Geography 1111 Lecture 12 Notes
Geography 1111 Lecture 12 Notes GEOG 1111
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This 5 page Class Notes was uploaded by Bridget Notetaker on Monday September 12, 2016. The Class Notes belongs to GEOG 1111 at University of Georgia taught by Hopkins in Fall 2016. Since its upload, it has received 17 views. For similar materials see Intro to Physical Geography in Geography at University of Georgia.
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Date Created: 09/12/16
Geography 1111 Lecture 12 Notes Precipitation Process: o Simplified process: Rain: water vapor condenses into liquid water droplets forming clouds These droplets coalesce (join together) and become heavy enough to be pulled down by gravity as precipitation and do not evaporate before the hit the ground as a raindrop H2O V--------------------> liquid water ----------------------------> rain (Clouds) Snow: water vapor condenses into liquid water droplets forming clouds o Also within the clouds water vapor changes to ice crystals by deposition o These ice crystals coalesce (join together) and become heavy enough to be pulled down by gravity as precipitation and do not melt before they hit the ground as a snowflake H 2 V--------------------> ice crystals ----------------------------> snow (Clouds) Cloud droplet average size is about 20 microns in diameter and fall very slowly, about 1000 m/48 hrs o They usually evaporate 1 raindrop = about one million cloud droplets, so cloud droplets must coalesce or join together into a raindrop to avoid evaporation Two basic mechanisms to explain precipitation formation: 1. Bergeron Process or Ice-Crystal Process: the primary process for forming precipitation in the middle and high latitudes and the only process to form snow Need freezing nuclei to initiate change of water vapor to solid water and thus need temperatures below -10ºC, where both liquid drops and ice crystals can exist o Also need supersaturated conditions (RH > 100%), so ice crystals can collect more water vapor than they lose and thus grow The basic process is water vapor changing to ice crystals by deposition, and these ice crystals joining with other ice crystals to make snow crystals and then snowflakes o If they melt while falling, then they become a raindrop 2. Collision-Coalescence Process: the primary process in tropical areas for raindrop formation and in mid-latitudes during the summer The basic process is water vapor changing to liquid water droplets (cloud droplets) by condensation, and these droplets coalescing with other droplets to form raindrops The maximum size a raindrop will achieve is about 5 mm o If it gets bigger, then it gets pulled apart by friction and drag forces as it falls A combination of Bergeron and Collision-Coalescence processes is often seen in thunderstorms to form precipitation o Even in summer, ice crystals will form in the top of the thunderstorms and then melt as they fall forming raindrops Fall or Terminal Velocities of Cloud droplets and Raindrops: (don’t worry about memorizing numbers) Type Diameter (mm) Velocity (kph)/(mph) Typical cloud droplet .02 .04/.03 Drizzle .5 7/4 Raindrop 2-5 23-33/14-20 Precipitation Types: o Remember that all precipitation comes from either nimbostratus or cumulonimbus clouds o Rain droplets average between .5 mm to 5 mm in size and are a liquid form of precipitation o Following the Bergeron Process: So, water vapor to ice crystal to raindrop (gas) (solid) (liquid) o Following the Collision-Coalescence Process: So, water vapor to raindrop (gas) (liquid) Snow, or snowflakes, are a solid form of precipitation and average 1 - 2mm in size o They only form via the Bergeron Process So, water vapor to snowflake (gas) (solid) Know the conversion of snow to rain for the purpose of precipitation total o An average of ***10 inches of snow is equivalent to 1 inch of rain*** is commonly used, but this will be dependent on the amount of moisture in the snow however o Wetter snow will be fewer than 10 inches and drier snow will be more than 10 inches Sleet and Freezing rain are two other solid forms of precipitation commonly occurring in winter o Sleet is essentially a frozen raindrop If it starts as a snowflake (Bergeron Process), then it melts to form a raindrop that then freezes (re- freezes) before it hits the ground As the snowflake falls it passes through air which is at a temperature above 32° F, melts, then passes through colder air again and freezes frozen ---------------> liquid ------------------------> frozen (cloud) (melts) (in atmo) (re-freezes before it hits ground) OR liquid --------------------------------------------------> frozen (cloud) (freezes in atmo before it hits ground) Freezing rain is similar to sleet except it freezes (re-freezes) after making contact with the ground o This usually involves supercooled raindrops which allows them to freeze on contact with solid objects/surfaces Commonly referred to as Ice storms frozen --------------------> liquid ----------------------> liquid/frozen (cloud) (melts in atmo as it falls) (liquid as it hits the ground then re-freezes) OR liquid -----------------------------------------------------> liquid/frozen (cloud) (liquid as it hits the ground then freezes) o Sleet and Freezing rain are typically not counted in precipitation totals Hail is another form of solid precipitation consisting of hard, rounded pellets or lumps of ice o They are only produced in large cumulonimbus clouds, thunderstorms o They form in the network of updrafts and down drafts contained within the thunderstorm which move the hail stone up and down within the cloud causing it to grow o Unlike sleet and freezing rain, hail is primarily a summertime phenomena It also is not typically counted in precipitation totals Global Pattern of Precipitation: o An idealized pattern of precipitation would be quite simple with more in tropical areas where the air is warmer and can hold more moisture versus polar areas where it’s cold and dry But other factors complicate the picture: Air Pressure systems greatly affect precipitation patterns with areas of Low Pressure typically being unstable (ITCZ) and areas of High Pressure consisting of subsidence and being stable (STH) Wind directions affect precipitation patterns by whether it is flowing onshore or offshore, diverging or converging, encounters mountain ranges, etc. Seasonality or shifting of pressure belts (ITCZ & STH) during the year which shifts wind directions Landmass and Ocean locations which can affect wind flow, differential heating, rain shadows, etc. So, throw all this together and we could model an Idealized Continent for precipitation BUT some exceptions to the Idealized pattern give a truer picture of precipitation patterns: o Sub-Tropical High pressure cells (STH’s), which sit over oceans, don’t have the same characteristics on both the east and west side East side of STH: shows the typical subsidence, temperature inversion, and stable and dry conditions They are also affected by upwelling of cold ocean currents Ex: Sahara of northwest Africa, deserts of Baja California and Mexico West side of STH: shows little subsidence, more uplifting, convergence, and warm ocean currents which leads to greater instability and wetter conditions o Ex: Southeastern U.S. Rain shadow deserts: formed by mountain barriers and orographic effects such that the leeward side is often much drier than the windward side o Ex: Great Basin (Nevada deserts) of western U.S., Patagonia in southern Argentina Monsoon: an annual cycle of dryness and wetness, with seasonally shifting winds produced by shifting atmospheric pressure patterns o Ex: Southern Asia (India)
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