Berkeley Frost: Spicule Watch


As related in earlier winters , sometimes Berkeley gets cold enough that frost settles over the town. Well, settle isn’t really the right word, since the frost crystals actually grows in what appears to the layperson to be a magical process of sublimation. The crystals are called spicules, which resemble little spikes or hairs when they form on a cold surface.

Speaking of our weather, one of our local TV weatherfolk, KTVU’s Bill Martin, referred to it as “Chicago cold” last night. And not once but twice he advised viewers that they’d want to take action to make sure plants, pets and “the elderly” were protected from the weather’s effects. The elderly? We brought our own resident grandparent in from the unheated shed in the backyard.

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Berkeley Frost

Oh, sure: You, wherever you are to the north or east of the San Francisco Bay shoreline, you have your cold snaps, your big old snowstorms, and your drifts. All that’s enough to make you forget how the cold season started some frosty morning a few months ago. Here on the Bay, frost happens every so often in the dead of winter, on some clear morning after a storm has passed. This morning was one of those frosty mornings for us to come out of our uninsulated bungalows and think that we’re in some kind of wintry solidarity with folks on the Columbia, near the East River, or on the shores of Lake Michigan and Lake Erie.

Today’s Frost Report

Down to freezing again overnight (yes, I’m conscious that most of the vegetation on which the frost forms here is green, not Northern Hemisphere winter brown). Those little crystals of ice? They’re called spicules (according to the OED, which refers to them as spicula, a spicule is “1. A sharp-pointed or acicular crystal or similar formation … b. esp. A formation of this nature caused by the action of frost”).

Jack Frost Nipping at Your Ankles


Frost this morning, leading to today’s inquiry: How does frost form if the air temperature is above freezing? Frost is ice, after all, so where does it come from if your reasonably accurate thermometer (ours: on the back porch, six feet above the ground) shows that it’s 38 degrees outside? What accounts for car roofs getting frosted when there’s no other sign of frost in the area?

I never thought about this much growing up in Illinois because when you saw frost, it was usually well below freezing. Here, I started to wonder about it because wintertime frost is common in our relatively mild bayside climes, mostly when the thermometer is showing a temperature five or six degrees or more above freezing.

The short answer (from a couple of just-OK references, here and here) is that frost only forms (it sublimates, from water vapor directly to ice) in the presence of freezing temperatures. The temperature that’s critical in the process is not the air temperature several feet off the ground, where most thermometers are placed, but at the surface where frost is formed. Among the factors that make ground temperatures significantly colder than the air several feet above are radiative cooling–the process by which the ground is surrendering heat energy into the atmosphere in the absence of some input (sunlight, for instance)–and the tendency of cold air to sink. So while it’s 38 degrees at an altitude of six feet, it can be 32 or below on the ground; if there’s sufficient moisture in the air, frost will form.

And the presence of frost on car roofs, etc., when there’s little or no frost nearby? The same general explanation holds; the difference is that exposed metal and glass radiate heat faster and more completely than ground surfaces and thus reach the frost point more quickly. A car roof is an example of a sort of micro-micro-climate, I guess.

[Update, 12/19: I found a second thermometer and measured the temperature at the ground to compare it to the temperature recorded on our indoor-outdoor thermometer, which has a sensor at a height of six feet above the ground. The latter recorded a low this morning of 35 degrees; at the same time, the ground thermometer, which was just half an inch above the ground on one of those Frisbee donut things (so that air could circulate under it and so that it would not be resting directly on the ground), showed a temperature of 27 degrees.]


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