The Ozone Layer

"The ozone layer" refers to the ozone within stratosphere, where over 90% of the earth's ozone resides. Ozone is an irritating, corrosive, colorless gas with a smell something like burning electrical wiring. In fact, ozone is easily produced by any high-voltage electrical arc (spark plugs, Van de Graaff generators, Tesla coils, arc welders). Each molecule of ozone has three oxygen atoms and is produced when oxygen molecules (O2) are broken up by energetic electrons or high energy radiation. For information on the history of the ozone layer for the layman, see the Short history of ozone depletion , National Oceanic and Atmospheric Administration's NOAA Ozone overview or NOAA on stratospheric ozone. For short and to-the-point answers, check out Robert Parson's Ozone overview, FAQ1

The Stratosphere

Variations in temperature and pressure divide the earths atmosphere into layers, shown below, and mixing of gases between the layers happens very slowly.

The altitudes on the diagram are logarithmic so an analogy in the glossary might give you a better idea of the relative thicknesses of these layers. Notice that the lowest 10% of the atmosphere holds 90% of the air. This is because gases are compressable. In a huge pile of feathers the bottom-most feathers become compressed under the weight of the feathers above them. Likewise the lower levels of the atmosphere are filled with compressed air while the upper levels, such as the stratosphere, contain very 'thin' uncompressed air. Although the stratosphere layer is over four times thicker than the lower atmosphere, the stratosphere holds so little gas that ozone is still considered one of the minor trace-gases of the overall atmosphere.

The ozone layer absorbs 97-99% of the sun's high frequency ultraviolet light , light which is potentially damaging to life on earth. Every 1% decrease in the earths ozone shield is projected to increases the amount of UV light exposure to the lower atmosphere by 2%. Because this would cause more ozone to form in the lower atmosphere, it is uncertain how much of UV light would actually reach the earths surface. Recent UV measurements from around the northern hemisphere indicate small UV increases in rural areas and almost no increase in areas near large cities.

Units used to measure ozone concentration

When describing the amount or concentration of gas, scientists resort to several different units: Dobsin unit (DU)- the principle unit for measuring ozone concentration. One DU is about twenty-seven million molecules per square centimeter ( the palm of your hand covers an area of rougly a hundred square centimeters). The ozone concentration over the US is about 300 DU and the antarctic hole during the late spring can drop to 117 DU.

Mixing ratios: within a specified volume, it is a fraction of the number of molecules of a particular gas divided by the total number of molecules in that given space. Terms of usually abreviated, like ppmv for parts-per-million or ppbv which is parts-per-billion . For example the concentration of HCl at 3 km is said to be about 0.1 ppbv; this means that if you selected a volume of air that contained 10 billion molecules of air, one of those molecules would be an HCl molecule.

Factors influencing Ozone concentrations

Stratospheric sulfate aerosols: large explosive volcanoes are able to place a significant amount of aerosols into the lower stratosphere, as well as some chlorine. Because more than 90% of a volcanic plume is water vapor most of the other compounds, including volcanic chlorine, get ''rained-out'' of the stratosphere. The effects of a large volcano on global weather are significant, which in turn can affect localized weather patterns such as the antarctic ozone hole. Many observations have linked the 1991 Mt. Pinatubo eruption to a 20% increase in the ozone hole that following spring[Solomon et al. 1993]) . The effects of a large volcanic eruption on total global ozone are more modest (less than 3%) and last no more than 2-3 years.

Stratospheric winds: every 26 months the tropical winds in the lower stratophere change from easterly to westerly and then back again, an event called the Quasi-biennial Ocillation (QBO). The QBO causes ozone values at a particular latitude to expand and contract roughly 3%. Since stratospheric winds move ozone, not destroy it, the loss of one latitude is the gain of another and globally the effects cancel out.

Greenhouse gases: to the degree that greenhouse gases might heat the planet and alter weather patterns, the magnatude of the stratospheric winds will certainly be affected. Some of the more popular senarios of global warming predict cooler stratospheric temperatures, leading to more polar stratospheric clouds and more active chlorine in the area of the antarctic ozone hole.

