Bill said:

I bought a couple dozen this year and they seemed to work as advertised. I
set my tomatoes out April 15 and didn't lose any until I (erroneously)
believed a NOAA weather report that stated a low of 34 deg F for the night.

Maybe it never went below that where they took their measurements, but I
lost half my Druzba tomatoes and a couple of each of the other varieties to
frost. From now on, no more taking chances. If the advertised low is below
40, I'm leaving the hot caps on.

Be wary of frost any time predicted low is less than 40 degrees and the sky
is clear. Be especially worried if the DEW POINT is at or near 32 degrees.
Get REALLY WORRIED if there is no wind. Radiative cooling can produce
patchy frost even when the official, measured low is higher than 32 degrees.


--
Pat in Plymouth MI ('someplace.net' is comcast)

Any technology distinguishable from magic is insufficiently advanced.
(attributed to Don Marti)