With cold temperatures settling in, a magical force has taken the water around us and transformed it into complex formations. Moisture in the ground and air; freeze and form intricate patterns. In parts of the northern states snow has already fallen and covered the ground in a white blanket while further south frost coats the ground on cold nights.
Photo Credit: Kate Keefe
9/07 (Mount Monroe)
The first sign of the upcoming winter is frost. Towards the end of September gardeners start watching the nightly lows and putting sheets over their garden to protect their vegetables from ice formation. Frost forms when the temperature near the ground reaches freezing. If the dew point temperature, the temperature in which water vapor condenses to form dew, is also near freezing, any moisture in the air near the ground changes directly from a gas to a solid, and deposits itself on anything that is also at freezing temperatures. These structures are usually plants, grass, park benches and even the roofs of well insulated homes.
Frost will form in valleys before it forms in higher elevations because heat rises and cool air sinks. Temperature readings are usually taken about six feet above the ground surface. Temperatures on the ground can be five to seven degrees cooler than the reported temperatures. On nights when it is either cloudy or windy mixing occurs and the temperature gradient disappears and frost doesn’t form. Ideal frost conditions include clear nights with little to no wind and air temperatures around freezing.
Photo Credit: Kate Keefe 10/07
(Needle Ice on the way up Mt. Willard)
Needle ice also forms in fine textured soils after the first hard freeze of the fall. The needles are fed by capillary action and are pushed out of the ground by the ground water. The surrounding soil temperatures must be above freezing to allow the water to be pushed out into the freezing air. These ice crystals usually grow in columns three to four inches high but can grow larger and end up looking like miniature chandeliers. These needles lift the cap of the soil and rocks and cause them to settle down slope as they melt.
Snow, as we generally think of it, is ice crystals that grow to be large and heavy enough to fall to the ground. Snowflakes are composed of one or more ice crystals. These crystals form when water vapor freezes around a particle which is known as the nucleating agent. The resulting ice crystal or snow flake grows when water molecules are transferred from water droplets in the air.
Photo Credit: Kate Keefe 11/07
(Footprints in fresh snow around Saco Lake)
Snowflakes are often portrayed in their beautifully symmetrical six pointed structure. These tiny crystals form six points because water molecules form a hexagonal structure when forming crystals. The six fold symmetry of the crystal comes from the symmetry of the water molecules in the crystal lattice. The crystal type is determined by the temperature and available vapor supply during formation.
Check back for more information on winter progresses. In the meantime check out where frost is forming or snow is falling in your area.