What is UV?

Ultraviolet (UV) light is part of a family of radiations called the electromagnetic (EM) spectrum. UV is just beyond the violet end of visible light and has smaller wavelengths and greater energy.

As with all electromagnetic spectrum radiations, UV travels at the speed of light. Humans cannot see it, but some animals, especially some insects, can see UV light and have body markings that reflect UV light.

Nature of science

Where possible, scientists try to find natural ways to classify things. UV is subdivided according to the effect it has on atoms. Even then, there is still debate about whether 320 nm or 315 nm should be used to mark the division between UVA and UVB.

UV is produced naturally by very hot objects such as our Sun. About 10% of the Sun’s energy output is UV. UV shines on the Earth along with heat and visible light. Our atmosphere reflects much of the incoming UV back out to space and absorbs most of the rest. Overall, then, only a small proportion of the Sun’s UV reaches us. The ozone layer at the top of the Earth’s atmosphere and oxygen within the atmosphere absorbs the more energetic UV with shorter wavelengths. It is some of the longer wavelength UV that reaches the Earth’s surface.

UV wavelength ranges

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This diagram shows the wavelength ranges into which UV is subdivided.

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Different characteristics of UVA, B and C

UVA

UVB

UVC

98.7% of the UV that reaches us on the Earth’s surface. Affected little by ozone.

1.3% of the UV that reaches us on the Earth’s surface. Highly affected by ozone.

0% of the UV that reaches us on the Earth’s surface. Scattered and absorbed by atmospheric oxygen, nitrogen and ozone.

Cause oxygen to form into ozone. Slowly regenerates the ozone layer.

Causes aging of the skin.

Responsible for sunburn and producing a tan.

Penetrates through windows and causes colours of dyes and paints to fade.

Blocked by the glass in windows and doors.

Destructive effect on skin cells – causes lesions on the skin.

Causes melanoma – the deadliest form of skin cancer.

Causes other skin cancers.

270–300 nm are the best wavelengths to stimulate the production of vitamin D.

Monitoring UV

It is important to monitor the amount of UV that reaches us at various places on the Earth. This can help us see how the UV level varies during the day, over a year and from year to year, so we can make predictions and give warnings when the UV level is high. A UV Index has been developed that uses the monitored UV to estimate the exposure level for humans. A UV Index number of 2 or less is a low exposure level and rises progressively to a UV Index number of 11+ (an extreme exposure level). One unit in this index is roughly equivalent to 25 millijoules of UV energy falling per second on a one square metre area.

In the article, Monitoring ozone levels find out more about NIWA’s research programme into monitoring atmospheric ozone and understanding and predicting the relationship between ozone depletion and climate change.