October 14 2013 Forms of Precipitation 1 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 level 3 Sleet Lowest later of troposphere will be below freezing 4 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 droplet that freezes upon contact 6 Hail Forms in cumulonimbus clouds where updrafts can reach 100 mph Occur in clouds with abundant supply of supercooled water Measuring Precipitation 1 Standard Rain Gauge commonly used measurable amount is 0 01 Inches or greater less than 0 01 is reported as a trace 2 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 recorder 3 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 data Measuring 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 rainfall