UV Radiation & UV Index

Solar UV Radiation

Ultraviolet (UV) radiation refers to the region of the electromagnetic spectrum that has a wavelength between 100-400 nm.  The sun emits ultraviolet radiation in three bands, UVA (315-400 nm), UVB (280-315 nm) and UVC (100-280 nm).  The ozone layer in the stratosphere prevents almost all UVC rays and up to 90% of UVB rays from reaching the earth's surface. Thus 94% of UV radiation reaching the earth’s surface consists of UVA rays and only 6% consists of UVB rays. 

Schematic diagram showing the interaction of ultra-violet radiation with the earth’s atmosphere before it reaches the surface.

The UV Index

The UV Index (UVI) is a simple and informative index jointly developed by the World Health Organisation (WHO), United Nations Environment Programme (UNEP), and the World Meteorological Organization (WMO) to measure the level of UV radiation exposure. The UVI describes the level of solar UV radiation at the Earth’s surface and is dependent on both UVA and UVB radiation.  UV radiation levels and therefore the values of the index vary throughout the day. The maximum UV radiation level typically occurs during the four-hour period between 11am and 3pm.

Exposure Categories of the UV Index

  

The UV index ranges from 0 to 11+ and are grouped into various exposure categories indicating the potential for harmful effects to the skin and eyes.

Measurement of the UV Index

The UV Index is measured with a UVS-E-T broadband radiometer (UVI sensor) located in an open field at the Changi Meteorological Station.  It is able to give a UV index value that approximates the erythemal (sunburn) effect of UV radiation on the human skin based on measured UVA and UVB radiation.

 

UVS-ET sensor used to measure surface UVI values

Factors Affecting Surface UV Radiation and the UV Index

Time of the day

It is common for the UVI in Singapore to reach Very High and Extreme levels in the four hour period between 11 am and 3 pm on a day with little cloud cover.  UVI values are typically close to zero during the night.

Time of year (season)

UVI values are higher during months where there is less cloud cover and when the position of the sun is directly over the equator.  The chart below depicts the average variation of daily maximum UVI values across the year.

    

Latitude

UV radiation and UVI values are highest near the equator and decrease with increasing latitude.  

               

The schematic diagram above shows the effect of latitude on UV radiation levels.  At lower latitudes, UV radiation passes through a shorter distance in the atmosphere and is focused on a smaller area as it reaches the earth’s surface due to the curvature of the earth.                           

Altitude

UV radiation increases with increasing altitude as the atmosphere is thinner at higher elevations and UV radiation is less filtered before it reaches the earth’s surface.

Cloud cover

Thick cloud cover can help to absorb or scatter most UV radiation and keep UVI values from reaching Very High or Extreme values around noon in Singapore. 

  
                                            

 

Ozone

Higher stratospheric ozone levels help to filter UV radiation as it helps to absorb and filter UV radiation before it reaches the surface.

Typical Daily Variation of the UV Index

It is common for the UV Index in Singapore to reach Very High and Extreme levels in the four hour period between 11 am and 3 pm on a day with little cloud cover.  A diurnal variation of UV Index on such a day is shown below.  The occurrence of rain and thick cloud cover can result in lower values of the UV Index as UV radiation gets filtered before reaching the surface.