Lecture 18

Cycles of the Earth

There is a good amount of food for thought in pages 528-534 of your textbook : do read them carefully, and meditate about the fragility of our eco-system. The main points that transpire from the reading are

• we leave in an (almost) closed environment. What is produced on earth, stays on earth
• similarly, apart from the very remote possibility of harvesting from outer space, all of the natural resources we can have access to are the ones that are presently contained within our planet. This is particularly relevant with respect to fossil fuels : it took millions of years for the coal and oil deposits to be formed, and we are burning them away in a few decades
• practically every aspect of human activity involves some consumption of energy. Luckily, the sun does provide a fair amount of necessary energy (at least in allowing the growth of living organisms), but a lot of energy is expended in producing (and then discarding) finished products.
• all of these items point to the need of wise recycling and of efficient and parsimonious consumption of energy

But, enough with gloomy thoughts, let us rather talk about the weather!

General Circulation of the Atmosphere

The energy that puts our atmosphere into motion comes from the sun, but the rotation of the earth and the distribution of land and water masses intervene to modify what would otherwise be a very simple pattern.

The main "engine" the powers the atmospheric circulation is the differential heat absorption for regions at the equator versus the polar regions. In the equatorial regions, due to direct overhead illumination, large amounts of solar heat are absorbed, while much less heat is collected in the polar regions. In the absence of earth rotation, this differential heating would produce a large "convective cell" : hot air rising at the equator, would move towards the poles, cool off and sink. This process would result in a general air circulation consisting of strong high altitude winds from equator to poles, and corresponding low altitude winds from poles to equator.

In reality, the rotation of the earth affects to a large extent this simple pattern, breaking up the single convection cell into three separate ones, and diverting the main South-North flow into easterly or westerly directions.

In the, more familiar to us, Northern Hemisphere, the three main circulation cells consist of an air flow mainly East to West at equatorial or tropical latitudes, a generally West to East flow ad intermediate (or "temperate") latitudes, and again a mostly East to West flow in the polar regions.

This is in direct agreement with our experience that, in North America, the weather comes from the West. But, if you have ever watched the Weather Channel during hurricane season, you might also have noticed that the hurricanes that eventually affect our SouthEastern regions are typically born off the coast of Africa and travel towards us in an East to West flow.

In the upper atmosphere, the boundary between the polar and the temperate zones is marked by the jet stream, a band of high speed winds flowing mainly in a West to East direction. As long as the flow is mainly West-East, there is no strong mixing between the cold polar and the warmer air of our latitudes, and this leads to rather stable conditions. On the other side, weather perturbations are associated with ripples in the flow of the jet stream, bringing cold air to the South and warm air to the North.

The next factor determining the patterns of air circulation and its effects on the weather is associated with differences in atmospheric pressure. The atmosphere is a very dynamic entity, and, if you think of it as a liquid in a container, it can "slosh around". But, unlike a liquid, these unbalances will result not in differences in height of the fluid, but in regions of different densities. A region of high pressure occurs when there is a larger than average number of air molecules per unit volume, i.e. a region of higher air density.

Remembering that atmospheric pressure in a given location is effectively given by the weight of the column of air above it, it is then obvious that higher density will result into higher pressure, since in such regions the amount of air molecules contained in a "tube" of air overhead is larger, and the tube will weigh more. Similarly, a low pressure region will correspond to lower than average air densities.

In a weather chart, high and low pressure zones are represented by joining with a line points of equal pressure: these are the isobars. To help you interpreting them, you might think of a comparison with a topographic map showing the lines of equal elevation: a high pressure zone would correspond to a mountain, a low pressure to a valley or a depression.

A situation of pressure differences cannot obviously be stable, since the "heavier" air will rush in to balance the weight of the "lighter" air. This motion of air masses will typically produce weather phenomena, since it results in the encounter of air masses of different temperature and humidity. Such an encounter between air masses is what we call a front.

In a coarse classification, we distinguish between cold and warm fronts according to the relative mobilities of the colder vs. the warmer air. We say that we have a warm front when the warmer air arrives to displace colder air, while a cold front results from an inrush of colder air.

In either case we will experience phenomena of cloud formation and precipitation since, when two air masses of different temperature and/or density meet, the lighter air will rise, in doing so it will get colder, and the water vapour it contained will condense into clouds (remember that the amount of water that can be present in the air as vapour depends on the air temperature: the lower is the temperature, the less is the amount of water that can be present in gaseous form).

It is important to realize that cloud formation is practically always due to air cooling associated with rising. In addition to the encounter between air masses of different densities we have just mentioned, air can be caused to rise because of

• strong solar heating of the ground and of the air near it. Hot air will expand, therefore decrease in density, and rise. This is what causes the formation of cumulus clouds and of local thunderstorms during hot Summer days
• the encounter with a mountain range. If a body of air moving near the surface encounters a mountain range, it will be forced to rise, and in doing so it will cool down and condensate its water vapour. This is why it always rains on the West side of the Pacific mountain ranges, while there are very dry conditions on the East side, since the very humid air from the Pacific loses most of its moisture when climbing up the West slopes.

In order to understand the weather patterns, you need one more piece of information. In the absence of extraneous factors, the flow of air from high to low pressure should be oriented along the line joining the high and the low centers, i.e. it should be roughly perpendicular to the lines of equal pressure. In reality, the effect of the earth rotation distorts the natural direction of flow, and the prevaling winds align themselves roghly parallel to the isobars. Moreover, the deflection is such that, in the Northern hemisphere, the circulation runs clockwise around a center of high pressure and counterclockwise around the low pressure. Next time you look at a weather chart you can verify the validity of these rules.

What about El Niño?
Meteorology is not an exact science, in fact we are far from being able to make a reliable, detailed long term weather forecast. In fact, just because of the complexity of the phenomena involved, one could state that there is a basic amount of intrinsic unpredictability since some minor variation of the conditions at a given instant and place could cause major differences in the weather evolution. This principle has sometime been referred to as the butterfly effect: with slight exageration, it has been stated that a butterfly fluttering its wings somewhere in Australia could eventually influence the formation of a storm in North America...

Still, progresses are made in meteorology, as in any other science, and a recent major breakthrough has been the realization of the importance of an anomalous behaviour in the water temperature in the South Pacific. El Niño, litterally "the child" but meant to represent "Christ child" since it was first observed to occur around Christmas, is an abnormal raise in the ocean temperature off the West coast of South America. Recent investigations have been able to correlate El Niño occurrences with major disruptions of the normal weather pattern, leading e.g. to exceptional rain falls in Southern California, droughts in the Indonesian tropical rain forest, etc. (i.e typically rain where its normally dry and viceversa). El Niño in not a new phenomenon, but probably you have heard it mentioned very frequently in recent times both because meteorologist have only recently realized its effects and, more importantly, because its occurrence seems to have increased in the last years. In the past, briefer El Niño episodes used to alternate with the normal "La Niña" conditions of cold waters off the Peruvian coast. Recently El Niño appears to have become more frequent and to last longer. Is this just a normal fluctuations in the weather pattern, or is it a serious consequence of the global warming (that, as we will discuss later, is attributed to our overall energy consumption and the greenhouse effect)? Noboby can yet tell for sure, but it is certainly an issue that deserves continued research.