Geography 1111 Lecture 15 Notes
Geography 1111 Lecture 15 Notes GEOG 1111
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This 4 page Class Notes was uploaded by Bridget Notetaker on Monday September 19, 2016. The Class Notes belongs to GEOG 1111 at University of Georgia taught by Hopkins in Fall 2016. Since its upload, it has received 26 views. For similar materials see Intro to Physical Geography in Geography at University of Georgia.
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Date Created: 09/19/16
Geography 1111 Lecture 15 Notes Thunderstorms: A thunderstorm (T-storm) is a cumulus cloud which has developed sufficiently to produce precipitation and thus be classified as a cumulonimbus (Cn) cloud o It also is producing the characteristic events of lightning and thunder o Severe T-storms are accompanied by strong winds and wind gusts, heavy rain, and sometimes hail and tornadoes o They are an indication of great instability in the atmosphere and show a great deal of vertical development Requirements for Formation: o Warm, moist air: releases Latent Energy when lifted and condensation occurs This provides buoyancy and maintains lift to develop updrafts o High surface temperatures: enhances instability, air parcel warming and uplift o These two conditions help establish and/or strengthen atmospheric instability which will strengthen the T-storm Stages of Formation: o Cumulus stage: the initial build-up of cumulus clouds fueled by updrafts (up to 160 kph) of warm, moist air cooling adiabatically Droplet formation is by the Bergeron and Collision- Coalescence processes Updrafts dominate during this stage o Mature stage: raindrops start to fall initiating downdrafts The process of entrainment (the influx of cool, dry surrounding air) helps to fuel the downdrafts Heavy rains, lightning and thunder are most intense during this stage It is also during this stage that hail or a tornado may occur Mixture of updrafts and downdrafts during this stage o Dissipating stage: occurs as the rate and amount of rain lessens with the loss of warm, moist air and latent heat energy The storm breaks up and the cloud mass evaporates On average they are of relatively short duration, 1 - 3 hours, but may last for up to 12 hours or more Downdrafts dominate during this stage Locations of Occurrence: o Thunderstorms occur in many parts of the world o They are a daily occurrence along the ITCZ o In the U.S. they form primarily east of the Rocky Mts. with Florida being the state with the greatest number of days per year with thunderstorm occurrence o The Great Plains is the region with the greatest number per year Lightning: flashes of light generated by the flow of tens of millions of volts of electrons (electrical charge) between oppositely charged parts of a cloud or between the cloud and the ground o What causes lightning? Charges are separated within the cloud as the thunderstorm develops with positive charges primarily near the top and negative charges at the base Lightning is essentially the clouds way of trying to equalize this charge difference or imbalance After a charge difference builds to millions or hundreds of millions, a lightning strike occurs to discharge the negative base of the cloud by moving positive charges to the base and negative charges to the top This is accomplished either by: within cloud lightning, Cloud-to-cloud lightning, or Cloud-to-ground lightning o The Lightning Strike: basic Cloud-to-ground lightning The first step is the establishment of an invisible step leader of charges formed from the cloud base toward the ground o This may contain several linked pathways, each reaching toward the ground Once one of these pathways reaches the ground, the connection between the areas of unlike charges is completed o The pathway which reaches the ground first is the main trunk of the lightning strike and is the most brightly illuminated The movement of opposite charges back cloud ward is the return stroke and what causes the illumination of the pathways, the flash of lightning This also illuminates all connected pathways Thunder: the sound emitted by rapidly expanding gases along a channel of lightning discharge o The lightning flash causes the surrounding air temperature to be heated to between 15,000º C to 33,000º C in a matter of milliseconds o The air being so quickly heated, explosively expands and you hear the pressure or shock waves (sound) waves as thunder o Thunder travels at a speed of about 1082 feet/sec (the speed of sound), so it takes about 5 seconds for it to travel 1 mile (5280 ft.) Tornadoes: violently rotating column of air or vortex which can be seen when filled by a funnel-shaped or tubular mass of cloud and debris o It extends downward from a cumulonimbus cloud in response to extremely low air pressure o Characteristics: Air (barometric) pressure at the center of the vortex can be upwards of 90 - 100 mb (estimated) below the surrounding air, thus yielding a very strong pressure gradient This strong pressure gradient yields winds upwards of 300 mph (estimated) The winds are measured on a modified version of the original Fujita Scale developed by Dr. Ted Fujita The Enhanced Fujita Scale ranges from an EF0 (winds 65-85 mph) up to an EF5 (winds > 200 mph) A tornado can travel horizontally at speeds up to 30-35 mph or more, with average distances traveled of 16 mi, but it go much further o Average diameters are 490 - 1700 feet, or about ½ mile wide They form in association with thunderstorms and are most likely to occur in the area of the thunderstorm in association with the updrafts Requirements for Formation: o As with thunderstorms, tornadoes require warm, moist air with a high degree of instability o They are more likely to be associated with a cold front and where the two air masses have a strong temperature gradient across the frontal boundary o Strong geostrophic winds, known as upper level support, can be can be essential to promoting greater surface uplift Tornado Formation: o Tornadoes will form in basically two different ways: Weaker tornadoes (EF0, EF1 & some EF2s) form as winds draw into the thunderstorm converge and form a vortex that is already in a vertical orientation, and is pulled upward into the cloud base Stronger tornadoes (EF2 – EF5) start out in a horizontal position As wind is drawn into the thunderstorm from tens of miles away, the air closest to the ground moves slower than the air above it, causing the air to tumble like a barrel rolling on the ground This mesocyclone is then drawn upward by the updrafts and becomes a vertically oriented column of spinning air of tornado Locations of Occurrence: o Tornadoes have occurred in many parts of the world, but they are most numerous in North America and especially the U.S. o The U.S. averages about 750-800 per year with the highest concentration in an area stretching from central Texas to Nebraska (Tornado Alley) o Even though tornadoes have occurred in every month of the year, they are quite seasonal with the SE U.S. tornado season in March/April (Spring) Thunderstorm and Tornado Watch vs. Warning: o Watch: means conditions are favorable for the occurrence of a severe thunderstorm or tornado in your area, within the time period stated in the watch announcement, but is not occurring as yet o Warning: means that this type of weather has been spotted by a trained observer, OR has been indicated by Doppler radar in your area
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