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Ultraviolet (UV) Radiation Information

UV Index Exposure level Recommended protective measures
0-2 Low
  • No special protective measures required
3-5 Moderate
  • 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
8-10 Very high
  • Avoid being outdoors in the sun during midday hours
  • If you have to stay outdoors, make sure to 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

Real-time UV radiation observation


*UV Index (updated every hour)

*UV Index (updated every 15 minutes)

The mean UV Index for today

Will be available after 6 p.m.

Notes:

  • * Rounded to the nearest whole number when greater than 1
  • '--' : data not available
  • The Observatory issues an UV index forecast for the next day at 5:00 p.m. every day. The forecast will be updated as necessary in case of weather changes, till 10:00 a.m. the next day.
  • The UV index data is the mean UV index for the past 15 minutes or the past hour. For example, the data of 7:15 is the mean UV index of 7:01-7:15; the data of 8:00 is the average UV index of 7:01-8:00.
  • Ultraviolet (UV) radiation comes from the Sun and on reaching the Earth after passing through the atmosphere it may have been modified by a number of factors. Therefore, apart from referring to the UV Index predicted by the Observatory, members of the public are advised to note the sky conditions, their altitude, and their whereabouts regarding the nature of ground surface, and take the appropriate protective measures against UV.
  • The data displayed is provisional. Only limited data validation has been carried out.

UVA radiation intensity (updated every hour)

UVA radiation intensity (updated every 15 minutes)

Notes:

  • UVA radiation intensity is measured in watt per square metre (W/m2), which shows the average radiant power received per square metre.
  • UVA radiation intensity will vary with time. In general, it will be highest around noon and the intensity in summer will be higher than that in winter.
  • The amount of radiation is the average amount of radiation in the past 15 minutes or the past one hour. For example, the data of 7:15 is the average radiation dose of 7:01-7:15; the data of 8:00 is the average radiation dose of 7:01-8:00.
  • The data displayed is provisional. Only limited data validation has been carried out.

Want to know more?

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 is invisible and cannot be felt.

Visible light is in between the wavelength of infrared and ultraviolet

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.

The main characteristics of UV ( 1 nanometre = 0.000000001 metre or 1x10-9 metre)
Characteristics UVA UVB UVC
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 More than 98% of UV radiation is UVA Less than 2% of UV radiation is UVB Negligible

Note: the following video and its transcript are Chinese only


Factors affecting the ground-level UV intensity

Factors affecting the ground-level UV intensity
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. In general, 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.

The UV index is a measure of the potential harm of UV 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. For Hong Kong, UV index can often exceed 10 on a sunny day in the summer.

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

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.

UV radiation can be classified according to their wavelength (in unit of nanometre, nm) as UVA (315-400nm), UVB (280-315nm) and UVC (100-280nm). UV radiation reaching the earth's surface consists of UVA and UVB only, as all UVC is absorbed by the atmosphere. Over 98% of the UV radiation reaching the ground is UVA and less than 2% is UVB.

The characteristics of UVA radiation

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.

Effect of UVA radiation on health

Excessive exposure to UVB may cause sunburn and is one of the major causes of skin cancer. In the past, the harmful effect of UVA is often neglected by people as it is much less powerful to induce sunburn than UVB. Recent 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. Both sunburn and tanning are signs of skin damage.

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.

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

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

Variation of surface UV radiation depends greatly on time of the year and time of the day. It also depends on a number of factors such as cloud cover, ozone concentration, aerosol, rain, fog, haze and so on. The methodology of UV forecast employed by the Hong Kong Observatory involves two steps:

  1. Find out the clear sky UV index using an equation with a number of astronomical factors and ozone concentration as input parameters. This equation was derived using past observation data collected since 1999.
  2. The UV index forecast is then obtained by multiplying the clear sky UV index by several factors, the magnitude of which depends on the forecast cloud amount, cloud type and the presence of rain, fog or haze. Details of the forecast methodology are documented in the Observatory's technical paper.

