Introducing the Atmosphere

A while back I introduced the ocean. a bit. Time for the atmosphere, which turns out to be much simpler than the ocean, or maybe that just is a matter of being more subtle.

In the ocean, we have names for areas bounded (usually) by land. This doesn't work for the atmosphere, since it covers the whole earth. It does turn out, however, that the circulation itself (as for the Southern Ocean) helps divide the atmosphere. For a first approximation, little air crosses the equator. (Some does, to be sure.) So we can divide the atmosphere to northern and southern hemisphere circulations. Even though the two hemispheres are greatly different in their amounts and distribution of land and ocean, it turns out that their major circulation is quite similar.

In both hemispheres, air rises from the equator, move towards higher latitude at elevation (5-15 km, 3-9 miles up) and then sinks to the surface around 30 degrees latitude (north or south). The surface circulation is from that sinking latitude back towards the equator. This is the Hadley cell. There's a similar cell between about 60 degrees latitude and the pole, called the Ferrel cell. In between the two is not so much a cell as a storm zone.

A couple of questions I haven't seen firmly answered (I do have leads on a couple of papers, but haven't read them yet) are: Why are the circulations so similar between the hemispheres? Why are there 3 zones of circulation instead of 2, 4, 5, 75, ...?

In the vertical ... how high is up? :-) There's actually a way of approaching this by way of a different question:

What is the atmosphere made of? If we sample air all around the world, and through all levels of the atmosphere, we discover a few striking things. First, only 3 gases account for almost the entire atmosphere -- Nitrogen (N2), Oxygen (O2), and Argon (Ar). They are, in order, 78%, 21%, and 1% of the atmosphere in terms of counting molecules, and 76%, 23%, and 1% by mass. Note that these figures have been rounded, but also note that they do add to 100%. A further feature is that this proportion is constant throughout the atmosphere -- until you get to about 100 km (about 60 miles) up. Above that point heavier gases separate from lighter ones.

So that's our answer to 'how high is up?' -- about 100 km.

But back to the matter of what the atmosphere is made of. We've all heard about water (H2O), Carbon dioxide (CO2), and Ozone (O3) and how important they are. They are indeed important. But, added up through the atmosphere, they account for only about 0.24% of the molecules. Considering all gases, only about 1 in 400 molecules is not one of the big 3 of Nitrogen, Oxygen, or Argon.

Yet it is these other, rare, molecules which account for almost all the interesting processes in the atmosphere! This includes rain, hurricanes, the fact that the earth is not frozen, protecting surface life from solar ultraviolet, constructing the stratosphere, ...

Ok, the stratosphere brings us back to the vertical structure of the atmosphere. Starting from the surface, we have the troposphere. Here's where almost all the weather happens, and temperatures generally decrease with height. (So it's usually cooler in the mountains than the flat land around them.) In the stratosphere, temperatures are constant or increasing with height. Above this is the mesosphere, where temperatures go back to decreasing with height. The top of the mesosphere is that 100 km (or so) boundary. Above this, things get very different, and usually are studied by people who aren't meteorologists or climatologists. (Aeronomers, chemists, physicists instead.)

The stratosphere being a warm place, compared to the air above or below it, is because of the ozone absorbing ultraviolet. Without the ozone (and the chemistry that maintains that molecule), there's nothing to be absorbing energy to warm the layer. At most, ozone is about 12 parts per million of the molecules in that layer (0.0012 % !)

Since my first cut is always to take a look at the largest scale things, the most common, and such, we're already done. 3 molecules make up almost all of the atmosphere (N2, O2, Ar), there are 3 circulation zones between equator and pole in each hemisphere, there are 3 layers in the vertical, and the atmosphere goes up about 100 km.

To understand most of what's interesting, we have to get much more subtle. Always a fun and interesting thing to find that understanding some very small part of a big system will let us understand a lot about that large system.

Introduction to the Oceans

Just a quick look at who they are and what they're made of. The fact that I do polar work leads to me taking a somewhat different accounting of the 'who' side than low and mid-latitude oceanographers tend to. So, for me, the oceans are: Pacific, Atlantic, Indian, Southern, Arctic. The Southern and Arctic oceans are not infrequently neglected. The Arctic can reasonably be called a sea rather than an ocean; that is, it is a body of water that has limited inflow and outflow (the Bering Strait on the Pacific side and Fram Strait on the Atlantic side).

The Southern ocean truly is oceanic rather than sea-like. We polar types consider it separate from the other oceans because of the Antarctic Circumpolar Current. South of this, the Southern Ocean waters are mixed pretty well with each other and are not much like the waters to the north. North of there, the other oceans have greatly limited transport with the Southern Ocean. Currents tend to flow from one of the other three to another of the others, rather than in to the Southern Ocean. Since the Antarctic Circumpolar Current, like all currents, is a dynamic feature, it moves. This means that the boundary location and size of the Southern Ocean (and, conversely, the Pacific, Atlantic, and Indian) fluctuates somewhat in time.

The elements in the ocean, as for most geophysical systems, are mostly a fairly select subset of the periodic table. Although there are over 110 elements known now, only 8 account for most of the ocean. If we take a kilogram of sea water (1000 grams, remember), then there are about:
858 grams Oxygen (O)
107 grams Hydrogen (H)
10.3 g Sodium (Na)
19.5 g Chlorine (Cl)
1.3 g Magnesium (Mg)
0.9 g Sulphur (S)
0.4 g Potassium (K)
0.4 g Calcium (Ca)

Aside: Sodium and Potassium's chemical symbol comes from their Latin names 'Natrium', and 'Kalium', respectively. Their symbols do make sense, at least if you know the right second language.

So, 8 elements, 8 symbols to remember out of the 110+, and you've got well over 99% of the ocean's content. Even better is that the composition of the ocean varies by very little throughout, at least for the major elements. Very scarce elements, like Iron (Fe -- Ferrum) do vary substantially, but they're so rare this doesn't affect the ocean's dynamics.

If we think instead of molecules, the main components are water (H2O), table salt (NaCl), additional salts (KCl, CaCl), and Magnesium Sulphate (MgSO4, epsom salts). Of these, water is the overwhelming majority. Table salt is most of the rest. Viewed in terms of its bulk composition, the ocean is fairly simple. But if we think about living things in the sea, it gets complicated very quickly. Living things depend on elements and chemicals which are very scarce.

The oceans, all together (and however we name them), occupy about 70% of the surface of the earth. This is one reason oceanographers take an interest -- it is most of the surface of our planet. (Calling it 'Earth' is poor naming, we feel. It should be 'Water', or 'Oceania' or something like that.) The average depth of the ocean is about 3730 meters (about 12,200 feet), farther below sea level than almost all land areas are above sea level (land averages about 800 meters, 2600 feet, above sea level; by the time you get to 12,000 feet above sea level, you're on the side of a good mountain).

Later, we'll get more detailed about the ocean. But here's a start on names.