The present atmosphere of the Earth is probably not its original atmosphere. Our current atmosphere is what chemists would call an oxidizing atmosphere, while the original atmosphere was what chemists would call a reducing atmosphere. In particular, it probably did not contain oxygen.
Composition of the Atmosphere
The original atmosphere may have been similar to the composition of the solar nebula and close to the present composition of the Gas Giant planets, though this depends on the details of how the planets condensed from the solar nebula. That atmosphere was lost to space, and replaced by compounds outgassed from the crust or (in some more recent theories) much of the atmosphere may have come instead from the impacts of comets and other planetesimals rich in volatile materials.
The oxygen so characteristic of our atmosphere was almost all produced by plants (cyanobacteria or, more colloquially, blue-green algae). Thus, the present composition of the atmosphere is 79% nitrogen, 20% oxygen, and 1% other gases.
Layers of the Atmosphere
The atmosphere of the Earth may be divided into several distinct layers, as the following figure indicates.
|Layers of the Earth’s atmosphere|
The troposphere is where all weather takes place; it is the region of rising and falling packets of air. The air pressure at the top of the troposphere is only 10% of that at sea level (0.1 atmospheres). There is a thin buffer zone between the troposphere and the next layer called the tropopause.
The Stratosphere and Ozone Layer
Above the troposphere is the stratosphere, where air flow is mostly horizontal. The thin ozone layer in the upper stratosphere has a high concentration of ozone, a particularly reactive form of oxygen. This layer is primarily responsible for absorbing the ultraviolet radiation from the Sun. The formation of this layer is a delicate matter, since only when oxygen is produced in the atmosphere can an ozone layer form and prevent an intense flux of ultraviolet radiation from reaching the surface, where it is quite hazardous to the evolution of life. There is considerable recent concern that manmade flourocarbon compounds may be depleting the ozone layer, with dire future consequences for life on the Earth.
The Mesosphere and Ionosphere
Above the stratosphere is the mesosphere and above that is the ionosphere (or thermosphere), where many atoms are ionized (have gained or lost electrons so they have a net electrical charge). The ionosphere is very thin, but it is where aurora take place, and is also responsible for absorbing the most energetic photons from the Sun, and for reflecting radio waves, thereby making long-distance radio communication possible.
The structure of the ionosphere is strongly influenced by the charged particle wind from the Sun (solar wind), which is in turn governed by the level of Solar activity. One measure of the structure of the ionosphere is the free electron density, which is an indicator of the degree of ionization. Here are electron density contour maps of the ionosphere for months in 1957 to the present. Compare these simulations of the variation by month of the ionosphere for the year 1990 (a period of high solar activity with many sunspots) and 1996 (a period of low solar activity with few sunspots):
WEATHER & CLIMATE
Not only does the Earth have a complex atmosphere, but that atmosphere has complicated motion and nontrivial behavior. We Earthlings call this weather in the short term and climate over the longer term. The following images illustrate some of the complex patterns that develop in Earth’s atmosphere.
| Left: GOES-7 image of weather patterns in the Americas and Eastern Pacific.
Middle: Hurricane Fran from GOES-8 (Ref).
Right: GOES-8 false-color image of water vapor in Earth’s atmosphere (Ref).
links to current weather satellite images:
- Current USIR (GOES 8)
- Current Western Hemisphere (GOES 8 visible-IR composite)
- Most Current GOES-9 IR MPEG of Eastern Pacific & Western U.S.
How does this complex behavior arise? The most general answer is that it is a consequence of (1) solar heating, and (2) effects associated with the rotation of the Earth. In the next section we address these issues explicitly.
Consequences of Rotation
The Earth is a spinning globe where a point at the equator is travelling at around 1100 km/hour, but a point at the poles is not moved by the rotation. This fact means that projectiles moving across the Earth’s surface are subject to Coriolis forces that cause apparent deflection of the motion.
The following diagram illustrates the effect of Coriolis forces in the Northern and Southern hemispheres.
|The Coriolis force deflects to the right in the Northern hemisphere and to the left in the Southern hemisphere when viewed along the line of motion.|
Solar Heating and Coriolis Forces
Since winds are just molecules of air, they are also subject to Coriolis forces. Winds are basically driven by Solar heating. As the adjacent (highly idealized) image indicates, Solar heating on the Earth has the effect of producing three major convection zones in each hemisphere.
If solar heating were the only thing influencing the weather, we would then expect the prevailing winds along the Earth’s surface to either be from the North or the South, depending on the latitude. However, the Coriolis force deflects these wind flows to the right in the Northern hemisphere and to the left in the Southern hemisphere. This produces the prevailing surface winds illustrated in the adjacent figure.
For example, between 30 degrees and 60 degrees North latitude the solar convection pattern would produce a prevailing surface wind from the South. However, the Coriolis force deflects this flow to the right and the prevailing winds at these latitudes are more from the West and Southwest. They are called the prevailing Westerlies.
The adjacent animation shows GOES-8 weather satellite images over a 72-hour period from Dec. 29, 1996, through Jan. 1, 1997. This is a geosynchrous satellite, which means that it orbits the Earth with the same period as the Earth’s rotation and therefore appears to be essentially motionless over a fixed position on the Earth’s surface. For GOES-8 this fixed position looks down on North and South America.
In these composite images red indicates visible light (reflected sunlight), green indicates the 11 micron IR channel (thermal emission), and blue indicates the 3.9 micron channel (thermal + sunlight). At night the images are blue and green. The three periods of daylight in this 72 hour sequence are clearly visible as red-orange regions moving from East to West (right to left). In the IR channels, the natural intensity pattern has been inverted: warmer is darker, so that cool cloudtops stand out brightly.
One can see clearly the pronounced cloud flows associated with the strong westerlies at mid-latitudes in each hemisphere. (This is taken in Northern hemisphere Winter, so the heavier cloud cover in that hemisphere is not surprising.) Less obvious are the easterly trade winds and the polar easterlies, though one can see vestiges of each if one looks carefully. Also apparent are the swirling motions associated with frontal systems. These are particularly pronounced at the boundaries between the mid-latitude westerly and polar wind flows in each hemisphere.
Here is a similar weather animation (1.49 MB animated GIF) using GOES-8/9 IR images for North America over a 2 day period from December 31, 1996 through January 1, 1997. The large weather systems that move ashore from the Pacific in this animation produced catastrophic flooding in California, Oregon, and Washington in early January, 1997.
Cyclones & Anticyclones
The swirling motions evident in the preceding animations are consequences of frontal systems anchored to high and low pressure systems, which are also called anticyclones and cyclones, respectively. The wind flow around high pressure (anticyclonic) systems is clockwise in the Northern hemisphere and counterclockwise in the Southern hemisphere. The corresponding flow around low pressure (cyclonic) systems is counterclockwise in the Northern hemisphere and clockwise in the Southern hemisphere. This is a consequence of the Coriolis force, as illustrated for the Northern hemisphere in the following figure.
|Low pressure systems (left) and high pressure systems (right) in the Northern hemisphere|
Here is a pronounced example of a cyclone: a movie of Hurricane Andrew (653 kB). This animation is a loop of infrared satellite images showing the path of Hurricane Andrew across Florida and into Louisiana from Sunday, August 23 through Thursday, August 27, 1992 (Credit: Nathan Gasser).