Category: Weather

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Jack Frost Nipping at Your Ankles

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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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Weekend Weather Oddment

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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.

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Midwest: Flocked

A beautiful NASA satellite shot of the Midwest after the snow- and icestorm that blew through a couple weeks ago. Worth taking a look at the larger image (just click the one below).

(By way of exit 670)

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Today’s Best Names

Typhoon Prapiroon. About to hit the island of Hainan, off China’s southwestern coast. Prapiroon is reported to be the Thai god of rain. When the storm hit the Philippines a couple days ago, it went by the name Henry.

David Tarwater. U.S. and University of Michigan swimmer and training partner of Michael Phelps. Tarwater? Sounds viscous.

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Flying Back

There will be plenty of East Coast trip postscripts to come, but for now: We’re sitting in a terminal at Kennedy airport; outside, it’s about 100, and even people who have been working inside all day are complainng about the heat. Outside, one big difference between city dwellers and suburban folk shows itself. The urban types are out on the streets, walking to the subway, shopping, whatever they have to do. It’s not like the sidewalks were packed in my brother’s neighborhood, but people were out and about, even if lots of us looked a little wilted. Out in the suburbs: No one on the street, anywere; people out there — and "out there" is probably any suburb you can think of — live strictly a doorway to doorway existence during the worst weather. Glad the power grid is holding up for everyone so far.

This morning, getting ready to leave John and Dawn’s place, we were talking about the latest bicycle fatality in the Oakland Hills. A guy out for a ride was hit head on up there on Skyline Boulevard, within a half mile or so of where I crashed in June, by a motorcyclist; the cyclist died of his injuries, the motorcyclist apparently walked away from the wreck. Not to place blame without knowing what really happened, but one of the risks bicyclists take riding up in the hills, a risk that’s increased a lot in the last 20 years, is that we share the road with motorcycle riders and motorists who treat the twisting roads like a raceway challenge. I’ve often worried about getting hit up there.

Anyway. At one point, John said, "Hey, did you hear about that Wired editor who died during the marathon?" I hadn’t. I looked up "wired editor marathon" onlne, and found a story on Wired News. The editor who died during the marathon was a guy named Bill Goggins. I knew him from my stint at the magazine in 1998 and from my days freelancing for the magazine. Bill was 43, and the news accounts say that he collapsed at mile 24 of the San Francisco Marathon last weekend and couldn’t be revived. A friend who saw him at mile 21 said he was smiling and running strong, and a mutual friend had seen him twice in recent days and said he seemed fine. The thing about Bill, whom I never got to know well enough, was that he was brilliant and funny and charming and had a big heart that was right there for anyone to see. Forty-three. Hard to believe. See you, Bill, wherever you are.

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Heat, a Re-examination

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The last day of July, the first of August, it’s supposed to be hot. Today, it’s an unremarkable 90 or so here in Brooklyn. I’m sitting in my brother and sister-in-law’s unairconditioned kitchen about a mile south and east of the Brooklyn Bridge. Not suffering. But tomorrow we’ll be getting what folks to the west have been dealing with for the last couple of days (112 in Bismarck?!). The National Weather Service is warning it will get up to about 100 Tuesday and Wednesday, that it will be plenty humid, and that we’ll have high ozone levels as the air in the region stagnates. (Add rum and guns, then stir for a swell party!)

The last few days, Kate and I have been staying in a friend’s house  near the northern New Jersey shore. It’s got central air conditioning, and the system has been running ever since we arrived there last Thursday. It struck me this morning as I walked outside for the first time and shut the sliding glass door behind me that around here, the ability to cool the air in homes and cars and public places of all kinds is just as vital as the ability to heat it in the winter. In the suburbs, anyway, you don’t see homes open to the elements on a hot day any more than you’d see a place with its windows flung open when it’s zero outside. Yet, the weather’s the weather. It may be incrementally hotter on average than it was a generation or two or three ago, but everyone here endured long, stifling stretches of heat then without refrigerating every living space, just as most of the world’s people do today. (We went to France in August 2003 at the tail end of the country’s extended heat wave; I knew air conditioning was uncommon there, but I hoped against hope that somehow our little hotel would be an exception; instead, when we got to our room, we found that the windows hadn’t been opened for days and the place was like an oven — and what was worse was that for several days afterward, there wasn’t enough of a breeze to cool anything off.)

I’m not arguing for some kind of sweaty, hair-shirt virtue in living without air conditioning. Just makes me wonder sometimes what would happen if we all suddenly had to do without (which ties into my fear for the next couple of days; I’m concerned that the power demand here will cause a blackout and shut down the air-traffic-control system and keep us from flying back Wednesday to our effete little climate back in Berkeley). I do remember that before we had our first air conditioners, in 1966, the remedy for hot nights was staying up late watching movies with our mom and taking cool showers before we headed off to bed. Somehow, we slept.

(Picture: Hamilton Avenue and West 9th Street, Brooklyn. It wasn’t really 99 degrees.)