Why is there Frost on the Ground?

This past Monday morning offered a great science teaching moment for my daughter.  While waiting with her for the school bus to arrive she asked "Why is the ground all white?  Did it snow last night?"  No, that's not snow, it's frost on the ground and we see frost on the ground whenever it's cold outside.  There was also frost on all of the nearby trees.  So the real question isn't why is the ground white.  The real question here is why does frost appear on the ground/trees?

Each night the grass or trees radiate infrared energy outward.  This is similar to your body radiating infrared energy (what we call body heat).  If the grass loses enough energy, water vapor from the air can condense into ice on the grass blades.  This is frost.  Two things need to occur for frost to appear.

1.  The temperature must be below freezing (32 degrees F).  If the temperature is above freezing, dew (water) droplets can form.  You'll see this many times during the spring/summer/fall.  

2.  There must be enough water vapor (moisture) in the air.  If the air is extremely dry, there will be little moisture to condense as ice on the grass.  In this case, frost will not form.  

It's funny how science introduces itself to you.  I didn't wake up this morning thinking I would be explaining frost to my daughter, but I did.  Science is amazing!