I've had this question several times in the last couple of days...since we had the scattered frost over the weekend.  How can you get frost to form when the temperature is 34, 35...even 38 degrees?  It's not a miracle of nature, or anything, it's actually just simple physics.  The first thing is that cold air is heavier than warm air.  So, when the air is clear, heat escapes into the upper atmosphere and the cold air begins to settle toward the bottom.  Also, when the air is calm, there is no wind to mix up the air and keep it all the same, so the coldest air settles right to the ground.  It can be several degrees colder at your feet than at your head in this situation.  For whatever reason, early Meteorologists decided to take the official temperature at 5 feet off the ground.  This means that it can be 32 degrees at the ground, and 38 degrees at 5 feet above the ground.  This would allow frost to form on the grass when the official temperature is 38 degrees.

Mike Hoffman