ATMO 336 1st Edition Lecture 18Outline of Last Lecture I. New midterm material starts nowOutline of Current Lecture II. Lightinga. Defining lightingb. Cause fires due to virgaIII. Electric charge and charge separation sets the stage for lightinga. Neutral particle ionizedb. Regions of cloud with different chargesIV. Lighting happensa. Intracloud lightningb. Good conductorsc. Stepped leaders and short circuitd. Lighting rodsV. Unusual types of lightinga. Positive lightning= negative induction at groundb. Upward triggered lightningVI. Thunder and distance to lighting strikea. FormulaVII. Lightning safetyVIII. Lightning prediction and detectiona. Electric fieldb. Lightning direction finderCurrent Lecture- Lightningo Flow of electrical current through air= atmospheric gasses heated to temp as high as 30,000 degrees C (5 times hotter than surface of the sun)= hot gas emits flash of light and expands explosivelyo 53 people killed per year, usually struck just before or after the occurrence of precipitationo responsible for 30% of power outageso cause fires= rain falling from thunderstorm will often evaporate before reaching ground (Virga). Lightning strikes this dry ground and starts fire= dry lightning (without any rain)- electric charge and charge separation sets the stage for lightingThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o neutral particle becomes ionized= when electron removed from it= now two different charged particles (one negative and one positive) like charges repel each other, oppositely charged particles attracted togethero super cooled region of thunder cloud is where electrical charge is separated. Collisions between precipitation particles separate the electrical charge needed for lighting colder than -15 degrees C= particle becomes negatively charged after colliding with snow or ice crystal warmer than -15 degrees C= polarities reversed in the charge separation process. Large volume of positive charge builds at top of thunderstorm, negativecharge accumulates at the middle of the cloud.- Positive charge builds at ground= induction charge (induced by layer of negative charge region overhead and thus flow away from the area, leaving this positive induction at ground)- Lightning happenso Electrical attractive forces between charge centers gets high enough, then lightning occurs.o Most lightning happens in the cloud= intracloud lighting (positive charge at top of cloud, negative charge in middle of cloud clash)o Good electricity conductors= water, metals, and ground = positive induction charge, including peopleo Air= poor conductor of electricity, thus a large amount of energy is needed in order to pass through it (why lighting is so powerful)o Cloud to ground lighting acts to neutralize charge between cloud and the ground Stepped leaders= charge build up and electrons begin surging away from negative charge region of cloud Short circuit= connection between stepped leader and the upward discharge, lighting will then flow through this channelo Lightning rods= make use of upward connecting discharge made of conducting material and stick up above structure they are meant to protect, ground the electrical current- Unusual types of lightingo Occasionally lighting will travel from positive charged region in top of thunderstorm cloud= positive lighting (5% of all cloud- ground lighting strikes) Happens when top of cloud (anvil) moves away from lower part of cloud= negative induction at the ground More common near the end of storms Carries more current than normal lighting (unusually loud and long lasting clap of thunder)o Upward triggered lighting= lighting starts at ground and travels upward Initiated by mountains and tall objects (i.e. skyscrapers)o Lighting has been observed in dust storms and volcanic eruptions- Thunder and distance to lighting strikeo Some energy is in form of visible radiation which is what we seeo Some energy goes into heating and expanding the air, which generates sound waves thatwe hear as thundero Other energy released as longer wavelength radiation (cannot see) called radio waveso Measure time between when you see lighting strike and when you hear thunder to estimate distance Distance to lighting= (speed of sound )x (time to hear thunder) Distance to lighting= (1 mile/5 sec)x (10 sec)= 2 miles- Lightning safetyo Stay away from tall objectso Stay out of watero Crouch, don’t lie downo Automobiles are safe shelter due to the metal body that conducts current around you- Lighting prediction and detectiono Likelihood of cloud to ground lightning estimated measuring the electric field build up in the air (electric field = measure of strength of build up)o Lighting direction finder= cloud to ground lighting that has already occurred can be located using this Uses triangulation by noting the direction from which radio waves arrive and thetime the signal was detected Shows general motion of lighting producing storms and density of lighting strikes Cannot be used to predict when and where new developing storm will first produce
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