Drizzle that falls in liquid form but freezes upon impact to form a coating of glaze. In U.S. aviation weather observations, this hydrometeor is encoded ZL. The physical cause of this phenomenon is th
e same as that for freezing rain.
The advancing boundary between frozen (or partially frozen) ground and Unfrozen Ground. In the usual case, where the Active Layer extends to the Permafrost Table, two Freezing Fronts will be present d
uring annual freezing of the ground, one moving downward from the ground surface, the other moving upward from the Permafrost Table. The Freezing Front may not coincide with the 0C isotherm (Cryofront) due to freezing-point depressions.
The cumulative number of degree-days below 0C for a given time period. Four main types of air freezing indices have been used: Approximate Freezing Index - calculated from the mean monthly air tempera
tures for a specific station without making corrections for positive degree-days (T > 0C) in spring and fall (Boyd, 1979); I_af = SUM (NT) where N = number of days per month for months with a mean monthly temperature below 0C during one complete year. T= mean monthly temperature. Total annual Freezing Index - calculated by adding all the negative mean daily air temperatures (C) for a specific station during a calendar year (Harris, 1981); I_af = SUM (T) where T = mean daily air temperatures (C) below 0C for one complete calendar year. _ Seasonal Freezing Index - calculated as the arithmetic sum of all the negative and positive mean daily air temperatures (C) for a specific station during the time period between the highest point in the fall and the lowest point the next spring on the cumulative degree-day time curve (Huschke, 1959); I_af = SUM (T) where T = mean daily air temperatures (C) during the time period between the highest temperature in the Fall and the lowest temperature in the Spring. _ Design Freezing Index - calculated by taking the average of the seasonal freezing indices for the three coldest winters in the most recent 30 years of record. If data for 30 years are not available, then the index is based on the coldest winter in the latest 10-year period of record (U.S. Army/Air Force, 1966).
A measure of the combined duration and magnitude of below freezing temperatures occurring during a specific freezing season and calculated by accumulating the number of degree-days below 0 degrees C a
nd subtracting from that total the number of degree-days above 0 degrees C over the same period.
As used by the U. S. Army Corps of Engineers, the number of Fahrenheit degree-days (above and below 32F) between the highest and lowest points on the cumulative degree-days time curve for one freezing
season. For a critical review of the topic see Sakari Tuhkanen (1980). "The freezing index is used as a measure of the combined duration and magnitude of below freezing temperatures occurring during any given freezing season. The index determined for air temperatures at 4.5 feet above the ground is commonly designated as the air freezing index, while that determined for temperatures immediately below a surface is known as the surface freezing-index." (Also called "coldness sun.")
Commonly, and in aviation terminology, the lowest altitude in the atmosphere, over a given location, at which the air temperature is 0C; the height of the 0C constant-temperature surface. This simple
concept may become slightly complicated by the existence of one or more "above- freezing layers" formed by temperature inversions at altitudes higher than the above-defined freezing level. In cloud physics terminology, this is more accurately termed the melting level, for melting of ice always occurs very near 0C, but liquid cloud drops may remain supercooled to much colder temperatures.
Any particle immersed within supercooled water, initiating the growth of an ice crystal to be compared with particles nucleating directly from the vapor phase (deposition nucleus). Similar particles m
ay nucleate at somewhat different temperatures (a few degrees) depending on the process. Observations of natural freezing nuclei indicate that there is normally present in the atmosphere a large variety of such particles with varying activation temperatures (temperatures at which they become effective nucleators). Certain bacteria from vegetation (pseudomonas syringae) nucleate ice at temperatures as high as -2C; mineral particles (e.g., clays: kaolinite and montmorillonite) at -10 to -20C; artificial nuclei (e.g., silver iodide, lead iodide, and metaldehyde), as smoke, can be found to nucleate at intermediate temperatures, i.e., -5 to -10C. The origin, distribution, and composition of these particles is highly variable; some are composed of a mixture with a hygroscopic component that dilutes prior to nucleation of the water by the freezing nucleus.