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About Frost (updated)

Posted December 15, 2007, 5:16 PM.

As we enter the winter season in Southern California, the potential for morning frost returns to the coastal and inland areas as the nights and mornings get colder.

Frost is actually frozen dew.

Dew forms whenever the relative humidity is high enough to allow condensation. Another way of saying this is that condensation occurs when the temperature falls below the dew point.

If it's cold enough, and wind conditions allow, condensation in the form of water droplets will freeze, forming frost. Generally, if there is enough wind, frost can't form.

Why do we see frost most commonly on lawns and roofs, and not necessarily in other places? Because condensation forms from radiation effects that make those things cold, colder than the actual ambient air temperature.

Which brings up another interesting point - frost is able to form up to air temperatures of 38°F locally, as long as the dew point is high enough, and the wind is relatively calm.

Frost is rarely a problem in places like Fallbrook and De Luz, and also, interestingly, the warmer portions of the Riverside area and the San Jacinto Valley. In those places, frost-intolerant plants thrive.

The dew factor makes frost appear less frequently in the eastern inland valleys, which tend to be drier, than the western inland valleys. So, the Temecula and Elsinore Valleys, for instance, would tend to get frost more often than the drier Menifee and Perris Valleys, even though the latter locations are generally colder this time of year.

How long does the frost season last? It differs, depending on the location, but in the inland valleys it's generally between mid-November and mid-March, with December and January having the highest chances.

Frost can be damaging to many kinds of plants. Prime examples are tropical plants like the bird of paradise. Precautions for smaller plants include covering them with something like burlap or a towel (this prevents condensation from forming on the leaves, hence no frost), or bringing them inside if they are portable.

Some other plants, such as tomatoes, will die when exposed to frost.

Thanks to Katy Parkhurst who contributed to this story.

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