Posted February 4, 2007
An arctic airmass moving into the region Sunday night and Monday will set us up for a very cold Tuesday morning. It will also bring some very dry air into the region, so that by Tuesday morning when we realize our coldest temperatures, there may not be enough water vapor in the air for any frost to form on cold surfaces. However, in locations where the air is especially still or there is a pond or creek nearby, there is at least some chance we could see frost form, and with the temperatures we expect on Tuesday that means the surfaces that would be most prone to frost should fall within a range where the formation of something a little more complex than your average fuzzy layer of tiny crystals is possible. A couple of years ago we had a similar situation, except in that instance recent rain had left the ground and lower atmosphere a bit more moisture laden before the very cold temperatures developed.
The result was frost that looked like a jumble of the classic images of a snowflake (actually snow crystals) all adhered to the tops of cars, mailboxes and so on. The reason for this is that the favored temperature range for development of "dendritic" crystals is about -10 to -20 degrees Celsius (14 deg F and colder). This is usually located in a layer somewhere a few to several tens of thousand feet up in the atmosphere around here, but on an especially cold morning this "layer" may intersect the ground. Sometimes when this happens it is too dry for frost and nothing much becomes of it. On that day two years past, conditions were just right to get the "fancy frost" of the title, and a nice mix of plate-like and column-shaped crystals were visible, along with quite a few dendrites. They were only around for a short time, and as soon as a little wind stirred and the relative humidity decreased, they quickly sublimated away (turned to water vapor without melting first). You can see some of the shapes to look for in outline form in the image above, or see a more detailed rundown on some of the possibilities at this address:
Will we have a complex "snow-frost" Tuesday morning? The odds are poor because of the low humidities expected, but it's worth taking a close look when you bundle up and head out for work or school.
Incidentally, we'll be at least in range of a record low that morning, although for now we're expecting to not quite reach it. As of Sunday, we're looking for 13 Tuesday morning, versus a record low of 10 for the date.