What is UV radiation?

The sun emits electromagnetic radiation of different energy or wavelengths. Some of the radiation, such as that making up the colours of rainbow, is visible which our eyes can see. The part of radiation beyond the violet end of visible light is called ultraviolet (UV) radiation which our eyes cannot see.

UV radiation can be classified as UVA, UVB and UVC according to the energy or wavelength. UV radiation reaching the earth's surface consists mostly of UVA and some UVB, as all UVC and most of the UVB are absorbed by the atmosphere.

Wavelength (nanometre, nm) 315-400 280-315 100-280
Absorption by the ozone layer Nil, i.e. it goes through the ozone layer Mostly absorbed by the ozone layer Practically all absorbed by the ozone layer
Amount reaching the Earth's surface >98% of UV radiation is UVA <2% of UV radiation is UVB Negligible

There are many factors affecting the intensity of UV radiation at surface, including position of the sun, amount of ozone in the atmosphere, cloud, rain, fog and haze, ground reflection, and altitude above sea level.

Factor Influence on UV intensity at the Earth's surface
Position of the sun Varies with time of the year, time of the day and the latitude. The higher the sun's position, the higher the UV intensity.
Amount of ozone in the atmosphere Ozone absorbs UV radiation. The more abundant the ozone in the upper atmosphere, the less UV radiation reaches the Earth's surface.
Clouds, rain, fog and haze UV radiation is both absorbed and scattered by clouds, rain, fog and haze.
Ground reflection Most natural surfaces such as grass, soil and water reflect less than 10% of UV radiation. However, fresh snow strongly reflects around 80% of UV radiation. Sand also reflects 10-25% of UV radiation.
Altitude above the sea level The higher the altitude, the higher the UV intensity. It is because the atmosphere gets thinner as we go up, and absorbs less UV.

Measurement of Ultraviolet Radiation

A broadband UV sensor is used by the Hong Kong Observatory to measure the intensity of the UV radiation at the King's Park Meteorological Station. Both the UV radiation transmitted directly through the atmosphere and that scattered by gases and particles in the atmosphere are measured. This type of sensor has a response which approximates the response of skin to UV radiation of different wavelengths.

Since the wavelength of UVA is longer than that of UVB, UVA is less scattered by air molecules as compared to UVB. As a result, the variation of UVA is less than that of UVB during the same time interval. This phenomenon is more prominent during the few hours after sunrise and before sunset when the sun elevation is low. For the same reason, the difference in UVA intensity between summer and winter is smaller.

The Hong Kong Observatory’s broadband UV sensor at King’s Park Meteorological Station

Principle of operation for UV sensor

There are two main types of UV sensors in the market, broadband UV sensor and spectrophotometer. Solar radiation passes through the glass dome on broadband UV sensor. The filter inside the instrument will filter the UVA and UVB radiation, which further strikes a UV sensitive phosphor and be transformed to visible light. A solid-state photodiode will then measure the light intensity. Broadband UV sensor is the most popular type of UV sensor. Spectrophotometer, however, can scan the UV spectrum in every wavelength and obtain the integrated UV intensity. However, calibration of the instrument is an issue which affects its popularity.

Schmatic diagram of broadband UV sensor
Schmatic diagram of broadband UV sensor

UV Index

UV sensor measures the intensity of UV radiation. UV Index can be obtained by multiplying the UV intensity by the conversion coefficient of the individual sensor. The UV index is a measure of the effect of solar ultraviolet radiation on human skin. The higher the UV index, the greater the potential for damage to the skin and eye, and the less time it takes for harm to occur.

The UV index and the corresponding exposure level as categorized by the World Health Organization
UV Index Exposure Level
0-2 Low
3-5 Moderate
6-7 High
8-10 Very High
>=11 Extreme

For Hong Kong, UV index can often exceed 10 on a sunny day in the summer. During the period with clouds and rain, the UV index will be lower. The Hong Kong Observatory has been measuring and disseminating UV index since 1999. In 2006, it started to provide forecast of the maximum UV index for the next day. When the index is measured or forecast to be 11 or above, the Observatory will also advise the public to avoid prolonged exposure under the sun.

Effect of UV radiation on health

A moderate exposure to the sun helps our body to produce vitamin D which could bring increased absorption and better utilization of calcium and phosphorus required for healthy bones and minimized risk of bone fracture. However, excessive exposure to UV radiation from the sun may cause painful sunburn, wrinkling and premature ageing of the skin as well as an increase in the risk of skin cancers and cataracts. UV radiation also undergoes photochemical reaction with volatile organic compounds to form ozone. High concentration of ozone can irritate the respiratory system.

Erythemal action spectrum shows a different effect on human skin between UVA and UVB:

Erythemal action spectrum

Simply speaking, although over 98% of UV radiation reaching ground is UVA also with stronger intensity, its erythemal weighting factor is not as high as for UVB. Therefore, both UVA and UVB pose a significant effect to human skin.

Measured UV intensity across the solar UV spectrum and the erythemal action spectrum
Measured UV intensity across the solar

Studies showed that over-exposure to UVA may lead to darkening, aging and wrinkling of the skin, as well as potential development of skin cancer. WHO pointed out that cataract is possibly related to over-exposure to UV radiation.

Protective measures against UVA include wearing appropriate clothing, wearing sunglasses, using an umbrella and applying sunscreen lotion. At present, the SPF (Sun Protection Factor) is an international standard for measuring the degree of protection against UVB offered by a sunscreen lotion.  Although there is no international consensus on the measurement of UVA protection by sunscreen lotion at the moment, the PA (Protection Grade of UVA) is commonly used in Asia. There are 3 PA grades: PA+, PA++, and PA+++, with each additional plus (+) indicating a higher protection.  When buying a sunscreen lotion, people should note the SPF as well as the PA and choose one that can block both UVA and UVB.

UV Index Exposure level Corresponding UVA intensity*(W/m2) Recommended protective measures
0-2 Low 0-20
  • No special protective measures required.
3-5 Moderate 21-35
  • Seek shade during midday hours when staying outdoors
  • Use a sunscreen lotion that can block both UVA and UVB (with SPF 15 and PA+, or above)
  • Wear long-sleeved and loose-fitting clothing, wear a broad brim hat, use an umbrella, wear sunglasses
6-7 High 36-45
8-10 Very High 46-55
  • Avoid being outdoors in the sun during midday hours
  • If you must stay outdoors, seek shade
  • Use a sunscreen lotion that can block both UVA and UVB (with SPF 15 and PA+, or above)
  • Wear long-sleeved and loose-fitting clothing, wear a broad brim hat, use an umbrella, wear sunglasses
>=11 Extreme >=56
* The reference values are obtained from an analysis of UVA radiation data collected in Hong Kong by the Observatory during 2010. There exists a certain degree of correlation between the UVA intensity and UV index in Hong Kong.


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