However above the troposphere, where atmospheric pressure is a small fraction of its sea-level value, the presence of ozone causes temperature to rise with height until the altitude of the stratopause is reached.
Above the stratosphere, temperature declines with height in the mesosphere, but rises again in the thermosphere due to the effect of radiation and charged particles from the Sun on what little atmosphere is left near the boundary with space. Learn more about how scientists in Antarctica collect data about how the atmosphere changes with height. Most of the stratospheric ozone is produced at tropical latitudes, but high altitude winds spread it over the whole planet.
It is continually forming and breaking down, and its distribution over the planet is not uniform or constant. Instead, there are seasonal and longer term variations in the quantity of stratospheric ozone in different parts of the world. However, over the long run the natural processes of formation and breakdown are balanced: it is only in recent decades that human activities have led to ozone being destroyed much faster than it can be formed, thereby creating the ozone hole that exists today.
This problem occurs primarily in the summer in cities with a high amount of traffic when sunlight interacts with car exhaust fumes containing nitrogen oxides. Ozone is measured as the total amount that is present in a column of overlying atmosphere in Dobson units.
One Dobson unit can be thought of as the amount of ozone that would be present if it formed a layer 0. A typical Dobson reading for the ozone layer is about Dobson units, meaning that the ozone layer would only be about 3mm thick if brought down to sea-level. The Dobson unit is so named because of the Dobson spectrophotometer which is used to make the measurements.
It works by comparing the ratio of two different wavelengths of UV radiation — one being more strongly absorbed by ozone than the other — and using the observed ratio to calculate the amount of ozone overhead. This method has been used to measure the ozone layer at Halley Research Station since In , British Antarctic Survey scientists published results showing a steep decline in the levels of ozone over Halley since the s, particularly during the austral spring, and the existence of the ozone hole was revealed.
Since then, the extent of the ozone hole has been monitored continuously using both ground-based and satellite-based techniques. The image from the link above shows the size and shape of the ozone hole as measured in October Notice that the hole where values are only around Dobson units covers most of Antarctica, and areas of depleted ozone under units extend beyond the continent.
The ozone hole has developed because people have polluted the atmosphere with chemicals containing chlorine and bromine. The primary chemicals involved are chlorofluorocarbons CFCs for short , halons, and carbon tetrachloride. CFCs in particular have been used for a wide range of applications, including refrigeration, air conditioning, foam packaging, and making aerosol spray cans. Because these chemicals are so inert, they are able to stay in the atmosphere long enough to be carried upwards to the stratosphere where they can damage the ozone layer.
The actual processes by which CFCs and other ozone depleting chemicals destroy ozone are complex and require certain weather conditions to exist. A simplified description of the process involving CFCs is as follows:. The fact that most of the ozone depletion happens over Antarctica also requires some explanation. CFCs and other ozone depleting gases may come from anywhere, but it is in the south polar stratosphere where the conditions become most favourable for ozone destruction.
The key factor is the presence of stratospheric clouds and the lack of atmospheric mixing between the south polar latitudes and air from elsewhere during the austral winter and early spring. Normally there are no clouds in the stratosphere because there is so little water vapour present. As long as it remains dark, nothing happens; but when spring arrives, UV radiation from the Sun reaches the Antarctic Circle and starts the process of chlorine release and ozone destruction.
This continues until the stratospheric clouds disappear due to warming of the south polar atmosphere as summer approaches. By summertime, stratospheric air from lower latitudes is able to penetrate the polar latitudes, and thereby replenish the ozone layer above Antarctica. Hence, there is a seasonal cycle to the ozone hole over Antarctica with the lowest ozone levels recorded in late September and early October.
The ozone layer protects life from harmful UV-B radiation which can cause cancer and stunt the growth of plants. As UV radiation can penetrate into the surface of the ocean, marine organisms especially phytoplankton can also be damaged. False-color view of total ozone over the Antarctic pole. Purple and blue represent areas where there is the least ozone, yellows and reds where there is more ozone. Yes and no. The ozone layer, which lies high in the atmosphere, shields us from harmful ultraviolet UV rays that come from the Sun.
Human activities effectively punched a hole in it, through the use of gases like chlorofluorocarbons CFCs in spray cans and refrigerants, which break down ozone molecules in the upper atmosphere. Animals Wild Cities This wild African cat has adapted to life in a big city. Animals This frog mysteriously re-evolved a full set of teeth. Animals Wild Cities Wild parakeets have taken a liking to London.
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