why is the hole in the ozone layer over antarctica
Introduction Why is the ozone hole over Antarctica? That is one of the first questions that comes to mind when people think about the ozone hole. Every winter and spring since the late 1970s, an ozone hole has formed in the stratosphere above the Antarctic continent. In recent years this hole has become both larger and deeper, in the sense that more and more ozone is being destroyed. As summer approaches, the hole repairs itself, only to reform during the following spring. Measuring the Ozone Hole The most common ozone measurement unit is the Dobson Unit (DU). The Dobson Unit is named after atmospheric ozone pioneer G. M. B. Dobson who carried out the earliest studies on ozone in the atmosphere from the 1920s to the 1970s. A DU measures the total amount of ozone in an overhead column of the atmosphere. Dobson Units are measured by how thick the layer of ozone would be if it were compressed into one layer at 0 degrees Celsius and with a pressure of one atmosphere above it. Every 0. 01 millimetre thickness of the layer is equal to one Dobson Unit. The average amount of ozone in the stratosphere across the globe is about 300 DU (or a thickness of only 3mm at 0пC and 1 atmospheric pressure! ). Highest levels of ozone are usually found in the mid to high latitudes, in Canada and Siberia (360DU). Why is the Hole over the Antarctic?
Observed ozone over the British Antarctic Survey station at Halley Bay first revealed obvious decreases in the early 1980s compared to data obtained since 1957. The ozone hole is formed each year when there is a sharp decline (currently up to 60%) in the total ozone over most of Antarctica for a period of about two months during southern hemisphere spring (September and October). Man-made emissions of CFCs occur mainly in the northern hemisphere, with about 90% released in Europe, Russia, Japan, and North America. Gases such as CFCs that are insoluble in water and relatively unreactive are mixed throughout the lower atmosphere and rise from the lower atmosphere into the stratosphere; winds then move this air poleward. Normally, chlorine and bromine is inactive, locked up in stable compounds, and does not destroy the ozone. However, during the Antarctic winter months (June to August) when the region receives no sunlight, the stratosphere becomes cold enough (-80пC) for high level [ice] clouds to form, called Polar Stratospheric Clouds (PSCs). These PSCs provide an ideal catalytic surface on which the chlorine can react with the ozone, thus destroying the ozone layer. This reaction requires sunlight, and therefore only begins when the Sun returns to Antarctica in spring (September to October), before the PSCs have had a chance to melt.
The ozone hole disappears again when the Antarctic air warms up enough during late spring and summer
Annual Decline in October Stratospheric Ozone over Antarctica During the southern hemisphere winter, Antarctica is isolated from the rest of the world by a natural circulation of wind called the polar vortex. This prevents atmospheric mixing of stratospheric ozone, thus contributing to the depletion of ozone. Although some ozone depletion occurs over the Arctic, meteorological conditions there are very different to Antarctica and so far have prevented the formation of ozone holes as large as in the southern hemisphere. A hole in the Earth's ozone layer that forms over Antarctica each September due to warm air was the smallest this year since 1988, scientists from and the National Oceanic and Atmospheric Administration ( ) announced on Friday. В According to NASA, the ozone hole reached its peak extent on September 11, covering an area about two and a half times the size of the US -- 7. 6 million square miles -- and then declined through the remainder of September and into October. The NOAA's ground-and balloon-based measurements also showed the least amount of ozone depletion above the continent during the peak of the ozone depletion cycle since 1988. В "The Antarctic ozone hole was exceptionally weak this year," Paul A. Newman, chief scientist for Earth Sciences at NASA's Goddard Space Flight Centre in Greenbelt, Maryland, said in a statement.
В "This is what we would expect to see given the weather conditions in the Antarctic stratosphere," Newman added. The smaller ozone hole in 2017 was strongly influenced by an unstable and warmer Antarctic vortex -- the stratospheric low pressure system that rotates clockwise in the atmosphere above Antarctica. В In 2016, the ozone hole reached a maximum 8. 9 million square miles, 2 million square miles less than in 2015. The average area of these daily ozone hole maximums observed since 1991 has been roughly 10 million square miles. Scientists said the smaller ozone hole extent in 2016 and 2017 is due to natural variability and not a signal of rapid healing. First detected in 1985, the Antarctic ozone hole forms during the Southern Hemisphere's late winter as the returning sun's rays catalyse reactions involving man-made, chemically active forms of chlorine and bromine. These reactions destroy ozone molecules. In the stratosphere, roughly seven to 25 miles above Earth's surface, the ozone layer acts like sunscreen, shielding the planet from potentially harmful ultraviolet radiation that can cause skin cancer and cataracts, suppress immune systems and also damage plants.
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