Just a quick update regarding the previous post on "Fancy Frost." As I thought would likely be the case, our very cold morning on Tuesday also proved to be too dry for frost, at least around my neighborhood, and that was consistent with the fact that around the time of our minimum temperatures (15 at the RDU airport, for example) the dew point was -1, a sizable difference from the temperature and indicative of air that lacked sufficient moisture for much frost formation.
However, since writing the previous entry, I did dig up a couple of photos I took back in early 2005 that illustrate how different frost can appear when it forms in the temperature range that is favored for "dendritic crystal growth." There are three photos taken on Jan 19, 2005, when the low temperature at RDU was 13 degrees. In this case, however, the dew point was around 10 degrees, so it was possible that exposed surfaces (my mailbox, in this case) would fall to a point that equals or briefly undercuts the dew point, allowing for a thin layer of air above that surface to become saturated with respect to ice and therefore allow frost to form. Because it happened at a temperature in the crystal growth (also known as "snow growth") zone, the frost occurred in complex patterns including six-sided plates and dendrites.
As a point of comparison, I took another photo of the same mailbox a couple of weeks later, on Feb 1, 2005. On that morning, we had a more typical frost that occurs with temperatures more common in our area. In this case, our low for the day was 28 degrees, and the dew point was 26. Instead of the large, complex crystals from the other case, we see the more common situation with numerous, tiny and relatively simple crystals, resulting in kind of a "fuzzy" coating of ice.
Didn't happen this go around, and we're often too dry for it, but the more intricate frost pattern is something to keep an eye out for when you hear that our low temperatures will be somewhere below about 15 degrees...