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NATURE OF STORMS thunderstorms = strong, rapid upward movement of warm, unstable, moist air 1. formed of cumulonimbus clouds 2. each day approximately 44,000 occur across Earth’s surface 3. always accompanied by thunder and lightning 4. formation: a. must be an abundant source of moisture in lower levels of atmosphere 1) as moisture condenses, it releases latent heat 2) keeps cloud warmer that air around it; critical in maintaining upward motion of cloud b. mechanism must lift air so that moisture can condense and release latent heat c. portion of atmosphere through which cloud grows must be unstable 1) air must continue to cool with increasing altitude for growing cloud to stay warmer than surrounding air 5. limits to growth: a. process will continue until rising air meets a layer of stable air that it cannot overcome b. until rate of condensation, which diminishes with height, is insufficient to generate enough latent heat to keep cloud warmer than surrounding air 1) limits most cumulonimbus clouds to height of around 18000 m c. typical storms only last about 30 minutes; individual storms are only about 24 km in diameter 6. classified according to mechanism that caused air to rise a. air-mass thunderstorms = if air rose because of unequal heating of Earth’s surface within one air mass 1) form within a warm, moist air mass 2) widely scattered and form mostly over land 3) unequal heating of surface reaches maximum during mid-afternoon 4) occur mostly in spring or summer usually mid-afternoon 5) usually last less than an hour; often single storms 6) mountain thunderstorms = due to air mass rising due to orographic lifting moving air up side of mountain 7) sea-breeze thunderstorms = local air-mass storm due to extreme temperature differences between air over land and air over water b. frontal thunderstorms = produced by advancing cold fronts and more rarely, warm fronts 1) with cold fronts, cold air pushes warm air rapidly up steep cold front boundary a) produce line of storms, sometimes hundreds of km long, along edge of front b) squall lines present c) can persist long into night 2) with warm fronts, warm air mass slides up and over cold air mass a) boundary between 2 air masses is not steep, thus air rises gradually b) if warm air behind front is unstable & moisture levels are high, mild storms can develop 3) occur most often in spring and summer 4) often stronger and may last for several hours 7. develop in 3 stages: classified according to direction in which air is moving a. cumulus stage: 1) warm, moist air starts to rise nearly vertically upward 2) creates updrafts which move moisture to upper reaches of cloud 3) moisture condenses into visible cloud droplets & releases latent heat 4) droplets will grown larger & eventually fall back to surface as precipitation b. mature stage: 1) as precipitation falls, it cools air around it 2) air now more dense, so sinks rapidly to ground with precipitation creating downdrafts 3) updrafts and downdrafts from convection cell that produces gusty winds associated with storms 4) equal amounts of updrafts and downdrafts exist side by side in cumulonimbus cloud c. dissipating stage: 1) updrafts need steady supply of warm, moist air at surface 2) once supply run out, updrafts will slow & eventually stop 3) precipitation can no longer form 4) will last until cloud runs out of previously formed raindrops SEVERE WEATHER severe thunderstorms = produce some of most violent weather conditions 1. supercells = multicell storms that are very organized and powerful a. self-sustaining, extremely powerful storms b. intense, rotating updrafts c. can last for several hours and updrafts as strong as 240 km/h 2. thunder is not usually dangerous a. caused by a pressure wave that accompanies lightning which produces sound wave b. since sound waves travel slower it is heard after the lightning has been seen c. takes 3 seconds to travel 1 km d. cold possibly cause hearing damage, crack plaster in buildings e. rumbling happens when thunder echoes from the mountain sides f. greatest distance it can be heard is 16 km lightning = electricity caused by rapid rush of air in cumulonimbus cloud 1. lightning bolt = forms when friction between updrafts and downdrafts within cloud separates electrons from some of their atoms either in cloud or near ground a. travels between two regions of opposite electrical charge b. stepped leader = invisible channel of negatively charged air moves from cloud toward ground to relieve electrical imbalance c. return stroke = channel of positively charged ions that rushes upward from ground to meet stepped leader from cloud d. will always follow the shortest path available between charged surfaces e. return stroke surges from ground to cloud, illuminating channel with about 100 million volts 2. most will occur outside the rain band of a storm a. single bolt can reach temperatures up to 30000oC b. usually strikes the tallest object in an area 3. heat lightning = glow of lightning so far away that its thunder cannot be heard 4. can produce heavy rains, high winds, hail, and tornadoes a. lightning is leading cause of fire in the western U.S. b. on the average, more people are killed by lightning than any other type of severe storm 5. safety tips: a. Go indoors, if possible. If you are traveling stay in your car. b. Stay off bicycles, motorcycles, scooters, golf carts, or farm equipment. c. If you are swimming, get out of water. Get off small boats. d. Avoid standing near or being the highest object in an area. 1) Outdoors, the best protection is in a cave, ditch, or canyon. 