Chapter 24 Water in the Atmosphere Section 3 Precipitation Notes 24-4

Precipitation Precipitation: any moisture that falls from the air to the earth’s surface –Liquid or solid –Rain: Liquid; between.5 mm and 5 mm Drizzle: smaller drops; close together; fall slowly –Snow: Solid; ice particle Snowflakes are just ice particles that combine together

Precipitation Snow: –Colder the temperature the smaller the flake –Bigger flakes occur when the temp. is closer to 0°C Sleet: –Solid; clear ice pellets –Forms when rain falls through freezing air –Glaze ice: rain that freezes on the surface of the earth during an ice storm

Precipitation Hail: solid lumps of ice –Alternating clear and cloudy layers Clear: passes through a layer of very moist air Cloudy: water droplets with air bubbles –Usually forms in cumulonimbus clouds From a continuous up and down movement inside the cloud –Rain is upswept to lower temp. and frozen –As it falls more water is added –Frozen precip. Is swept up again and again –Eventually it is large and heavy enough to fall out

Websites otes/chapter7/51_Sleet/A_51.htmlhttp://apollo.lsc.vsc.edu/classes/met130/n otes/chapter7/51_Sleet/A_51.html hail.htmhttp:// hail.htm

Precipitation Causes: –Droplets or ice crystals are large enough to fall out Average cloud droplet: 20 micrometers or m Must increase size 100 times this diameter to fall out of cloud Slightest air movement keeps it suspended

Precipitation Causes: –Coalescence: Differences in the size of cloud droplets Original size depends on the size of the condensation nuclei As larger droplets move down, they fall into other smaller droplets –They collide and combine –Becoming so large, they fall out of the cloud Tropical region rains form this way

Precipitation Causes: –Supercooling: Water droplets with a temp. of less than 0°C –May fall as low as -10°C Water droplets do NOT freeze –Few freezing nuclei: condensation nuclei with a crystalline structure similar to ice Molecules evaporate from water droplets and condense on ice crystals –Increase in size until they fall out as rain or snow Rain and snow in middle and high latitudes form this way

Precipitation Causes: –Cloud Seeding: Freezing nuclei are added to supercooled clouds –Silver-iodide: from planes or burners on ground –Powdered dry ice: cools the droplets causing ice crystals to form; from planes May increase precipitation in some conditions and decrease in others Could be used to release precip. from clouds before a storm becomes too large.

Measuring Precipitation Rain gauge: –Instrument that measures the amount of rainfall Only measures rain that falls in one spot Precipitation varies so much it is hard to get an accurate measure of a larger area

Rain Gauge

Measuring Precipitation Snow Board: –Board that allows for a flat surface from which you can measure the inches of snow 10 cm of snow = 1 cm of water Depending on the water content of the snow

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