Ultraviolet (UV) / Ozone Frequently Asked Questions

This page contains answers to frequently asked questions regarding UV radiation and the ozone layer; scroll down the page to view all the FAQs.

Ultraviolet radiation

What is ultraviolet (UV) radiation?

Ultraviolet radiation is the process by which shorter wavelength electromagnetic (radiant) energy is transferred from one place to another, but typically from the sun through the Earths atmosphere to the Earths surface; less typically from a tanning bed lamp to a persons skin. Radiant energy (measured in joules) or irradiance (the radiant power measured in watts), is classified according to wavelength measured in nanometres (one nm is a millionth of a millimetre). The shorter the wavelength, the more energetic. In order of decreasing energy, the principal forms of radiant energy are gamma rays, X rays, UV (ultraviolet), visible (or light), infrared, microwaves, and radio waves. There are three categories of UV:

  • UV-A, between 320 and 400 nm
  • UV-B, between 280 and 320 nm
  • UV-C, between 200 and 280 nm
How harmful is UV radiation?

Generally, the shorter the wave-length, the more biologically damaging UV radiation can be if it reaches the Earth in sufficient quantity.

  • UV-A reaches the Earth from the sun in the greatest quantities. Recent information suggests that UV-A can damage the skins immune system. UV-A from the sun passes right through the ozone layer to the Earths surface with little attenuation.
  • UV-B radiation is potentially very harmful. Fortunately, most of the sun's UV-B radiation is absorbed by ozone in the stratosphere.
  • UV-C radiation is potentially the most damaging because it is very energetic. Fortunately, all UV-C is absorbed by oxygen and ozone in the stratosphere and never reaches the Earth's surface.
What factors affect the amount of UV radiation that reaches the Earth?

Although the ozone layer is the one constant defence against UV penetration, several other factors can have an effect:

  • Ozone: Lower ozone values in the atmosphere lead to higher UV levels, and conversely, higher ozone values lead to lower UV levels.

    Ozone levels are affected by weather patterns in the upper atmosphere. Areas of converging air in the upper atmosphere generally have slightly higher ozone values resulting in slightly lower UV levels, and areas of diverging air in the upper atmosphere have slightly lower ozone values resulting in slightly higher UV levels.

  • Latitude: Since the sun's UV energy impacts the Earth's surface at the most direct angle over the equator it is the most intense at this latitude.

    Also ozone is transported by large scale circulations in the upper atmosphere, from equatorial regions towards the polar regions, resulting in generally higher values of ozone, and therefore lower levels of UV.

  • Time of day: On any day the greatest amount of UV reaches the Earth around midday when the sun is at its highest point. Up to 50% of daily UV radiation levels is received between 11am and 2pm.

    Daily changes in the angle of the sun influence the amount of UV that passes through the atmosphere. When the sun is low in the sky, solar energy must travel a greater distance through the atmosphere and may be scattered and absorbed by water vapour and other atmospheric components.

  • Time of year (Season): During winter months, the sun is lower in the sky than in summer, and the length of the day is shorter, hence UV is less intense.
  • Altitude: The air is thinner and cleaner on a mountain top, therefore more UV reaches the mountain top than at lower elevations. UV levels increase by 4% for every 300 metre rise in altitude. Some ski resorts in Australia are situated near 2000m altitude and will receive UV levels up to 25% more than locations situated at sea level along a similar latitude.
  • Cloud cover: Clouds can have a marked impact on the amount of UV that reaches the Earth's surface; generally, thick clouds reflect and absorb more UV than thin cloud cover. UV can pass through thin clouds. However, sides of clouds can also reflect UV and focus solar energy and can increases the amount received at the Earths surface in some situations.
  • Air pollution: Similar to the way clouds shield the Earth's surface from the suns UV, urban smog can reduce the amount of UV energy reaching the Earth by reflecting UV back towards space or absorbing UV.
  • Land Cover: Incoming UV is reflected from most surfaces. Reflected UV can damage people, plants, and animals just as incoming UV does. Conversely, you will still receive about 40% of UV radiation levels in water at a depth of 50cm. The following table of UV reflectances is sourced from Sliney, DH. 1986.
    Table of UV reflectances
    Surface Reflectance (%)
    Snow (new) 88
    Snow (old) 50
    Sea surf (white foam) 25 to 30
    Sand (dry) 18
    Sand (wet) 7
    Concrete footpath 8 to 12
    Boat deck (wood to fibreglass) 6 to 9
    Open water or ocean 3 to 8
    Soil 4 to 6
    Lawn grass 2 to 5
Is there any link between the temperature and UV levels?

