Sun intensity is caused by the angle of the sun.  The angle the sun’s rays hit the Earth changes at different times of year, time of day and where the place is on the Earth.  The angle formed when the sun’s rays hit the Earth is called the angle of incidence.  If the sun hits from straight overhead it’s a 90 degree angle and it’s most intense (strongest).  As the sun is lower in the sky, the angle is shorter.  The rays spread out and cover more of the Earth’s surface, which makes the rays less intense (weaker).The only place on Earth that gets 90 degrees of sunlight regularly and that’s at the equator.  The farther away from the equator you go, the less intense rays there are.  According to “”, the poles get about 40% less solar radiation than the equator yearly.  The tilt of the axis also has an effect on where the sun’s rays hit.  At summer solstice, the rays hit more directly on the Northern Hemisphere at the Tropic of Cancer.  The other side of the Earth is tilted away and has winter solstice.  The sun’s rays are at a shorter angle with the rays more spread out. On the summer side, the sun is higher in the sky and so it is more intense.   Its rays strike the ground at a higher angle than the side having winter. “This explains why the risk of sunburn is greatest in summertime. It also explains why temperatures are warmest in summertime, since the sun delivers heat energy, ”  says Amy Harris of  We learned that by looking at the latitude of where the sun is directly overhead and what our latitude is, we could figure out the angle of the sun during the different seasons.  It was about 50 degrees in spring and fall, about 74 degrees in summer and about 27 degrees in winter.  This is how we will set our model up to test!The last thing that might affect the chances of sunburn is the time of day.  No matter where you are on the globe OR the season, the sun’s rays are most intense in the middle of the day; around noon.  A suntan is the body’s natural defense against UV rays that can harm your skin.  If you get so much sun that your body is overwhelmed, then you get a sunburn.  Your body has a pigment called melanin, that’s job is to absorb ultra violet rays and send heat away from the area.  UV rays can overwhelm the melanin and cause a toxic reaction.  Too many UV rays can mess up the body’s DNA and cause cells to reproduce wrong, which can cause cancer in the long run.  Sunburns usually show up a few hours after too much sun exposure sends extra blood to the injured area.  Having five or more sunburns can make you more likely (two times) to get melanoma, a form of skin cancer.We are using two strains of yeast in our project.  One has it’s DNA altered so it does not respond normally to UV radiation.  Normally, yeast will grow well in the presence of the DNA but with the UV sensitive yeast, the yeast cells cannot repair themselves and so they are damaged and die.