Published: 2008-04-30 13:35:23
Updated: 2008-04-30 13:35:23
Posted April 30, 2008
By David Drumwright
MIKE MOSS SAYS: David, That is a pretty common question, but it doesn't hurt to cover it again! The basic issue at play here is that the important temperature in frost formation is that of the object upon which the frost forms, be it your car, some grass, a roof, the top of a mailbox, etc. On a calm, clear night with rather dry air in place, these surfaces may cool very rapidly by way of longwave infrared radiative heat loss. Air on the other hand is a less efficient radiator, and the air just a few feet above such surfaces can cool at a slower rate, remaining several degrees warmer. This is called a surface temperature inversion, since the temperature increases with height through a layer that starts at the surface. Reported air temperatures are usually measured about 4-5 feet above the ground.
All this being the case, it is not unusual for frost to occur on surfaces facing open sky, while air temperatures remain in the mid 30s to around 40. The presence of frost is an indicator that the surface of your car became much cooler than the air above, and also an indicator that there was enough water vapor in the air immediately adjacent to the car that the dew point was at least equal to the temperature of the surface. If, for example, the car cooled to 31 degrees, then the dew point had to be at least 31 degrees for the frost to form. Had the air been drier, with a dew point of 26 for example, you would not have seen frost on a 31 degree car.