There is no link between UV levels and how hot or cold the temperature is. There are differences between UV levels during Summer and Winter, but this is mostly due to the angle of the sun in the sky. UV levels generally peak around the middle of the day, whilst the temperature may still be rising towards its maximum in the afternoon. The graph below outlines this relationship.

A graph showing the relationship between temperature 
and UV index between sunrise and sun set in Melbourne 
on the 5th of January 2008. The Y axis on the left 
shows the temperature in degrees Celsius from 20 to 40 
degrees. The X axis (across the bottom) give the time 
of day between sunrise at 6 in the morning and sunset 
at 9 at night. The Y axis on the right show the UV index 
between zero and 15. A blue line representing the UV index 
shows that it is at 0 at sunrise and sunset, and at 3 at 
approximately 9 in the morning and 5 in the afternoon and 
peeks at 12 at 1 in the afternoon. The temperature, 
represented by a red line, on the other hand, starts at 
about 22 degrees at 6 in the morning and slowly climbs to 
a peek of around 37 degrees at about 7 in the evening then 
falls to about 32 degrees at 9 at night.
How does the ozone layer protect us from skin damage

Ozone's unique physical properties allow the ozone layer to act as our planet's sunscreen, providing an invisible filter to help protect all life forms from the sun's damaging UV. All UV-C and most incoming UV-B is absorbed by ozone and prevented from reaching the Earth's surface. Without the protective effect of ozone, life on Earth would not have evolved the way it has.

How does UV-B exposure affect people?

Exposure to UV-B, causes skin cancer, hastens skin aging, and can cause eye damage. The human immune system can also be weakened by exposure to UV-B. However it is important to note that UV-B has always had these effects on humans. In recent years these effects have become more prevalent because Australians are spending more time in the sunshine and are exposing more of their skin in the process. An increase in the levels of UV-B reaching the Earth as a result of ozone depletion may compound the effects that sun worshipping habits have already created.

Although fair-skinned or fair-haired individuals are at highest risk for skin cancer, the risk for all skin types increases with exposure to UV-B. The effects of UV-B on the human immune system have been observed in people with all types of skin.

The incidence of skin cancer, per head of population, in Australia is ten times higher than America and more than twenty times higher than in the U.K. More than 60% of our population will be treated for skin cancer in one form or another in their lifetime.

Skin cancer is very dangerous. It can be fatal if not detected early and treated correctly, usually by surgery. Over one thousand Australians die every year from melanoma and other forms of skin cancer.

How does UV-B exposure affect plants and animals?

Excessive UV-B inhibits the growth processes of almost all green plants. There is concern that ozone depletion may lead to a loss of plant species and reduce global food supply. Any change in the balance of plant species can have serious effects, since all life is interconnected. Plants form the basis of the food web, prevent soil erosion and water loss, and are the primary producers of oxygen and a primary sink (storage site) for carbon dioxide.

UV-B causes cancer in domestic animals similar to those observed in humans. Although most animals have greater protection from UV-B because of their heavy coats and skin pigmentation, they cannot be artificially protected from UV-B on a large scale. Eyes and exposed parts of the body are most at risk.

How does snow affect UV levels?

Snowcover can increase UV levels by approximately 90% in fresh snow, and by approximately 50% for old snow. The Bureau of Meteorology provides snow adjusted UV values for locations in snow covered areas between 1 June and 15 October each year.

Sun protection is recommended by Cancer Council Australia whilst on the snow.

How does solar UV exposure change during the year?