Sunspot cycle: ozone is created by solar UV radiation. The amount of UV radiation produced by the sun is not constant but varies by several percent in a rougly 11year cycle. This 11year cycle is related to magnetic changes within the sun which increase the solar UV output, and is heralded by an increase sunspots which appear on the surface of the sun. Comparisons of yearly ozone concentrations show a small 11 year variation in global ozone of about 2%. Episodes of unusual solar activity, solar storms and large solar flares, could certainly alter this value.

Stratospheric chlorine, coming mostly from man-made halocarbons. Careful subtracting of other natural factors yields a net decrease of 3% per decade in global ozone,1978-1991; due most likely to catalytic degredation by stratospheric chlorine.

Decrease in global ozone The measurement period is from November 1978 through November 1987, and combines depletion due to natural and man-made causes. This analysis and graphic comes from the United Nations Environmental Protection Agency(UNEP).


Twenty-five years ago if you made a trip to the local library and perused the periodical section for articles on global warming, you’d probably have come up with only a few abstracts from hardcore science journals or maybe a blurb in some esoteric geopolitical magazine. As an Internet search on global warming now attests, the subject has become as rooted in our public consciousness as Madonna or microwave cooking. Power plants, cattle, and cars are some of the major contributors of greenhouse gases such as carbon dioxide and methane.

Perhaps all this attention is deserved. With the possible exception of another world war, a giant asteroid, or an incurable plague, global warming may be the single largest threat to our planet. For decades human factories and cars have spewed billions of tons of greenhouse gases into the atmosphere, and the climate has begun to show some signs of warming. Many see this as a harbinger of what is to come. If we don’t curb our greenhouse gas emissions, then low-lying nations could be awash in seawater, rain and drought patterns across the world could change, hurricanes could become more frequent, and El Niños could become more intense. Some possible effects of global warming are the inundation of low-lying islands due to rising sea levels, increased frequency of severe storms, and the retreat of glaciers and icecaps.

On the other hand, there are those, some of whom are scientists, who believe that global warming will result in little more than warmer winters and increased plant growth. They point to the flaws in scientists’ measurements, the complexity of the climate, and the uncertainty in the climate models used to predict climate change. They claim that attempting to lower greenhouse emissions may do more damage to the world economy and human society than any amount of global warming. In truth, the future probably fits somewhere between these two scenarios. But to gain an understanding of global warming, it is necessary to get to know the science behind the issue. More yet to be updated!!

Black Hole

What is a Black Hole?

A black hole is one of the strangest objects in space. It is an area in space where gravity is so strong that even light cannot escape from it. Since light cannot escape from a black hole, it appears black. Light can travel faster than anything we know of - at a speed of 186,000 miles (300,000 kilometers) per second. If light cannot escape from a black hole, nothing else that we know of can. A black hole is not really a hole and it is not empty. It is filled with a lot of material crammed into an extremely small space. This is what gives a black hole its super strong gravity. The term black hole is used because these objects look like black holes in space - since they put out no light.

How are black holes made?

Scientists think that black holes are created in places where matter gets extremely dense (where a huge amount of material is crammed into an extremely small space). This can happen in the centers of large galaxies or when a giant star collapses and shrinks during the final phases of its life. When matter gets so dense that light cannot escape from it, the region that it is in becomes a black hole.

Are there really black holes in space?

Since black holes do not put out any light, we cannot see them. However, we can see the effect that they have on the area of space around them. Since black holes have extremely high gravity, they pull in surrounding material at very high speeds, causing this material to become very hot and emit X-rays. By finding this very hot material which is spiraling into black holes, astronomers can locate where some of them are. Also, astronomers study the motions of objects in space to see where there is material that might be moving as if a black hole were affecting it. So far, evidence had been found for the existence of black holes in the centers of several large galaxies, and in binary star systems (where two stars orbit each other).

How big is a black hole?

Black holes come in many sizes and their size depends on how much material is in them (their mass). Some are the remains of a giant star which collapsed. A star has to be much more massive than our Sun to become a black hole. These types of black holes are only a few miles across. Black holes have also been discovered in the centers of some galaxies. These black holes are very large and contain the same amount of material as 100 million or more suns. These types of black holes are several million miles across.