Tuesday, October 7, 2008

Frost, Freeze, and Winterization

The Front Range of Colorado was under a frost advisory this morning and most of us woke up with temperatures between 35 and 40 degrees.

Not everyone saw frost -- but as a precaution we were placed under the frost advisory.

Visible white frost forms on cold, clear, calm mornings when the dew-point temperature is at or below freezing.

When the air temperature cools to match the dew point, now called the frost point when specifically trying to forecast for frost, water vapor in the air changes directly to ice without becoming a liquid first -- a process called deposition.

These delicate, white crystals of ice are called hoarfrost, white frost, or simply frost.

Frost has a treelike branching pattern that easily distinguishes it from the nearly spherical beads of frozen dew that can sometimes occur.

In dry climates such as here in the west, the air temperature can rapidly drop below freezing without the process of frost ever happening -- this is called a freeze, or sometimes a black frost, because it can be so damaging to crops.

So for those in the colder climates who have not winterized your home, and especially irrigation systems, it is definitely time to do so.

I would make plans to turn the water off and have the sprinklers blown out sometime within the next 1 to at most 3 weeks.

One important note for CoCoRaHS observers -- frost and dew are not precipitation. These processes are taking place at the ground due to physical processes involving temperature and water vapor.

Precipitation as you know takes place up at cloud level and falls to the ground.

It is good, however, to note things like dew and frost in your comments because it does help "paint the picture" of what was happening weather-wise that day at your station.

For example, if you have zero precip but stated it was a frost covered morning -- I know it was clear and calm at your house. The dew-point was probably between about 29-32 and the air temperature was similar long enough for frost to form.

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