SINGAPORE - A day after ultraviolet (UV) radiation spiked to extreme levels, UV readings dropped to mostly moderate and high levels on March 28, with a peak to very high levels around 1pm and 3pm.

On March 27, UV index readings peaked at 12 on the extreme band at about 12.45pm. On March 28, the highest reading was 10 on the “very high” band at 1.15pm.

The lower UV index levels, compared with the day before, could be due to some overcast skies as parts of the island saw showers in the afternoon.

UV radiation refers to invisible rays that are emitted by the sun, with clouds able to block the rays from reaching the earth’s surface.

The highest UV Index reading so far in 2024 is 14, recorded at 1.30pm on Feb 15, the Meteorological Service Singapore (MSS) said in response to queries. The maximum level on the UV index is 15.

So far in March, 22 days have seen extreme UV levels, with the index reaching up to 13 on several days between noon and 2pm, added MSS, which is under the National Environment Agency (NEA).

Heat stress levels have remained low and moderate on March 27 and 28, indicating that UV and heat are affected by different factors.

The Straits Times explains the relationship between heat and UV rays, and whether exposure to UV rays is affected by climate change.

What is the UV index?

The UV index measures the level of solar UV radiation at the earth’s surface on a scale of zero to 11 and above, which indicates extreme and intense UV rays. It is common for the UV index to reach very high and extreme levels between 11am and 3pm on a day with little cloud cover, NEA said on its website. The months of February, March, April and September tend to see the highest UV radiation.

The UV Index reaching extreme levels is a common occurrence in countries near the equator, weather and climate scientist Koh Tieh Yong told ST in 2021, when UV radiation hit extreme levels.

MSS said Singapore generally experiences the most number of days with extreme UV Index in the month of March, when the sun is directly over the equator and UV radiation is more intense. 

Singapore experiences the least cloud cover from February to March. Apart from cloud cover, latitude and time, altitude and ozone levels also influence UV index levels.

The UV index is measured at the Changi Meteorological Station and reported every 15 minutes between 7am and 7pm.

Are UV radiation and heat stress related?

There is no direct link between the UV index and heat stress, said MSS, adding that UV radiation produced by the sun is different from the sun’s heat, which we can feel.

“It is possible for the UV Index to be high on days when heat stress levels are low and vice versa,” it added.

Professor Matthias Roth from the National University of Singapore’s geography department said that while UV radiation is part of the wavelength spectrum of solar radiation, it does not contribute much to the overall amount of radiation that is linked to heat stress.

Heat stress is measured by the Wet Bulb Globe Temperature (WBGT), which considers the effects of humidity, air temperature, wind speed and solar radiation.

Non-breathable, thick clothing and exertional activities can also contribute to heat stress.

According to NEA’s heat stress advisory, WBGT between 31 and 33 deg C indicates moderate heat stress. This means people outdoors need to start taking precautions by slowing down activities, taking breaks and drinking more fluids to prevent heat exhaustion and heat cramps.

Extremely high UV radiation can attack the skin and eyes. Short-term exposure to intense UV rays can lead to sunburn, while long-term exposure increases the risk of skin cancer.

Prolonged exposure can also suppress the immune system, cause cataract and lead to cancer around the eyes.

How should we protect ourselves outside?

Don’t forget to have umbrellas, hats and sunglasses with you outdoors. Apply sunscreen of at least SPF 30 every two hours.

Opt for a hat with a wide brim that protects the eyes, face and neck. Long-sleeved shirts and pants can prevent UV rays from hitting your skin.

How does climate change affect UV levels?

Historically, concerns regarding climate change and UV radiation have centred on the depletion of the ozone layer due to the release of certain greenhouse gases, said Prof Roth.

Holes in the ozone layer increase the amount of UV rays that can reach the earth’s surface. But irreversible depletion of the ozone layer has been avoided by the success of the Montreal Protocol, which was signed in 1987.

Prof Roth said: “Although the Montreal Protocol has been a success, understanding of the UV-ozone-climate links is not complete and more research is necessary to understand the potential risks caused by climate change and rising temperatures.”

Adverse effects such as pollution, dust, smoke from wildfires and other airborne and waterborne particles related to climate change reduce UV light from penetrating the earth’s surface, according to the European Climate Adaptation Platform Climate-Adapt.