How To Protect Our Skin from Seasonal UV changes
Why talk about sun protection in winter? We are approaching the shortest day of year which is also day that the UV (ultraviolet radiation) will be at its lowest for the year. Knowing a little bit about Earth’s relationship with the sun allows us to understand how the intensity of the sun’s rays can change during the year, and the importance of protecting our skin.
What causes the change in the sun’s intensity?
The Earth revolves around the sun, taking a year to complete one orbit. But as it does so, it also spins on its own axis (an imaginary line that extends from the north to the south pole), making one complete turn in 24 hours – this explains our alternating pattern of day and night.
The Earth is tilted by 23.5o in relation to its plane of orbit, and it’s this tilt that causes the change in seasons, affecting the duration of daylight and the angle of the sun’s rays at different locations.
This means that at different times of year, different places on Earth receive more or less sunlight. The tilt also affects the distance through Earth’s atmosphere that the sun’s rays travel. Earth’s atmosphere is approximately 100km thick. Rays entering the atmosphere directly overhead (90° angle – such as at midday in Summer) travel a distance of roughly 100km, but if the sun is lower in the sky (such as at dusk or in Winter), the path length through the atmosphere is longer, resulting in more absorption, reflection and scattering, which reduces the intensity of the sun’s energy at the Earth’s surface.
The closer a place is located to the poles, the greater its seasonal variation.
In summer, the sun’s rays have less atmosphere to travel through to reach the Earth’s surface. Because of the tilt away from the sun in winter, the sun’s rays have further to travel so the intensity is less.
The Earth is tilted towards the sun during summer and away from it during winter. In the southern hemisphere, the winter solstice occurs around the 20th/21st June which is when the south pole is tilted farthest away from the sun. The June solstice marks the point when the sun’s rays are directly overhead at the Tropic of Cancer (the imaginary line around the Earth marking the northernmost limit that the Sun’s rays will ever be directly overhead). The sun reaches it’s lowest point in the sky at noon, and has the shortest hours of daylight. This is also when UV reaches it’s lowest point. After this solstice, you can expect UV levels to be on the rise again.
Summer solstice occurs around 21st/22nd December, when the south pole is tilted closest to the sun. The December solstice is when the sun’s rays are directly overhead at the Tropic of Capricorn (the imaginary line around the Earth that indicates the southernmost position that the sun’s rays can ever be directly overhead, as the Earth revolves around the sun). The sun reaches it’s highest point in the sky at noon and it is the longest day of the year in terms of daylight hours.
As Earth travels around the sun, it reaches a position on its orbital path known as an equinox, when the Earth’s poles are neither tilted towards nor away from the sun. The equinox marks the occasions when the sun passes directly over the equator (the imaginary line drawn around the Earth, halfway between the North and South poles, dividing it into northern and southern hemispheres), and the duration of daylight is virtually the same everywhere on the Earth’s surface. The name equinox comes from the Latin: aequus, equal; nox, night.
In the southern hemisphere, this happens around 20th March, signalling the transition to when night becomes longer than day, and again around the 22nd September, signalling the transition to when day becomes longer than night.
What is solar ultraviolet radiation?
Ultraviolet radiation (UV) is one of the three major components of sunlight, which include visible light and infrared (heat). UV rays reaching the Earth are a combination of UVA and UVB.
Why is UV important?
UV in sunlight (solar UV) has both positive and negative effects. Although UVB plays a role in vitamin D production, UV can also damage skin cells, with overexposure causing both visible (sunburn) and invisible damage, resulting in premature ageing, eye damage (e.g. cataracts) and increasing skin cancer risk. However, the good news is that most skin cancers can be prevented by adopting sun protective behaviours.
Sun protection is recommended whenever the UV Index is 3 or above.
Seasonal UV changes
So, because of the tilt of the Earth, the UV changes accordingly. It’s higher in summer and higher the closer you are to the equator. In Western Australia, areas north of Perth will reach UV levels of 3 and above every day of the year.
For Perth and south, there are some days (but not many!) in the middle of year when the UV Index will be low enough for you not to worry about sun protection*.
So what affects UV?
o Latitude: Only at places located on or within the tropics (between the Tropic of Cancer and Capricorn), can the sun ever be directly overhead: the closer to the equator, the greater the UV levels.
o Season: The amount of solar UV reaching Earth is influenced by the height of the sun in the sky and therefore, changes with the seasons. During winter, the sun is lower in the sky than during summer months. The length of the day is also shorter, and so UV is less intense.
Cancer Council WA acknowledges that Aboriginal and Torres Strait Islander people have traditional land, animal and climate knowledge that has been passed down through generations. In WA, according to the Noongar calendar, it is currently Makura (June-July) which is the season of fertility. The Yawuru peoples of Broome call this time of year Barrgana, the cold season. This video gives a great description of the Noongar seasons www.facebook.com/watch/?v=541801876990722 or visit www.bom.gov.au/iwk.
o Time of day: Solar UV levels change throughout the day, rising during the morning and reaching their peak when the sun is at its highest point in the sky above the horizon (solar noon), before declining during the afternoon and evening.
Think UV, not heat!
UV cannot be seen or felt, so you need to defend yourself against overexposure. Both occasional and chronic UV exposure can be damaging; sunburn is the most harmful, but frequent non-burning exposures can also increase skin cancer risk.
UV levels are influenced by many factors, including: your location (latitude and altitude), time of day, time of year, cloud cover and reflection. UV levels ARE NOT INFLUENCED BY TEMPERATURE!
Protect your skin: know the UV index
The UV index measures the UV level at the surface of the Earth, and gives an indication of the potential for skin damage. The UV index ranges from zero upwards – the higher the number, the greater the risk. Remember the amount of time spent outside and personal vulnerability e.g. fair skin type, are also important considerations.
Watch the UV index
Make sun protection part of your daily routine whenever the UV Index is forecast to be 3 and above. Use the SunSmart widget on Dougal, the free SunSmart app for mobile devices, on Bureau of Meteorology weather forecasts or a www.myuv.com.au
*unless you’re an outdoor worker (ie you spend most of your time outside) or you’re in the snow. Then you need to cover up regardless