Weather Graphics Hurricane Science - Weather Graphics
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Hurricane Science
Recent studies involving hurricane science point an accusing finger at global warming as a contributing factor in what seems to be an increase in the intensity of tropical storms. Another study predicts that global warming may decrease the number of hurricanes that occur. Before exploring how global climate changes are likely to alter the physics of hurricanes and their development, we need first to examine the mechanics of hurricane formation.
There are six regions where hurricanes are known to form. Three are in the Pacific, and are referred to as the Northern Pacific region, the Western Northern Pacific region and the Eastern Pacific region. The North Atlantic region and the Southern and Northern Indian Ocean regions are the other three. Hurricanes start out in these regions as tropical storms. If the sea surface temperature is 79 degrees F or warmer, vast amounts of water vapor can condense rapidly up into the atmosphere through evaporation. While the rate of condensation is not drastically affected by the rate of evaporation, it IS affected by the temperature of the air immediately above the water's surface. The warmer it is, the more evaporation, the greater amount of water vapor gets sucked up into the storm. This causes a rapid ascent of the storm and an expansion of air. Expansion, in turn, causes air to cool. This temperature drop results in a drop in dew point--the point at which a particular body of air experiences condensation and subsequently rain formation.
As the storm ascends, it leaves a zone of low pressure below and surrounding air rushes in to fill the void. The onrushing gusts collide and are forced upward adding even more moisture to the air. Hence, more condensation, more rain. Condensation releases heat, so as the condensation increases, so does the heat, which further fuels the low pressure area below the storm which results in a tropical depression. If this build-up of energy, moisture, wind velocity and rain continues long enough it develops into a hurricane. The earth's rotation creates a force known as the Coriolis force. In the Northern Hemisphere, winds that flow inward to fill the low pressure void are deflected to the right, and a counter clockwise rotation is formed. In the Southern Hemisphere, the Coriolis force causes winds to be deflected to the left resulting in a clockwise rotation.
At the center of this fierce phenomenon, there is a small area were the weather is strangely calm, with clear sky and light breezes. This, of course is the infamous "eye of the storm." Occurring when winds have reached about 80 miles per hour, this "eye" can be anywhere from 20 to 40 miles in diameter. It extends down into the storm like the opening of a cylinder. The area surrounding the eye is known as the "eyewall," a dangerous combination of dense cumulonimbus clouds, thunderstorm clouds and extremely strong winds.
Hurricanes gain momentum as they move across the ocean. Once they reach land, however, they quickly lose energy. Unfortunately, before winding down, these fierce products of nature can unleash winds as high as 185 miles per hour onto the coast and surrounding cities and towns. The most dangerous storms are known as Category 4 or 5, a measurement from the Saffir-Simpson Hurricane Scale, developed originally for Atlantic and Northern Pacific regions. Other regions use different scales to measure the storms, which are more widely known as cyclones or typhoons depending on the region. To be considered a Category 5, the storm must have winds exceeding 155 miles per hour (69m/s; 135kn'; 249km/h)
Some would argue that a recent increase in dangerous hurricane conditions in the U.S. is simply a natural peak in the North Atlantic hurricane cycle. Researchers from the Georgia Institute of Technology would beg to differ, having published a study which suggests that this is a global trend affecting hurricane regions around the world. What they found was an increase in Category 4 and 5 storms across the board.
These scientists assembled data going back 35 years, combining data on hurricane intensity and sea surface temperatures, their analysis showed that globally there has been little to no noticeable change in the number of hurricanes per year. However, what the data did reveal was a troubling global increase in tropical storms that have reached a Category 4 or 5 in intensity. In the early 1970's records indicated that only 20% of these storms reached the Category 4 or 5 status. By the 1990's about 35% of these stormed reached dangerous Category 4 or 5 levels. By analyzing this data against the sea surface temperatures, they noted a correlation between rises in sea surface temperatures and storm intensities while taking into consideration other relevant environmental factors thought to influence these storms, such as humidity, wind shear and zonal stretching deformation. Since increasing sea surface temperatures are driven by global warming, this study adds even more evidence that global warming is causing stronger hurricanes--2005 being the worst season on record.
While these dire predictions might indicate the the worst is yet to come, a more recent study indicates a brighter picture. One researcher with the American Geophysical Union, Anand Gnanadesikan, has looked into how changes in the color of ocean waters might affect the prevalence of typhoons and hurricanes. His study predicts that hurricane formation may decrease by as much as 70% over time, due to predicted drops in the phytoplankton population and decreasing chlorophyll concentrations. A reduction in chlorophyll is thought to allow sunlight to penetrate more deeply into the ocean, which would leave the surface of the ocean cooler. A drop in surface temperature in Gnanadesikan's model is said to affect hurricanes in several ways. Cold water providing less energy and a change in air circulation patterns leading to more dry air aloft, which creates less favorable conditions for hurricanes to grow.
Utilizing a computer model, Gnanadesikan ran two possible scenarios. First, real conditions in the North Pacific using chlorophyll concentrations observed by satellites. A second scenario looked at chlorophyll concentrations in the North Pacific Subtropical Gyre--a large clockwise circulation pattern that occurs in most of the North Pacific. Here, the chlorophyll concentrations were set at zero. This resulted in modified air circulation and heat distribution patterns both within and beyond the gyre. Along the equator these new patterns showed an increase in hurricane formation by about 20%. However, the simulation showed a 70% decrease in storm formations further north over and near the gyre. In this scenario more hurricanes would hit the Philippines and Vietnam but, fewer would reach landfall in South China and Japan. In general, the North Pacific region produces more than half of the world's hurricanes.
Gnanadesikan admits that the complete absence of chlorophyll in areas of the world's oceans is a drastic scenario. Still, studies show that there has been a steady decline in global phytoplankton populations, which would certainly indicate that Gnanadsiken's study deserves a look.
Weather Graphics Hurricane Science - Weather Graphics
By: ezmickel934
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