Frost

[chickendoc]

I was having a discussion with a coworker about the frost we had this weekend. Supposedly the low this weekend was only 38°F. How does frost form when the air temp is higher than 32°F?

[Mangetout]

I believe this may be what is called 'radiation frost'; the ground (and other objects) cool to a lower temperature than the air by losing heat through radiation.

[chickendoc]

Mangetout:

Radiation frost sounds like a plausible explanation to me. I'll look it up.

Thanks

[Philster]

When the news reports 38 as the low, they are reffering to air temps. Of course, things like car windows and car hoods can cool below 38.

[Q.E.D.]

A thermometer indicates the temperature where the thermometer is, usually a few feet above the ground. Because cold air sinks relative to warmer air and because the ground can cool very quickly the temperature at ground level can be cooler than a few feet higher where the thermometer is, even though the thermometer indicates a temperature that is above freezing, it can easily be below freezing a few feet lower.

And

Certain materials like glass and car metal radiate heat quickly and therefore cool quickly. Also, areas like rooftops or high plateaus, because of their exposure lose heat through re-radiation very quickly and tend to receive frost before sheltered areas.

Fromthis article.

[chickendoc]

quote:
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Certain materials like glass and car metal radiate heat quickly and therefore cool quickly. Also, areas like rooftops or high plateaus, because of their exposure lose heat through re-radiation very quickly and tend to receive frost before sheltered areas.
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What about my pepper and tomato plants?