Frost Crystal on Car
January 21, 2007
Early in November (2006), cool overnight temperatures in North Salem, New York resulted in the deposition of a very light frost on the freshly waxed surface of my car. Notice in the detail photo that while the surface is generally covered with a very fine grained frost, there are 3 larger needle-like ice crystals growing out from a central point (likely a speck of dust that served as a deposition nucleus), and that the area adjacent to the larger crystals is completely free of ice! The entire roof and trunk were peppered with these little structures. Water molecules cling more securely to larger crystals than to smaller ones, and over time there's a net deposition on the larger crystals and net sublimation of the smaller adjacent crystals -- water molecules are 'siphoned' off of the small crystals and deposited onto the larger ones. See image link below for more on this.
Growth of liquid water droplets in clouds can occur in a similar way - larger droplets grow at the expense of smaller water droplets around them. And in a cloud that contains both liquid water droplets and ice crystals, the ice crystals rather quickly "take up" the water molecules. So, in a relatively short time only ice crystals exist in the cloud.