Answer to "frost" question

FROST -- Brian E., from Madison, WI noted "...it was cold the other night and we were a few degrees away from a frost. That triggered a question that has perplexed me for some time now -- Why does frost tend to form on top of picnic tables and not on the bottom? -- 9/25/99

(1) Any object not at absolute zero will radiate energy. You may have experienced this effect standing near a fireplace (with a fire burning). Even a few feet away, the side of you facing the fire got warm. Energy was radiated from the fire and you absorbed it. On your opposite side, no heat energy was absorbed, so you felt cold.

(2) Although the Earth (and everything on it, including picnic tables) is always radiating energy out to space, we notice it more at night because there is no energy coming in from the sun to balance things.

(3) The top of picnic bench radiates energy out to space. Unless there are clouds present (to absorb the outgoing energy and re-radiate back to Earth), the picnic table will lose energy and cool down. As it does, it cools the air next to it (by a process called conduction). As the air temperature next to the top of the picnic table reaches the dew point temperature (assuming the air is moist enough to begin with), the humidity reaches 100% and condensation occurs. If the temperature is warm enough, the picnic table will be dewy. If its cold enough, the top will be frost-covered.

(4) Underneath the picnic table, the picnic table is also radiating energy away. But it is also receiving energy back from the ground. This ground-based energy warms the underside of the table enough so that dew and frost usually don't form there. The few degrees difference in temperature between the top and bottom of the picnic table often spells the difference between dew/frost and none.

The bottom line to this is that one has to examine the energy balance on an object. If there is enough energy loss, and the air is moist enough, the object cools and dew or frost will form. If not, then dew or frost won't.

You can see the same effect if you look at a car(s) parked in a driveway. A car (or the side of a car) closest to a house will often have less dew or frost than one that is further away or which is facing to a more open space. Take a look and see if you find this to be true.

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