2006_02_20_hoarfrost.jpgIt is not the natural state of a New Yorker to look up at the sky. The skyscrapers block our view and we need to be alert to what our fellow pedestrians are doing. Gothamist has tried to get people to look at the halos, sun dogs and iridescent clouds with limited success. Luckily there are meteorological phenomena to see on the ground. On our way to work in the morning Gothamist walks up the 116th Street stairs in Morningside Park. The top set of stairs are made of pink granite slabs. The mortar between the steps has long since disappeared so air and water move into the cracks (take note Parks Commissioner Benepe!). When it gets cold after warm days when water has seeped into the cracks these little frost patches appear on the steps.

Usually we are running late and don't have time to stop and photograph the frost, but on Monday morning we had a few minutes to spare and took this picture (big version on Flickr). This type of frost is known as hoar frost. It forms on the steps when moist air gets trapped beneath the steps. At night the surface of the steps get much colder than the air beneath them. If the dew point temperature of the trapped air is greater than the temperature of the steps the first few millimeters of air above the steps becomes saturated and ice crystals, or frost, forms on the tiny imperfections of the granite surface. The Morningside Park hoar frost only forms a handful of times each winter so we were glad to get a picture. Hoar frost can be spectacularly and delicately beautiful, as witnessed via the Flickr hoar cluster and these amazing photos from Sioux Falls, South Dakota. You can see more pedestrian examples of hoar frost if your freezer is not frost free.

Your next chance of seeing hoar frost in the park is Monday morning. It will be cold enough but there may not be sufficient moisture available. There's a slight chance of rain or snow over the next couple of days starting tonight. The precipitation, if it occurs, will be light. Temperatures will be on the mild side through Friday, when a cold front will pass through and cool us down considerably.

Finally, we just like this false-color summer satellite image of the North Slope of Alaska we saw on NASA's Earth Observatory.