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Ch 11. summary pt 2.docx

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Earth Sciences
ERTH 2415
Claire Samson

Chapter 11 SummaryPart 2 How a Thunderstorm WorksAir temperature drops upward from the ground surface through the troposphere at average rate of 6 Ckm This is a stable rate of decrease in the troposphereWhen lapse rate the actual rate of cooling with height is greater than 10Ckm the atmosphere is unstableAtmosphere instability increases as the temperature differences increase between warm air on the bottom and overlying cool airWarm low latitude air is less dense and it wants to rise upward Air will rise as long as it is less dense than the surrounding airIf bottom air is warm and moist the warm air may rise enough to pass through the lifting condensation level allowing condensation of water vapour to begin This marks the cloud base for a thunderstorm Individual thunderstorms form on sunny days in late afternoonearly evening when temperature of ground surface and lower troposphere are the highest Microbursts An Airplanes EnemyMicrobursts Sudden violent downbursts of winds confined to a small area of tens of metres to4km in diameterCommonly mistaken for a weak Tornado because winds often exceed 200kmhDangerous to airplanes because they are so close to the ground that an unexpected downdraft can push the plane to the groundDoppler radar is commonly used to detect microbursts Thunderstorms in CanadaDistribution of thunderstorms in Canada shows a nonuniform patternSouthern Ontario has the fewest because it is humid Has the warmest summers and it is away from the effect of the great lakesA severe thunderstorm is issued when 1 Wind gusts of 90kmh or more 2 Hail of 2cm in diameter or larger 3 Rainfall of 50mm or more within one hour or 75mm or more within 3 hours HailLayered iceballs dropped from severe thunderstorms that fall at speeds exceeding 150kmhRequirements for hail are 1 Large thunderstorms with buoyant hot air rising from heated ground 2 Upperlevel cold air creating maximum temperature contrasts 3 Strong updrafts needed to keep hailstones suspended aloft while adding coatings of ice onto evergrowing coresDestructive large hail abounds in the colder prairies especially central AlbertaLayering on hailstones indicated that it has travelled through parts of the thunderstorm cloud with greater and lesser amounts of supercooled liquid contentJuly is the month of peak hailstorm activity range from May October
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