Ultraviolet Radiation (UVR)

How Ultraviolet Radiation (UVR) Behaves During a Day

Diagram of how UVR behaves during the day, peaking at 1pmThroughout a day UVR behaves in a predictable way. It is highest at solar noon, which is around 1.30pm during daylight savings months. Typically, during a New Zealand summer you will need to protect yourself at least between 11am and 4pm.

What is it?

The sun radiates energy in a wide range of wavelengths, most of which are invisible to human eyes.

UVR that reaches the Earth's surface is in wavelengths between 290 and 400 nm (nanometers, or billionths of a meter). This is shorter than wavelengths of visible light, which are 400 to 700 nm.

UVR affects nearly all living organisms. We have to live with the harmful effects as well as the helpful ones.

  • Radiation at the longer UV wavelengths of 320-400 nm, designated as UV-A, plays a helpful and essential role in formation of Vitamin D by the skin, and plays a harmful role in that it causes sunburn on human skin and cataracts in our eyes. 
  • Radiation at shorter wavelengths of 290-320 nm, designated as UV-B, causes damage at the molecular level to the fundamental building block of life- deoxyribonucleic acid (DNA).

Without the layer of ozone in the stratosphere to protect us from excessive amounts of UV-B radiation, life as we know it would not exist.

What Determines How Much Ultraviolet Radiation Reaches the Earth's Surface?

The amount of UV radiation reaching the Earth's surface varies widely around the globe and through time. Several factors account for this variation at any given location:

  • Cloud cover 
  • Ozone in the Stratosphere 
  • Oblique angle of sunlight reaching the surface 
  • Aerosols 
  • Water Depth 
  • Elevation 
  • Reflectivity of the Earth's Surface

Amounts of UVR in NZ

Unlike many countries that have a relatively constant level of UV, New Zealand has significant peaks and troughs of intensity; with a huge variation between summer minimum and winter maximum levels.

Results from a recent study show peak UV intensities in NZ are approximatley 40% higher than those at a corresponding site in North America. Implications of the large geographical and temporal variability in UV radiation (PDF).

Another recent study highlights the connection beween higher UVR levels and altitude Enhanced UV exposure on a ski-field compared with exposures at sea level (PDF 267 kb).

References

Jeannie Allen
September 6, 2001. NASA.