Walk Softly - Just Chillin’ by the Window!

by Geoffrey Carpentier

I must admit, sometimes I just wonder why things are as they are. Case in point, it’s winter and cold outside. But as I stand near the window, with the sun beaming in, I feel warm, yet when I touch the window, it’s still cold. How can that be? Even though the air may be frigid outside, somehow the sun is clearly heating the window, and its heat is being transferred through the glass to make me warm and cozy. I thought I might explore this and find out what’s really going on.

Turns out this isn’t exactly a simple answer. It has everything to do with the wavelength of the sunlight as it passes into or out of a warm space. One feels warm when sunlight passes into the house through glass windows because of how different forms of solar energy interact with glass, our bodies and the objects in our homes.

The sun emits energy as electromagnetic radiation, primarily in three forms: visible, infrared, and ultraviolet. Standard window glass is highly transparent to visible light (so we can see clearly through it) and to most short-wave infrared radiation, allowing nearly 90 percent of its energy to pass directly into our homes. When these light rays strike an opaque or solid object inside the house, such as our bodies, furniture or walls, they are absorbed by the object. This absorption causes molecules in the object to vibrate faster, converting the light energy into thermal energy (heat). Once objects inside have warmed up, they try to cool down by redirecting that energy back out. However, they radiate outward at a much longer wavelength than the sunlight which entered, so the glass of the window traps the heat in the house, simply because this long-wave thermal radiation doesn’t move through the glass as effectively as the short-wave UV light which entered previously.

Basically, the sun does heat up a room, even in the dead of winter, and that heat persists in the home, rather than simply moving back out through the glass, due to the differing wavelengths of incoming and outgoing light. Conversely, when you are outside, you might feel the heat of the sun, but since the outside air is constantly moving, heat is carried away from your body, due to convection, so we cool off more rapidly. Inside the house, the air is relatively still, so we stay warmer longer.

Another question I have pondered in the past is: Can I get a sunburn or a tan from sunlight passing through glass or do I have to be outside in direct sunlight? Short answer – yes. That said, the intensity of the burn or the tan will be significantly less than if you were outdoors. It depends on the type of glass (laminated or tinted) and the duration of exposure. While ‘standard’ glass has a natural protective effect roughly equivalent to SPF 30, prolonged exposure next to a sunny window can still lead to a burn or a tan. Sunlight contains three main types of ultraviolet radiation that affect your skin: (1) UVC is the most dangerous, but fortunately, it doesn’t reach us as it is blocked by the ozone layer and other protective atmospheric features. (2) UVB Rays (which cause sunburn and tanning) are generally blocked by standard window glass (97 percent blocked). Because most of these UVB rays are filtered out, you are generally protected from immediate burning. (3) UVA rays, which cause skin damage and cancer, can penetrate the glass; between 50 and 75 percent of UVA rays pass through the windows. While UVA rays are less likely to cause a quick, painful burn, they penetrate deeper into the skin and cause long-term damage, including premature aging (wrinkles), tanning, and increased skin cancer risk.

One last question: can I get a tan or burn on a cloudy day? You sure can but the intensity varies with cloud cover and density. So, there you have it, folks; it may be cold out but don’t be chillin’ by the window!

Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff online on LinkedIn, Facebook and Instagram.