"You want it to be one way. But it's the other way."
It was freakishly warm here today, meaning the warmest day on record for the date around most of the Bay Area. Temperatures in the 70s were common. A few places got into the 80s. I remember years where I’ve waited well into April before we’ve had our first 70-degree day.
Now, it’s nearly midnight. Still 65 degrees. The warm northeasterly is still blowing across the hills and down across the flatlands. Not like any January night I remember in these parts.
The National Weather Service reports, and eyewitness accounts confirm, that we’re having a storm. The forecasters have this to say about the current atmospheric proceedings: “A WALLOP OF A STORM CONTINUES TO BARREL ITS WAY THROUGH THE BAY AREA EARLY THIS MORNING.” That’s right — a wallop. Wind gusts up to 75 mph. Rain blowing sideways. If you live east of here, and nearly everyone does if you look at the map the right way, the wallop is headed your way.
More later. I have to brave the tempest for a trip into the city. [Pictures (click for larger versions]: Above, Codornices Creek in northwest Berkeley, over its banks. Below, the entrance to Golden Gate Fields.]
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”).
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.]
Yesterday, from a hill on Van Fleet Avenue in the Richmond Annex portion of Richmond. All afternoon, towering clouds were visible out over the Pacific, beyond Marin County and San Francisco. A storm was moving south and just brushed the coast below us. From where we were, the lighting was striking all day.
Rain in Berkeley today. Light rain, for sure, but still: it’s drizzling down; gurgling in the drainpipes; creating adventurey driving conditions for weather-challenged Bay Area commuters. The reason it’s worth mentioning in a forum as august as this here scribblefest: It’s a rather rare occurrence — one of the local weatherpersons said on the radio that it’s only rained on June 8 14 times since the start of official meteorological record-keeping hereabouts 150 years or so ago.
Film at 11.