October 14, 2013Forms of Precipitation1. Rain-Restricted to falling drops of 0.5 mm (0.01 inch) or greater, in diameter-Less than 0.5 mm is considered a drizzle-Raindrops begin to evaporate as they enter unsaturated air below the cloud-If all evaporates before reaching the ground, it is called Virga (Seen as streaks of precipitation) Fig 7.14.2. Snow-Much rainfall begins as snow-For snow to reach the ground, the entire temperature profile needs to be below freezing-Freezing level is higher above 12,000 ft. in summer (No snow)-Snowflakes falling from a cloud melt before reaching the ground-Freezing level is much lower in winter-Thus, falling snowflakes have better chance to reach the surface-Because snow scatters sunlight more effectively than rain, sky appears darker above the melting level3. Sleet-Lowest later of troposphere will be below freezing4. Freezing Rain-Formed by freezing of supercooled rain on contact with exposed surfaces-Coating of ice is clear and smooth-Sometimes called Glaze-Storm that produces Glaze is termed an “Icing Storm”5. Snow Pellets-Sometimes called Graupel-Less than 5mm (0.2 in) in diameter-Ice particles in a cloud that grow by accretion. Remain frozen and reach the surface as snow pellets.-Accretion: Growth of precipitation particle by collision of an ice crystal with a supercooled liquid dropletthat freezes upon contact.6. Hail-Forms in cumulonimbus clouds where updrafts can reach 100 mph-Occur in clouds with abundant supply of supercooled waterMeasuring Precipitation:1. Standard Rain Gauge-commonly used-measurable amount is 0.01 Inches or greater-less than 0.01 is reported as a trace2. Tipping-Bucket Gauge-Consists of two buckets-Each time a bucket fills up with 0.01 inch of rain, it tips, sending an electrical signal to the remote recorder3. Weighing Gauge-Collects rainfall in a cylinder that sits on a sensitive weighing platform-When cylinder fills up, electrical impulse is transmitted to a pen that records the dataMeasuring Snowfall:Two measurements are taken:1. Depth (With calibrated stick)2. Water Equivalent (General ratio is 10:1)Ex.) 10 inches deep = 1 inch of