Points to note:

  • The Observatory issues an UV index forecast for the next day at 5:00 p.m. every day. The forecast will be updated as necessary in case of weather changes, till 10:00 a.m. the next day.
  • Ultraviolet (UV) radiation comes from the Sun and on reaching the Earth after passing through the atmosphere it may have been modified by a number of factors. The accuracy of UV index forecast will also be affected by these factors. Therefore, apart from referring to the UV Index predicted by the Observatory, members of the public are advised to note the sky conditions, their altitude, and their whereabouts regarding the nature of ground surface, and take the appropriate protective measures against UV.

Factors affect the accuracy of UV index forecast

While UV levels may be highest under cloudless skies, they can also be high with cloud cover. There have been instances of very high UV levels under broken clouds, which can enhance UV levels by reflection from their sides.

There are uncertainties in the accurate forecasting of weather elements such as clouds, rain, fog and haze. Besides, there are further uncertainties associated with where you are and how high. The atmosphere gets thinner as we go up, and absorbs less UV. With every 1000 metres increase in height, the UV levels can increase by up to 12%.

The amount of UV received is also influenced by the nature of ground surface. Pay attention to the following two points:

  1. Sand on the beach can reflect some of the incoming UV,
  2. While UV levels at 0.5 metre beneath water can still be 40% as intense as at the water surface

For the above reasons, apart from referring to the UV index predicted by the Observatory, members of the public are advised to note the sky conditions, their altitude, and their whereabouts regarding the nature of ground surface, and take the appropriate protective measures against UV.

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.

In respect of skin cancer, about 80% of all cases may be attributable to excessive exposure to sunlight. Specifically, studies suggested that UVA is responsible for the wrinkling, ageing and immediate tanning effects, as well as potential development of skin cancer. Overexposure to UVB may cause sunburn and is one of the major causes of skin cancer.

In respect of cataracts, the World Health Organization estimates that up to 20% of cataracts may be due to UV overexposure. Therefore, one must not underestimate the potential harm UV overexposure may bring.

For more health information related to UV, please visit the webpage of the Central Health Education Unit of the Department of Health.

Protective measures against UV radiation

To reduce the harm from UV radiation, the most important thing is to minimize direct exposure of the skin and the eyes to sunlight.

Avoid staying outdoors for prolonged periods on days when the UV index is high (6 or above).

If staying in the sun is inevitable, take the following precautions:

  • Check the latest UV index and its forecast
  • Wear a broad brim hat
  • Seek shade
  • Wear UV blocking sunglasses
  • Use an umbrella
  • Use a broad-spectrum sunscreen lotion blocking both UVA and UVB (with a Protection Grade of UVA (PA) of PA+ or above, and a Sun Protection Factor (SPF) for UVB of 15 or above). Apply liberally and reapply after swimming or sweating
  • Wear long-sleeved and loose-fitting clothing

Note: the following video and its transcript are Chinese only

For more health information related to UV, please visit the webpage of the Central Health Education Unit of the Department of Health.

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.

The following are the UV index and the corresponding UVA intensity, as well as the protective measures

UV index and the corresponding UVA intensity, as well as the protective measures
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 have to stay outdoors, make sure to 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.

Children are at a higher risk of suffering UV damage than adults, and therefore require special protection. To enjoy fun in the sun with children, parents should bear in mind the following tips:

  • Encourage children to play in the shade.
  • Make sure children wear appropriate clothing (e.g. a hat and sunglasses) when they go outdoors.
  • Help children apply sunscreen lotion generously and pay particular attention to the most exposed body parts (e.g. face, neck, shoulders, back, knees and tops of feet).

For more health information related to UV, please visit the webpage of the Central Health Education Unit of the Department of Health.