Solar UV exposure changes throughout the year in line with the seasons. The highest exposure levels are experienced in the Summer months, and the lowest levels in the Winter months. During Spring and Autumn, solar UV exposure levels may change rapidly day to day. View our climatology maps for more detailed information.

Where can I find UV information for overseas locations?

If you are travelling overseas, UV forecasts are available from the majority of Weather Services organisations. The World Health Organisation has compiled a list of countries that provide UV forecasts.


What is ozone?

Ozone is the triatomic form of oxygen (that is O3 rather than the usual form O2) and is a naturally occurring trace gas in the earth's atmosphere.

Where is ozone found in the atmosphere?

About 90% of ozone is concentrated in the lower part of the stratosphere, between about 15 and 30 kilometres above the earth's surface, where it is sometimes referred to as the ozone layer. Ozone is also found naturally, at lower concentrations, in the troposphere. In the boundary layer, human made pollutants can cause the production of excess ozone.

Ozone is found in the atmosphere over the entire globe although the amount varies with location and season.

Why is ozone important?

Ozone is the major absorber of UVB (Ultraviolet radiant energy in the wavelength range 280-320 nanometres) in sunlight, absorbing approximately 90% of it. Many experimental studies of plants and animals, and clinical studies of humans, have shown the harmful effect of excessive exposure to UVB radiant energy. In humans these effects include increased incidence of skin cancer and cataracts.

In the 1970s, scientists first raised concerns that the production and use of synthetic substances known as chloroflourocarbons (CFCs) might lead to a reduction of ozone in the stratosphere leading to an increase of UVB radiant energy. It was with the discovery of the Antarctic Ozone Hole, however, in the mid 1980s, that the issue of ozone depletion due to human activity rose to great prominence, leading to the signing of the Montreal Protocol.

Ozone is also a radiatively important gas in the atmosphere which absorbs both incoming ultraviolet and outgoing infrared radiant energy, and thus has a significant impact on the earths climate.

Finally, ozone remains a very useful tracer for following air movements in the stratosphere in fact this is the reason it was originally monitored.

In the troposphere, excess ozone is generated by human pollution and is harmful to human, animal and plant health.

What is meant by ozone depletion?

During the 1980s and early 1990s, ozone levels around the world dropped steadily, and scientists believe this reduction was primarily due to the human production of Ozone Depleting Substances (ODSs). Averaged over the globe, the size of the reduction was approximately 5%, although this amount varied considerably with location and season.

However, it was also discovered in the mid 1980s that a much greater depletion of ozone had started taking place each year over Antarctica in springtime, and this phenomenon became known as the Antarctic Ozone Hole and received great publicity.

During the most severe period of the Antarctic Ozone Hole (usually late September or early October) total column ozone can drop by as much as a half or even two-thirds at some locations.

The Antarctic Ozone Hole has continued to form each year from the 1980s until today.

What are Ozone Depleting Substances?

Ozone Depleting Substances (ODSs) are gases that contain chlorine or bromine atoms in forms able to reach the stratosphere, where they can take part in chemical reactions that destroy ozone on a large scale. The best known are the chloroflourocarbons (CFCs), formerly used extensively in refrigeration, air-conditioning, foam-blowing and as aerosol propellants, and the halons (hydrocarbon gases containing bromine), which were widely used as fire extinguishers. Production of all major ODSs is now regulated by the Montreal Protocol. Most ODSs are made by human activity but some do have natural sources as well, including methyl bromide and methyl chloride.

How much has ozone been depleted over Australia?

Ozone depletion in the 1980s and early 1990s was more severe in the southern hemisphere than the northern. Observations suggest that, in southern mid-latitudes (which includes Sydney, Canberra, Adelaide, Melbourne and Hobart) the amount of depletion caused by ODSs during this time was about 5%. The amount of depletion was observed to be smallest in the tropics and to increase with distance from the equator. It is important to remember, though, that total ozone varies considerably from year to year due to natural variations, not all of which are currently fully accounted for. This means that it takes many years for clear trends to be discernible. Further long-term ozone changes can be caused by changes in transport rather than chemistry. Nonetheless, there are now tentative early indications that ozone is recovering following the success of the Montreal Protocol and its amendments.

