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Chapter 13 The Nature of Storms Thunderstorms • Three conditions required: – Source of moisture – Rising air mass – Unstable atmosphere • Air mass t-storms T-storm days per year – Orographic (mountain) – Sea breeze – Convection • Frontal t-storms – Advancing cold fronts (squall line) Sea breeze Three Stages of Thunderstorm Development Lightning and Thunder • Transfer of electricity (similar to static electric shock) • 5 times hotter than the sun’s surface • Thunder is the sonic boom made by expanding air • Forest fires, injuries, and deaths • Lightning causes more annual deaths (on average) than tornadoes, hurricanes or floods Lightning Strikes Supercells • Severe t-storm • Fastest winds on Earth • Can last for several hours • Strong winds • Hail • Downbursts • Tornadoes There are about 100,000 thunderstorms in the United States each year. About 10% of those are severe; fewer still are supercells. Tornado Development • Horizontal rotating air is lifted vertically by t-storm updraft • 80% of all tornadoes are weak (F0 or F1) Tornado Classification Tornado Frequency Map • “Tornado alley”—Texas, Oklahoma, Kansas, Missouri • Most common in spring during late afternoon and evening • Only about 1% of all tornadoes are violent (F4 or F5) • Safety: Shelter in a basement or interior room; NOT in a car or mobile home Hurricanes/Tropical Storms • Begin to form over warm tropical oceans • Air rises and pulls in more warm humid air • Coriolis effect causes cyclonic rotation • Wind speeds are used to classify tropical storms/hurricanes Floods and Droughts • Floods and flash floods can occur after storms or a hurricane moves through an area • Coastlines, floodplains, and narrow river valleys are most susceptible • Drought—extremely dry weather—can also cause damage • Droughts are usually caused by sustained high pressure systems Heat Waves/Cold Waves • Heat waves are often produced by high pressure systems that cause droughts • High humidity adds to the discomfort of high temperatures (see next slide) • Cold waves are also produced by large high pressure systems • Windchill magnifies the effect of cold weather for people Heat Index
