At the equator the sun's rays strike the earth's surface at essentially a 90 degree angle. That means energy is absorbed at the equator more dramatically than at any other latitude, so it gets very hot. Contrary to what many believe, the fundamental cause of warm and cold areas on the earth is not our distance from the sun, the makeup of the atmosphere, clouds, or a canopy. It is the way the sun's rays strike the earth. Hitting a baseball perpendicularly to the bat will cause a home run, but hitting the ball at an angle on the top of the bat will produce a foul ball. It is very much the same situation.
The increased energy and heat at the equator causes the air to rise. As it rises clouds form and rain falls to the surface of the earth and at the equatorial latitudes causes what is called a tropical rain forest. Once the rain has fallen the hot air is very dry and will move away from the equator. In the earth's design, by the time it gets to 30 degrees latitude the air cools and begins to sink to the surface of the earth. This air is very dry and the result is that a massive percentage of the world's deserts are located at 30 degrees north or south latitude.
As the air rises at the equator, it must be replaced, and air moving towards the equator to replace it creates what we call the trade winds. The air coming down at 30 degrees latitude flows both to the equator and the opposite direction producing in the United States a prevailing wind direction and storm direction from the southwest.
This explanation is greatly oversimplified. The earth's rotation exerts a force called the Coriolis force which causes the wind to turn as it flows (see the drawing to the left). The tilt of the earth causes the sun to be directly overhead right at the equator only twice a year--moving as far north and south as 23½ degrees. Oceans, continents, mountains, and even pollution can modify this basic system but in simplified terms, this is how it is designed.
The problem then for living things which are found at 30 degrees latitude is how to get enough water to survive. In this journal we have written from time to time about the individual design features of living things that allow them to conserve or find water in arid and inhospitable climates. The other part of the story however, is how water balance occurs in these areas that are naturally a desert climate. When certain areas of the oceans of the world have the sun directly overhead, they heat up. This causes massive amounts of water to evaporate and produces a low pressure system. We call these low-pressure systems tropical depressions. The Coriolis force of the earth's rotation spins these systems and consolidates them so that a massive water machine develops over the ocean. These storms drift across the Atlantic evaporating more water and gaining strength, and the Coriolis force moves them up to the 30 degree north latitude. A similar process happens in the southern hemisphere. This huge bubble of water comes ashore as a tropical depression or hurricane. This water is vital to the natural ecology of plants and animals native to 30 degrees latitude.
It is interesting that when there are no hurricanes or tropical depressions in the 30 degree latitude areas, the result is a problem for all living things. Drought, dropping water tables, surface collapses, saline invasion of fresh water areas, and a plethora of other problems cause suffering for man as well as all other living things.
I am sure that there are readers of this column living in New Orleans, Homestead, Florida, or some other place that have suffered terrible destruction and pain from a hurricane who will object to the portrayal of a hurricane as a design feature of the earth. We certainly empathize with those who have suffered because of these storms, but most of the horrific damage from these storms happens because of human error. In the original coastal areas of North America for example, there were massive mangrove swamps which eliminated storm surge and blunted the effect of wind and rain. Swamps and marshes had their own set of physical properties that minimized the damage from hurricanes. Mankind has destroyed the mangroves, drained the swamps, and built cities below sea level surrounded by levies. Human structures made of light-weight materials are not going to withstand heavy wind. It is interesting that in the Bahamas most of the buildings are made of cement, and hurricanes are accepted as a normal event in the lives of those who live in these islands. Hurricanes are storms that arrive slowly with adequate warning, and need not cause the loss of human life or significant losses of property. We have seen tragedy because of human greed and carelessness that need not have happened.
How difficult would it be to design this weather machine? How many variables are there in this system? The first item that has to be designed is the miracle of water itself. Its molecular weight (18) is less than the molecular weight of air (30) making the density change possible to produce weather systems, evaporation, and the Hadley Cells shown in the drawing (to the right above). The cohesive and adhesive properties of water which are dependent on the chemical properties of oxygen and hydrogen are essential for the processes we have been describing. Water's high specific heat, its very high heat of vaporization, and the most unusual closeness of its freezing and boiling points are also critical.
From an astronomical standpoint there is a huge number of variables that have to be carefully chosen for the system to work. The mass of the earth controls the thickness of the earth's atmosphere. The rotation rate of the earth allows the air masses to move properly, and avoids the possibility that the wind velocities will go to catastrophic levels. The gases that make up our atmosphere (primarily nitrogen and oxygen) provide a density and set of physical and chemical factors essential for life and for the weather systems to function. The size of our sun, its temperature, the type of star it is, the kinds of radiations it emits, its stability, and its distance from the earth are all critical factors.
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