant pressure to produce saturation. Don't forget that constant pressure tidbit. The dew point is a measure of the actual amount of water vapor in the air. Another measurement meteorologists use is the dew point depression, which is defined as the ambient temperature minus the dew point.

Without further ado, I'll begin my writeup on my next topic: surface station models. It'll be a short one, but could be one of the most important things to study for the test, so the ~4 Facebook friends I have that are taking the class should take a solid gander.


Remember the politics analogy? "What is wind?" Well, you could say that it is the air moving. And actually, you'd be close. Wind is defined as the velocity of a percel of air with respect to the surface of the Earth. The Earth is moving too, folks, but if the air is moving in the same exact manner as the Earth, there is no wind because there is no relative motion between the atmosphere and the Earth.

Because wind is velocity and not speed (speed is a scalar, meaning it is only assigned a numerical value, while velocity is a vector, meaning it has a numerical value and a direction), our equation for wind is v = ui + vj + wk, where v is the velocity vector, and i, j, and k are the unit vectors in the x, y, and z direction. The u, v, and w coefficients respectively associated with the unit vectors actually determine the direction of the vector. ui denotes zonal flow (east-west) and vi denotes meridional flow (north-south).

We don't use the k vector when talking about wind direction. For example, the wind is from the northeast, not from the northeast plus a specified height per unit distance.

No comments:

Post a Comment