Saturday, November 24, 2007

Tis the Season

Well early this morning did it, finally we had a freeze. In my neck of the woods, we hit around 28 degrees Fahrenheit. Now we could argue about shades of gray, hard freeze or not. I'd say then we had a soft freeze. Cold enough to wilt and blacken the more tender annuals. Not cold enough long enough to set back the flowering perennials much.

The difference between a frost and a freeze? Well a freeze is much easier to define: when the temperature drops below 32 degrees F for a sustained period of time. This often happens during the early morning hours at this time of year, after much of the heat the city has accumulated radiates out into the night sky. Freezes tend to come on clear, still nights. Clear allows much heat to radiate upwards, still allows this process to continue unfettered. Clear and still also indicates cold and dry near winter time. Dry, meaning low moisture content in the air, allows the air to cool off even faster because water vapor retains heat. So at this time of year, after a cold front passes, the wind dies down, and the skies clear we have a greater chance of a freeze. Freezes will kill most annuals, so many of which are tropical in origin. Freezes may kill native annuals, but often it takes a hard freeze. Perennial leaves will blacken and die depending on the length and depth of the freeze.

Frost is a phenomena dependent on a variety of factors. When the temperature drops to freezing, usually in the dark, early morning hours, and the moisture content of the air happens to be high, we may have a frost. Frosts involve the coalescence of dew point and temperature. The frost can occur at a range of freezing temperatures, but the frosted surface must be colder than the dew point, the dew point colder than the air temperature, and the dew point must be below freezing. Simply put, frost is water vapor that condenses directly into a solid (ice) on things like plants, cars, and ground whose surface temperature is colder than the dew point. Frosts occur most frequently where there is little to no wind.

In spring we worry about late season frosts, which could take down tender young plants. In autumn, we usually accept the fate of our plants after the long season. However, if it is vegetables you want to protect in spring or fall, simple row covers of plastic or even soda bottles filled with water may do the trick. Both these devices hold heat and lessen the chance for frost on an extremely local level. I've grown broccoli under a clear plastic tent for weeks in freezing temps until they were ready for harvest. While broccoli tends toward cold hardiness, most plants can't be protected from a long, hard freeze. It is then time to let go.

This frost formed one December before dawn and was gone by 9 am

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