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After his Thursday morning dog walk, Rich Stack asked me this question.
Temperatures don’t have to be below freezing for frost to form:
One unique characteristic of frost is that temperatures don’t need to reach 32° for it to form. Frost can appear on a roof due to a combination of temperature, dew point, humidity and most importantly radiational cooling.
In Sugar Land, the low temperature was 40°, in Richmond it was 39°, and in Cypress it was 38°. Yet, we received photos and questions about frost from all these areas.
The dew point in each location was in the lower 40s, with humidity near 90%. As the temperature dropped below the dew point, moisture in the air condensed into liquid on the roof. Higher humidity made more moisture available to condense.
However, what’s most important is the temperature where frost actually formed—on the roof, for example. On Thursday morning, the temperature reported by the recording station (typically displayed on our phones) is taken six feet above the ground and remained above freezing. But the temperature at the surface where frost forms was at or below freezing due to radiational cooling.
Roofs radiate heat efficiently during calm, clear nights, causing the temperature at or near the ground to drop several degrees lower than the temperature measured six feet above the ground. When the surface temperature of the roof reached 32°F or lower, frost formed.
What can a patchy frosted roof indicate?
If you notice patchy frost on your roof tomorrow or Saturday morning, it could be a warning sign. This might indicate inadequate attic insulation, with heat escaping from your home in certain spots and melting the frost. If you see this, it’s a good idea to have your insulation checked by a professional.
This question came to us through the click2houston.com help desk. If you have a weather question, you can submit it there or email me directly at ayanez@kprc.com. I’d also love to see your weather photos—share them with us at Click2houston.com/pins.
