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Unformatted text preview: Precipitation Precipitation This chapter discusses: 1. Processes that generate falling precipitation from 1. clouds clouds 2. Types of precipitation, such as rain and snow, and 2. methods for its measurement methods Growth of Cloud Drops Growth
Atmospheric Atmospheric vertical winds and eddies can keep small and light cloud droplets and condensation nuclei aloft. nuclei As the cloud droplets knock and join together, they grow larger and their weight increases, causing them to fall as precipitation. precipitation. Figure 8.1 Evaporation & Droplet Size Evaporation Figure 8.2 Water molecules more easily evaporate from a curved surface than a Water flat surface, which creates a greater concentration of molecules above the droplet, known as increased equilibrium vapor pressure. above Vapor Pressure & Saturation Vapor
Smaller drops have Smaller greater curvature and require greater vapor pressure to keep water molecules from evaporating away. evaporating As the drop size As increases, the required relative humidity (RH) for equilibrium decreases. equilibrium If the required RH is If exceeded, the drop will grow. will
Figure 8.3 Collision & Coalescence Process Collision
Both a) collisions Both that join together small cloud droplets b) coalescence that attaches faster and larger droplets with smaller slower droplets work together to assemble nearly 1 million cloud droplets into a raindrop large enough to fall to earth. earth.
Figure 8.4A Warm Cloud Processes Warm
Collision and Collision coalescence operates in warm clouds (> 15° C) to produce rain, and is affected by the clouds liquid water content, droplet sizes, cloud thickness, updrafts, and drop electrical charges. charges. Figure 8.5 Ice Crystal Process Ice Cold clouds may drop Cold below –40° C before small droplets freeze into small ice embryos that can ice serve as condensation nuclei. nuclei. Figure 8.6 At very low At temperatures, vapor can also condense as ice onto nuclei formed by: nuclei a) deposition a) b) freezing c) contact, c) primarily in the glaciated region of the cloud. Molecules from Water to Ice Molecules Figure 8.8 Ice crystals have lower saturation Ice vapor pressures than liquid droplets, creating a gradient of high to low water molecules from liquid to ice that encourages ice growth. that This growth is critical to the icecrystal precipitation process. Figure 8.9 Ice Particle Changes Ice As ice crystals fall and collide with super As cooled drops, they get bigger by accretion. cooled Falling icy matter is called graupel, and Falling aggregation describes the joining of two ice crystals into snowflakes. ice Figure 8.10A Cloud Seeding Cloud Figure 8.11 Artificial seeding, such as Silver Iodide, and natural seeding, such as Artificial cirriform ice crystals, are available to increase the number of condensation nuclei and encourage precipitation. condensation Ice Crystal Growth Ice Ice crystal growth Ice is the dominant cause of precipitation in nimobstratus and cumulonimbus clouds, both of which have lower liquid content than warm-layered clouds such as stratus. stratus.
Figure 8.12 Evaporating Rain Evaporating Figure 8.13 Rain falling Rain into low humidity air below will cause the drops to decrease in size, possibly evaporating into streaks of dry air as in this virga. this Viewing Precipitation Viewing
The darkness of The the cloud can sometimes be used to discern between snow or rain. snow Due to scattering Due snow is darker when viewed from a distance, but lighter than rain when viewed from below. below. In this image the In melting zone is identified. identified.
Figure 8.14 Fall streaks & Sublimation Fall
Ice crystals in Ice cirrus clouds that fall and sublimate into drier air, fall streaks are produced that indicate wind speed and direction. direction. This is similar This to rain evaporating in a virga. virga. Figure 8.15 Snowflakes & Snowfall Snowflakes Figure 8.17 Figure 8.18 Snowflakes are crystalline structures that can have plate, Snowflakes column, dendrite, or needle forms. column, Air temperature and humidity determine crystal form, and Air dendrite is the most common habit. dendrite Sleet & Freezing Rain Sleet
Environmental Environmental temperatures may reveal a warm zone between two freezing layers. freezing Snow falling into Snow the warm zone will melt and either a) fall as rain and refreeze on contact with the ground, or b) refreeze and fall as sleet. as Figure 8.19 Sleet & Freezing Rain - II Sleet Figure 8.22A Figure 8.22B Figure 8.22C Figure 8.22D Four vertical temperature profiles are shown to illustrate the phase change Four that a snowflake may experience in its path toward earth's surface. that Rime & Freezing Rain Rime Rime, a granular ice, Rime, accumulates when super cooled fog droplets touch a frozen surface. surface.
Figure 8.20 Freezing rain creates incredible Freezing strain on branches and other structures, resulting in costly damages. damages.
Figure 8.21 Snow Grain & Pellet Snow Figure 8.23 Flat and long snow grains fall as frozen drizzle, too small to bounce Flat or shatter. or Snow pellets are larger, and have a rounded layer of rimed ice that Snow creates air bubbles and a bounce. creates Hailstones & Damage Hailstones Figure 8.25 Figure 8.24 Updrafts in a towering cumulus cloud recirculate graupel Updrafts through an accretion and freezing process that produces large heavy hailstones. heavy Such storms cause regular property damage, but only 2 U.S. Such deaths in the 20th Century. deaths Coffeyville Hailstone Coffeyville Figure 8.26A Figure 8.26B Regular and polarized light images of the 14 cm diameter Regular hailstone that fell in Kansas in 1970. hailstone Hailstorms cause severe damage to crops and other structures. Hailstorms Measuring Rain w/ Standard Gauge Measuring Standard rain gauge uses a funnel Standard to collect rain and then stores it in a narrower tube, so that the gauge detection is amplified 10-fold. detection The 50 cm long tube, when filled, The represents only 5 cm of total rainfall. rainfall. Figure 8.27 Measuring Rain w/ Recording Gauge Measuring Figure 8.28 Tipping bucket and weighing rain gauges record precipitation rate Tipping at shorter time intervals, providing rain intensity data. at Snow intensity can be measured with depth recorders, or Snow accumulated totals with measuring sticks. accumulated Radar Rainfall Radar Figure 8.29A Figure 8.29B Doppler radar uses microwave transmission and reception to Doppler measure backscatter intensity, or reflectivity for large geographic areas. areas. This signal is converted into maps of precipitation intensity, while This phase shift data provide information on storm movement. phase ...
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- Spring '09