Sandy and Nilam. Those depressing visitors!

Hurricane Sandy began receding on October 31 2012 from the Eastern seaboard of the United States, and cyclone Nilam hit the Eastern seaboard of India on November 1 2012. Both are examples of naturally recurring phenomena that create havoc and cause losses worth billions of dollars, and hundreds of lives every year. The affected areas are still limping back to normalcy.

Cyclones ("hurricanes" in North America and "typhoons" in Japan) are giant storms that have high winds and heavy rainfall produced by a circling vortex of clouds that can be over a thousand miles across. Cyclones are caused by weather depressions. But they are also the cause of fiscal and mental depression because of the damage and chaos they cause.

You can better understand low pressure or depression by understanding pressure itself. 

The Earth is wrapped in a 75-mile-thick film of air; to scale, that is like the film of water on a wet basketball. Towering over any point on the Earth's surface is a column of air approximately 75 miles high. This column of air, with help from the Earth's gravitational pull, exerts a force. Force measured per unit area is called "pressure", and the air we breathe exerts a measurable atmospheric pressure on everything around us, at all times. 

The air is not equally dense at all altitudes - 75% of the air is available closest to the Earth, a zone going all around that is less than 11 miles high. This is the air made up mainly of Nitrogen and Oxygen that all creatures breathe. As you go higher, the density of air reduces as there are fewer and fewer molecules of air available, and it gets more and more difficult to breathe. On the surface of the Earth, this pressure - commonly known as atmospheric pressure and measuring 1 bar or 101,315 Pascals - is equivalent to a standing column of 76 cm of Mercury. In other words, the pressure exerted by 75 miles of atmospheric air equals the pressure exerted by 76 cm of the much denser fluid, Mercury.

An atmospheric depression, therefore, is a low pressure zone within the Earth's atmosphere. It is caused because differing sunshine from the tropics to the poles causes differing ocean temperatures and air temperatures, which in turn causes low and high pressure regions. The characteristic of a low pressure region is that air from surrounding areas rushes in to equalize the pressure. Put very simply, air rushing in to fill an atomospheric depression is what produces high winds, that sometimes go on to create a hurricane; the spinning, moving vortex is formed because the zone of depression itself keeps moving due to the rotation of the Earth.
The greater the difference in ocean temperature, the greater the depression, the greater the velocity of winds, and the greater the storms that are produced. Ever since the invention of barometers, it has been noticed that a drop in air pressure could mean the onset of a storm. Climate change in recent decades has meant higher and higher temperatures, which produce deeper depressions and bigger, more destructive storms.

Keep in mind that 1 bar, or 1000 millibar, is the normal atmospheric pressure at sea level. The atmospheric pressure at landfall of hurricane Sandy (940 millibar) was lower than the pressure of cyclone Nilam (990 millibar). The apparently small difference in pressure, however creates a much magnified effect at the scale of a storm - Sandy had winds as high as 110 mph compared to the 50 mph winds of cyclone Nilam. You can track global temperatures, see how they affect barometric pressures, and understand how depressions form and produce storm systems.

No comments:

Post a Comment

Post a Comment