Is ozone depletion the same as the greenhouse effect, global warming or climate change?

Ozone depletion and changes to climate due to the increased concentration of well-mixed greenhouse gases produced by human activity are essentially separate issues, in the sense that either one would still occur in the absence of the other, however they do interact with each other in numerous ways. Ozone is itself a greenhouse gas and there is evidence that stratospheric ozone depletion has had an impact on tropospheric weather patterns. Many of the substances regulated by the Montreal Protocol are also strong greenhouse gases. Conversely, the rate at which ODSs deplete ozone is heavily dependent on the state of the atmosphere including stratospheric temperature and circulation speeds. For example, cooler temperatures in the Antarctic stratosphere would lead to an increase in severity of the Antarctic Ozone Hole. Many of these linkages are not currently well understood and are the subject of current research.

Antarctic Ozone Hole

What is the Antarctic Ozone Hole?

The Antarctic Ozone Hole refers to a severe depletion of ozone which has been observed to take place over Antarctica in springtime every year since the early 1980s. The standard definition is the region where the total column ozone is less than 220 Dobson Units (DU).

It should be kept in mind that the word hole does not imply there is no ozone at all over a particular point.

When did the Antarctic Ozone Hole first appear?

In the early 1980s, the British Antarctic Survey noticed that springtime ozone values measured by their Dobson spectrometer at Halley since 1958 had been dropping since the 1970s. The Japanese station at Syowa also measured decreases in ozone. In 1985, the decreases over Halley were reported in a now-famous article by Farman, Gardiner and Shanklin published in the journal Nature. Satellite observations then revealed that the depletion was actually taking place over a wide area across Antarctica, and the term ozone hole came into widespread use.

When does the Antarctic Ozone Hole occur?

In each year, the Antarctic Ozone Hole typically first appears in August, reaches a peak size in late September and dissipates in mid-December, although there is some variability in this timing from year to year.

How large is the Antarctic Ozone Hole?

The maximum size of the Antarctic Ozone Hole reached each year increased rapidly from the early 1980s up until the early 1990s. Since then, however, the maximum area each year has almost always been in between 25 and 30 million square kilometres, the exceptions being the years 2002 and 2004 where it was somewhat less. (For comparison, the area of Australia is 7.7 million square kilometres). The largest areas so far observed have been 30 million square kilometres in 2000 and 29 in 2006. The years 1998, 2003 and 2005 also saw very large holes form.

What causes the Antarctic Ozone Hole to form?

Three essential ingredients are necessary for the Antarctic Ozone Hole to form: ozone depleting substances, cold temperatures and sunlight. Ozone depleting substances were produced in industrial areas all over the world but over the course of many years, make their way into the stratosphere and are eventually transported to the polar regions. While the stratosphere is very dry, at very cold temperature (below -78C), clouds can nonetheless form, known as Polar Stratospheric Clouds (PSCs). PSCs form over a large area of Antarctica during winter. On the surface of these clouds, chemical reactions take place which convert chlorine and bromine into highly reactive forms, which in the presence of sunlight, undergo further reactions which are able to destroy vast numbers of ozone molecules. The Antarctic Ozone Hole thus appears when sunlight returns to Antarctica at the end of winter.

Why does the Ozone Hole only occur over Antarctica?

The chemical reactions that lead to the formation of the Antarctic Ozone Hole take place on the surface of Polar Stratospheric Clouds (PSCs), which can only form in very cold conditions (-78C). Because of differences in the geography of the northern and southern hemispheres, the temperature over Antarctica in winter is somewhat colder than over the Arctic, resulting in PSCs being able to form over a much greater area and for a longer duration. This is the main reason why ozone depletion in the Arctic, while significant, does not result in a sustained ozone hole forming.

Does the Antarctic Ozone Hole ever come over Australia?

