Getting Into the Ozone

30/06/2014 13:14 BST | Updated 26/08/2014 10:59 BST

The summer solstice has been and gone and the nights are drawing in. Sorry for that. But don't despair, we've still got another two months of summer left - so let's chat about sunlight, the significance of the UV Index and why the ozone layer plays a crucial role our relationship with the sun.

This time of year is when we can expect our highest UV levels. The strength of UV radiation varies depending on where you are in the world, the time of year, the time of day and a number of different weather factors such as forecast cloud cover and ozone amounts.

Also from The Weather Channel: The UK beaches you should avoid

All this information is combined to come up with a UV index - which gives an idea of the amount of skin-damaging UV radiation expected to reach the earth's surface. In the UK it's rare to see the UV index reach 8, with even a 7 only occurring on exceptional days, mostly at the end of June.

Head further south to the Mediterranean and you might get a 9 or a 10. So right now, you can expect a 6 on a sunny day in Britain, making suntan lotion essential.

Protective shield

Sunscreen aside, one of the most important barriers between the sun's rays and skin damage is the ozone layer in the stratosphere, the atmospheric layer just above where life exists and weather occurs. Even at the low concentration of 12 parts per million, ozone is very efficient at absorbing the sun's UV radiation, protecting us from skin cancer and cataracts.

It also prevents damage to plants, crops and marine life.

So where does this crucial ozone come from? Turns out there wouldn't be an ozone layer without the sun's rays in the first place. It's all to do with how ozone is formed. This special form of oxygen is made up of three rather than the usual two atoms.

When radiation or an electrical discharge separates the two atoms in an oxygen molecule these separate atoms can then latch on to other oxygen molecules to form ozone.

Ozone destroyers

This means sunlight increases the number of ozone molecules and makes the ozone layer more effective. But there are things that can destroy ozone, and unfortunately the main culprit was made by us.

The ozone layer became a household name in 1985 when British scientists made a shocking discovery. The ozone levels above an Antarctic research station had declined by as much as 60% in just a decade. This so-called ozone hole became the definitive proof that chemical compounds called chlorofluorocarbons (CFCs) were not so environmentally friendly after all.

Too good to be true

CFCs were developed in the late 1920s/early 1930s for a variety of industrial, commercial and household applications, such as aerosol propellants, fire extinguishers and refrigerator coolants.

These inert compounds were thought to be non-toxic, non-flammable and non-reactive with other chemical compounds. By the early 1970s CFCs were being mass produced an an annual rate of one million metric tons and growing by 20 per cent each year.

But something wasn't right. In the 1960s, scientists noticed that actual measurements of ozone in the atmosphere were lower than the values predicted by their models. Something was going on, either naturally or artificially created chemicals were lowering ozone levels.

The science bit

It took another decade to work it out but the culprit was finally identified - chlorine. While CFCs are extremely stable and don't dissolve in the rain they can be broken down by UV radiation. After a period of several years, CFCs released into the atmosphere will reach the stratosphere, about 10km above the Earth's surface.

The strong UV light here breaks apart the compound to release chlorine atoms.

The chlorine then reacts with an ozone molecule to leave a molecule of oxygen and a molecule of chlorine monoxide. The chlorine on this molecule then reacts with a single oxygen molecule to leave another oxygen molecule and the free chlorine atom.

Our chlorine atom is now floating around the stratosphere ready to break apart another ozone molecule, continuing the cycle until the chlorine atom eventually breaks down.

Long-term effects

Unfortunately this might not happen for a thousand years or more and it's estimated that one chlorine atom can destroy over 100,000 ozone molecules. This process will remove ozone faster than it is created so ozone levels fall.

After an international treaty called the Montreal Protocol was signed in 1973, the manufacture of these chemical compounds was greatly reduced. Unfortunately CFCs can last for several decades in our atmosphere and ozone levels are expected to remain lower for many years to come.

Bad ozone

So far it sounds like ozone is the best thing ever, protecting us from skin cancer and cataracts. But there is a bad side to this good guy.

Ozone is also produced near the ground when sunlight interacts with atmospheric pollution in cities. This ozone is bad for our health, causing breathing problems and aggravating asthma.

The worst time of year is summertime, when pollution over a city builds up during the stagnant air conditions associated with areas of high pressure. So not only have we got high UV levels and high pollen levels right now, we've also got smog to worry about. Ah the joys of summer!