UNIT 5—LESSON 1
OVERVIEW Identify the characteristics of hydrometeors, (precipitation, clouds, fog, dew, frost, rime,glaze, drifting and blowing snow, and spray, tornadoes, and waterspouts).
Drifting and blowing snow
Hydrometers consist of liquid or solid waterparticles that are either falling through or suspended in the atmosphere, blown from the surface by wind, or deposited on objects. Hydrometers comprise all forms of precipitation, such as rain, drizzle, snow, and hail, and such elements as clouds, fog, blowing snow, dew, frost, tornadoes, and waterspouts.
Learning Objective: Identify thecharacteristics of hydrometers (precipita-tion, clouds, fog, dew, frost, rime, glaze, drifting and blowing snow, and spray, tor-nadoes, and waterspouts).
Precipitation includes all forms of moisturethat fall to Earth’s surface, such as rain, drizzle, snow, hail, etc. Dew, frost, clouds, fog, rime, glaze, spray, tornadoes, and waterspouts are not forms of precipitation, although they are hydrometeors. Precipitation is classified according to both form (liquid, freezing, and solid) and size (rate of fall). The size of precipitation drops deter-mines their rate of fall to a large extent.
Precipitation that reaches Earth’s surface aswater droplets with a diameter of 0.02 inch (0.5 mm) or more is classified as rain. If the droplets freeze on contact with the ground or other objects, the precipitation is classified as freezing rain. Rain falling from convective clouds is referred to as rain showers. Showers are usually intermittent in character, are of large droplet size, and change rapidly in intensity.
Drizzle consists of very small and uniformly dispersed droplets that appear to float while following air currents. Sometimes drizzle is re-ferred to as mist. Drizzle usually falls from low stratus clouds and is frequently accompanied by fog and reduced visibility. A slow rate of fall and the small size of the droplets (less than 0.5 mm) distinguish drizzle from rain. When drizzle freezes on contact with the ground or other objects, it is referred to as freezing drizzle. Drizzle usually restricts visibility.
Snow consists of white or translucent icecrystals. In their pure form, the ice crystals are highly complex, hexagonally branched structures. However, most snow falls as parts of crystals, as individual crystals, or more commonly as clusters and combinations of these. Snow occurs in meteorological conditions similar to those in which rain occurs, except that with snow the in-itial temperatures must be at or below freezing. Snow falling from convective clouds is termed snow showers.
Snow pellets are white, opaque, round (oroccasionally conical) kernels of snowlike con-sistency, 0.08 to 0.2 inch in diameter. They are crisp, easily compressible, and may rebound or burst when striking hard surfaces. Snow pellets occur almost exclusively in snow showers.
Snow grains consist of precipitation of verysmall, white, opaque grains of ice similar in struc-ture to snow crystals. They resemble snow pellets somewhat, but are more flattened and elongated. When the grains hit hard ground, they do not bounce or shatter. Snow grains usually fall in small quantities, mostly from stratus clouds, and never as showers.
Ice pellets are transparent or translucent pelletsof ice that are round or irregular (rarely conical) and have a diameter of 0.2 (5 mm) inch or less. They usually rebound upon striking hard ground and make a sound on impact. Ice pellets are generally subdivided into two groups, sleet and small hail. Sleet is composed of hard grains of ice which has formed from the freezing of rain-drops or the refreezing of largely melted snowflakes; it falls as continuous precipitation. Small hail is composed of pellets of snow encased in a thin layer of ice that has formed from the freezing of either droplets intercepted by the pellets or water resulting from the partial melting of the pellets; small hail falls as showery precipitation.
Ice balls or stones, ranging in diameter fromthat of a medium-size raindrop to two inches or more, are referred to as hail. They may fall detached or frozen together into irregular, lumpy masses. Hail is composed either of clear ice or of alternating clear and opaque snowflake layers. Hail forms in cumulonimbus clouds, and it is normally associated with thunderstorm activity and surface temperatures above freezing. Deter-mination of size is based on the diameter (in inches) of normally shaped hailstones.
Ice Crystals (Ice Prisms)
Ice crystals fall as unbranched crystals in theform of needles, columns, or plates. They are often so tiny they seem to be suspended in the air. They may fall from a cloud or from clear air. In a synoptic observation, ice crystals are called ice prisms. They are visible mainly when they glitter in the sunlight or other bright light; they may even produce a luminous pillar or other optical phenomenon. This hydrometer is common in polar regions and occurs only at low temperatures in stable air masses.