Science Buddies: "Ask an Expert"

I am doing a science fair project that envolves testing weather or not the sunscreen in make-up works as well as regular sunscreen. I take some Photosensitive Paper (black paper that turns white when exposed to the sun), put regular sunscreen on one piece, and the liquid makeup foundation (with the same SPF as the regular sunscreen) on another different piece. I label both pieces, and then expose them both to the sun for exactly 2min 30sec. Since the Photosensitive Paper is black, and it turns white when it is exposed to the sun, the sunscreen or the foundation will (or should) block the sun from getting to the paper, leaving the part where the sunscreen or foundation was black, because the sun wasn't exposed to it. I am curious, since it is winter, and it is cloudy and cold where I live (Colorado) if the weather will effect this experiment. Will the clouds block the passing of UV rays?

Hi Laura-
Clouds will block UV to varying degrees so you may need to adjust the time of exposure for your papers on very overcast days-- thin cloud cover will have very little effect on UV but heavy, low altitude and dense cloud cover will severely impact UV penetration. Nevertheless, your experiment is still doable- be sure to include experimental controls (e.g. include a paper that is not coated).

best
htb

So-
You think that it will be O.K. to do the project today? I live in Colorado, and it is Cloudy and 38 degrees farenheit today. I'm not sure if the weather is going to get any better. Would it really make any difference if I did it today, rather than some other time on Christmas Break?

In addition to my other question, I was wondering if higher SPF in a sunscreen just gives you longer sun protection, or better sun protection?

Laura,

SPF refers to skin protection factor. It is a measurement of how well the sunblock protects your skin from the UV rays. A SPF of 10 means that that sunblock offers ten times more protection than the pigments in your skin. Similiarly, a SPF of 50 means that the sunblock has a 50 times the protection of your skin alone. SPF does not indicate how long the lotion will be effective.

You can do you experiment any day that you like. I would reccomend that you do it several times, one several different days. Take note of the cloud cover on each day, and record it in your project notebook. you can then take the average of the results of all of the tests, and use that, as using the average will help to elimenate differences due to the cloud cover.