Tanning is a biological reaction where skin cells produce the pigment melanin in response to ultraviolet (UV) radiation exposure. This response is the body’s way of attempting to protect itself from damage. The potential for tanning at 6 PM depends entirely on the presence and intensity of these UV rays, which are not uniform throughout the day.
Understanding UVA and UVB Rays
The sun emits two primary types of ultraviolet radiation that reach the Earth’s surface: Ultraviolet A (UVA) and Ultraviolet B (UVB). These rays differ significantly in their wavelength, energy, and biological effects on the skin.
UVA rays have a longer wavelength and penetrate deeply into the skin’s dermis layer, primarily causing photoaging and contributing to the initial, rapid darkening of existing melanin.
UVB rays possess a shorter wavelength and higher energy, primarily affecting the epidermis, the skin’s outermost layer. Overexposure to UVB is the main cause of sunburn, and it stimulates the production of new melanin, leading to the delayed and more lasting tan. While both types of UV radiation are linked to DNA damage and skin cancer risk, their relative intensity changes dramatically depending on the sun’s position in the sky.
UV Intensity at Late Hours
At 6 PM, the potential for tanning is significantly reduced compared to midday, but it is not zero. This reduction is due to the solar zenith angle, which refers to the angle of the sun relative to a point directly overhead. In the late afternoon, the sun sits low on the horizon, causing its rays to travel through a much greater thickness of atmosphere before reaching the surface.
This increased atmospheric path acts as a natural filter, absorbing and scattering a large percentage of incoming UV radiation. The atmosphere is particularly effective at blocking the shorter-wavelength, higher-energy UVB rays. For instance, the UV Index (UVI) in the late afternoon might be between 2 and 3, presenting a low risk of sunburn. This means the potent UVB rays responsible for deep, delayed tanning are largely absent by 6 PM.
However, the longer-wavelength UVA rays are less readily scattered and absorbed by the atmosphere, meaning a significant amount of them persist even when the sun is low. Since UVA causes the immediate darkening of melanin, a tan is still possible in the late afternoon, though it will be slower and less intense than at peak hours. The presence of UVA rays also means that skin damage, including oxidative stress that contributes to aging, can still occur.
Factors Modifying Late-Day Exposure
Seasonal and Geographical Factors
The intensity of UV radiation at 6 PM is not universal and can be significantly altered by environmental variables. The season plays a major role, as the sun is higher and the day is longer during summer months, leading to higher 6 PM UV exposure than in autumn or winter. Geographical latitude also dictates the sun’s path; locations closer to the equator experience a more direct sun path, meaning late-day UV levels remain stronger.
Environmental Reflection and Altitude
The surrounding environment also modifies the actual amount of UV radiation the skin receives. Reflection from surfaces can dramatically increase exposure, even when the sun is low.
- Altitude increases UV intensity because there is less atmosphere above to filter the rays, with intensity increasing by approximately 10 to 12% for every 1,000 meters of elevation gained.
- Fresh snow can reflect between 50% and 88% of UV radiation.
- Sand and water surfaces can reflect 15% to 30% of UV radiation.
- Being near a reflective surface like a lake or ocean at 6 PM can considerably increase the late-day risk of UV exposure and the potential for tanning.