Some common misconceptions about UV radiation and the proper ways of protection
FALSE TRUE
Darker sunglasses offer more protection from UV radiation The most important thing to look for in sunglasses is how much UV radiation they filter out. It should be noted that there is no relationship between the colour of sunglasses and their UV filtering action. For adequate protection, one should wear sunglasses that are able to block at least 98% of the UV radiation.
You can't get sunburnt on a cloudy day Up to 80% of solar UV radiation can penetrate thin cloud cover. There are also occasions when broken clouds enhance UV radiation by reflection from their sides. In case of doubt, it is advisable to check the latest UV index through radio, television, the Observatory's website and Dial-a-Weather system (1878200).
You can't get sunburnt while in the water Water offers only minimal protection from UV radiation and reflections from water can enhance your UV radiation exposure. At half a metre under water, the UV radiation level is still 40% as intense as at the surface.
Sunscreens protect me so I can sunbathe much longer Sunscreen lotion should not be used to increase sun exposure time but to increase protection during unavoidable exposure. The protection provided by sunscreen lotion depends critically on their correct application. For more about sunscreen lotion and sunburn protection, please check out "Sunburn and SPF".
If you take regular breaks during sunbathing you won't get sunburnt UV radiation exposure is cumulative. The total health damage you get will be the sum of the effect of individual exposure. Therefore, to protect yourself the objective is to reduce the cumulative UV radiation exposure. This could decrease the chance of skin cancer.
If you don't feel the hot rays of the sun you won't get sunburnt Sunburn is caused by UV radiation which cannot be felt. The heating effect is caused by the sun's infrared radiation and not by UV radiation. Therefore, even if you don't feel the warmth, you may also get sunburn.

For more health information related to UV, please visit the webpage of the Central Health Education Unit of the Department of Health.

Protection grade for UVA (PA)

The SPF on a sunscreen lotion only tells us the time extension before a person gets burnt by UVB, but not UVA. An international standard has yet to be developed for protection against UVA, although the PA (Protection Grade of UVA) is usually used in Asia. There are 3 PA grades: PA+, PA++, and PA+++, with each additional plus (+) indicating a higher protection. So, when buying a sunscreen lotion, one should note the SPF as well as the PA.

Reference:

  • Nash, J.F., P.R. Tanner, P.J. Matts, 2006, "Ultraviolet A Radiation : Testing and Labelling for Sunscreen Products", Dermatologic Clinics, published by Elsevier Saunders, 24, 63-74.

Sun Protection Factor (SPF) for UVB

SPF is mainly a measure of UVB protection . It relates to how long, on a sunny day, it takes to get burnt by the sun's UVB radiation. For instance, SPF 15 means that with sunscreen lotion on, it will take 150 minutes to produce a detectable burn on a person who gets sun burnt in 10 minutes.

SPF is also applicable to other sunburn preventive measures. As a rough guide, the SPF of a wide-brimmed hat is 3 to 6, while that of ordinary summer clothing is 6 to 7. One can see from their SPF values that the degree of protection is not as high as sunscreen lotions (usually SPF 15 or above).

The Ultraviolet Protection Factor (UPF) measures the UV protection provided by fabric. Unlike SPF which is a measure of only UVB protection, UPF rates protection against both UVA and UVB. For example, a garment with a UPF of 50 allows 1/50th of the UV radiation falling on the surface of the garment to pass through it. In other words, it blocks 49/50ths or 98% of the UV radiation. However, an international standard for the concept of UPF has not been developed yet.

There are many applications of man-made UV radiation in our daily life. The following presents some examples.
Application Principle Type of UV radiation used
Disinfection of drinking water and sterilization of apparatus High energy UV radiation destroys the DNA of bacteria and micro-organism. UVC
Checking of counterfeit banknotes Genuine banknotes have fluorescent marks that are visible only under UV light. UVA
Hardening of inks and coatings Certain inks and coatings harden when exposed to UV radiation. UVA or UVB
Pest control Insects are attracted to the UV light and killed by the electric shock in bug zappers. UVA
Medical applications such as treatment of psoriasis (a kind of disease causing red and scaly skins) UV radiation can suppress the immune system and reduce inflammatory responses. UVA or UVB

Real-time past 15-minute mean UV index alert

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