No. The ozone hole has only ever been observed to be well south of the Australian mainland and Tasmania. In fact, during springtime, when the hole is in existence, ozone levels over southern Australian cities are at their highest.

However, after the ozone hole has broken up parcels of ozone depleted air mixed with mid latitude air move northwards. These parcels can move over the southern part of Australia and cause a reduction in total ozone values.

When will the Antarctic Ozone Hole recover?

The latest calculations suggest that Antarctic ozone will recover to 1980 levels around the period 2055-2080, assuming the Montreal Protocol continues to be strictly observed.

How are ozone levels measured?

Australia takes part in the Global Atmosphere Watch (GAW) international monitoring and research program coordinated by the World Meteorological Organization (WMO). The Bureau operates a network of ozone monitoring stations (at Melbourne, Brisbane, Darwin, and Macquarie Island), where the total amount of ozone above the station (total column ozone) is measured several times every day.

The total column ozone is measured with an instrument called a Dobson spectrophotometer, which compares the amount of sunlight at two ultraviolet wavelengths, one wavelength being strongly affected by ozone and the other one not. Total column ozone can then be calculated from the difference in the sunlight measured at the two wavelengths.

The Bureau also flies ozonesondes weekly from Macquarie Island and Melbourne (using the Bureaus training annexe at Broadmeadows), as well as a partial program at Davis in Antarctica funded in part by the Australian Antarctic Division. Ozonesondes are devices which measure the ozone concentration in air electro-chemically. Carried into the air by a hydrogen balloon, they are then able to determine the ozone profile at high resolution from the ground up to a height of as great as 35 kilometres.

Since the late 1970s the ground-based networks have been supplemented by satellite instruments which are able to measure total ozone and to a certain extent, ozone profile, at high temporal and spatial resolution. The global ground-based network is still essential however to provide a reliable and well-calibrated long-term record.

What is a Dobson unit?

Ozone is measured in Dobson Units (DU). 300 DU is equivalent to a 3 millimetre thick layer of pure ozone at sea level temperature and pressure. The Dobson Unit is named after G.M.B. Dobson, the English physicist who pioneered the study of stratospheric ozone.

How long has ozone been measured in Australia?

The CSIRO began ozone measurements in Melbourne in 1956 in cooperation with the Bureau of Meteorology and in 1982 total responsibility passed to the Bureau.

What is the Montreal Protocol?

After the discovery of the Antarctic Ozone Hole, governments around the world moved quickly to adopt the Vienna Convention for the Protection of the Ozone Layer in March 1985. Work then began on negotiating a protocol to control the production and use of ozone depleting substances across the world, leading to the adoption of the Montreal Protocol on 16th September 1987, now celebrated as The International Day for the Preservation of the Ozone Layer. Since then, a number of Amendments to the original agreement have also been adopted to hasten the phasing-out of ODSs and to increase the number of substances controlled by the Protocol. To date, the Montreal Protocol has been ratified by one hundred and ninety three countries.

Has the Montreal Protocol been effective?

Yes. By 2006 consumption of ODSs globally had been reduced by over 96% compared to 1986 levels. Although the atmospheric lifetimes of many ODSs are very long (greater than a hundred years in some cases), the total concentration measured in the troposphere has now been declining since the mid 1990s.

How can I stay updated with the latest news about the ozone hole?

The World Meteorological Organization (WMO) puts out regular Antartica Ozone Bulletins.

More information can be found from the following organisations:

Map legend:
      11+ (violet) extreme,
      8-10 (red) very high,
      6-7 (orange) high,
      3-5 (yellow) moderate,
      1-2 (green) low
      Protect yourself in 5 ways:
      1) Slip on sun-protective clothing, 
      2) Slop on SPF30+ sunscreen. Reapply every two hours, 
      3) Slap on a broad-brimmed hat, 
      4) Seek shade, 
      5) Slide on wrap-around sunglasses.

Check and Protect

It is important for all Australians or visitors to Australia, to look and listen for UV Index levels in local weather forecasts.


You should use a combination of sun protection measures to keep you safe from UV radiation - never rely on just one.

Further Information