2) If you are out in open with only isolated trees nearby, crouch in open as far away from trees as possible. e. Indoors, stay away from open doors and windows and metal objects such as pipes, sinks, stoves, and radiators. f. Do not use the telephone or plugged in appliances. downbursts = violent downdrafts concentrated in a local area 1. winds can be more than 160 km/h 2. based on size of area affected can be classified as: a. macrobursts = path of destruction up to 5 km wide; wind speeds more than 200 km/h; last up to 30 minutes b. microbursts = affects areas of less than 3 km; winds exceed 250 km/h; last average of 10 minutes; difficult to detect and prepare for hail = precipitation in form of balls or lumps of ice 1. causes almost $1 billion in damage; severe damage to crops 2. most frequently occurs in Central United States 3. most common during spring growing season 4. formation: a. water droplets exist in liquid state in parts of cloud where temperature is below freezing b. when supercooled water droplets encounter ice pellets, water droplets freeze on contact and cause ice pellets to grow larger c. strong updrafts and downdrafts will cause ice pellets of move up and down in storm growing in size until too heavy for updraft to keep in sky and will then fall to surface floods = storms may dump rain over limited location 1. wind currents that move weather system is weak and causes storm to move slowly 2. worsen if abundant moisture available throughout atmosphere 3. rain falls faster than ground can absorb it or faster than streams and rivers can transport out of an area 4. main cause of thunderstorm related deaths in U.S each year tornadoes = narrow, funnel-shaped column of spiral winds that extends downward from cloud base and touches ground 1. develops in low, heavy cumulonimbus clouds; usually form from supercells a. air made visible by dust and debri drawn into swirling column or condensed water vapor b. wind shear = when wind speed and direction change suddenly with height c. average diameter is between 150 m and 600 m d. length of its path on ground varies from a few meters to more than 25 km (15 miles) e. usually only last a few minutes but can last up to an hour or longer f. occur most often in spring during late afternoon or early evenings 2. vortex = small low-pressure center of thunderstorm a. develops down from the bottom of thunderhead as air below is drawn in b. rising air cools forming clouds along wall of vortex making funnel shape visible c. air spins in counterclockwise direction in Northern Hemisphere 3. when low pressure area touches the ground it acts like a giant vacuum a. updraft in center is very strong b. strongest winds are between 360 - 500 km/h (224 - 311 miles/ hr) c. destructive power due to the speed of the winds 4. waterspouts = tornadoes over water a. less powerful than those that form over land b. occur with weak thunderstorms and large cumulus clouds 5. classified by Fujita tornado intensity scale a. ranks according to path of destruction, wind speed, and duration b. scale runs from F0 to F5 c. F0 = winds up to 118 km/h d. F5 = winds more than 500 km/h e. most do not exceed F1 category 6. more common in Great Plains region of United States = Tornado Alley (Texas, Kansas, Oklahoma) a. can occur at any time and at any place 7. Safety tips: a. If you are inside stay away from outside walls, windows, and doors. 1) Go to the basement or an interior room on the lowest level. 2) If possible, get under a heavy table or mattress. b. In a public place, go to designated shelter area or sit close to an interior wall on lowest level. c. Get away from mobile homes or vehicles if possible. d. If you are outside in an open area, look for a ravine or ditch. Lie flat and protect your head with your arms. *Most injuries occur from people being trapped in collapsed building or being hit by flying debri. blizzard = snowstorm with high winds and low temperatures 1. brought in by strong mid - latitude lows 2. must have two conditions a. must be enough moisture b. temperature must be cold enough for snow 3. lows form and grow along the polar front 4. usually have stronger winds in winter 5. moisture supply for winter storms differ in different parts of the country Ex: Midwestern United States - Gulf of Mexico East Coast - Atlantic Ocean 6. need temperatures of -6oC (-21oF) or lower 7. winds often range from 50 - 75 km/h (30 - 45 mph) 8. blowing, powdery snow can cause visibility near zero 9. people and animals should avoid exposure TROPICAL STORMS: hurricanes = large, swirling, low pressure system that forms over tropical oceans 1. also called tropical cyclone = low pressure area that contains rising warm air a. air circulates toward center in counterclockwise motion b. in Western Pacific Ocean called typhoons c. greatest number occur in western North Pacific Ocean Hurricane Fran 2. usually form in regions of trade winds between 20oN and 20oS latitudes where water is very warm; originate along ITCZ a. as long as it remains over water, warm moist air will rise providing energy b. as water evaporates from ocean surface, latent heat is stored c. latent heat is released when air begins to rise and water vapor condenses into clouds & rain d. rising air creates area of low pressure at ocean surface e. wind speed increases & air pressure decreases as storm develops f. move according to wind currents that steer them g. last until it can no longer produce enough energy to sustain itself 1) usually happens when moves over land 3. stages: a. tropical wave = disturbance from ITCZ or as weak, low pressure system b. tropical depression = acquires cyclonic circulation round center of low pressure c. tropical storm = when winds speeds around low pressure center of tropical depression d. hurricane = air pressure continues to fall, winds around center reach at least 120 km/h (75 mph) 4. characteristics: a. eye forms at center 1) central area of sinking air 2) usually 15 to 50 km in diameter (9-30 miles) 3) no rain, very calm - almost no wind b. eye wall = area where highest/strongest winds are located 1) completely surrounds eye 2) area of intense thunderstorms 3) when it hits land locations to right side of storm receives worst damage c. storms average diameters of 300- 600 km (180- 360 miles) Hurricane Katrina eyewall 5. damage caused a. high winds, tornadoes, heavy rains, high waves, and flooding b. storm surge = quickly rising ocean water levels that cause widespread flooding 1) piles up water along shore and then blows it inland 2) causes the greatest damage of storm 3) more severe during high tide 4) can be from 4 to 20 feet or more 5) in Northern Hemisphere, occurs primarily on right side of storm; strongest onshore winds occur 6. rated on Saffir-Simpson hurricane scale a. uses barometric pressure in inches, wind speeds in mph, and potential for property damage b. gives an estimate of storm surge in feet c. Category 1: minimum wind speeds of 74 mph d. Category 5: winds in excess of 155 mph e. Category 3: considered to be a major hurricane 7. storm is named when its winds reach 39 mph (tropical storm) a. women’s names were used from 1953 to 1978 b. in 1979 men’s names were added as well as names from other languages c. countries affected by storms suggest names d. names are on a six year cycle e. when one causes great damage its name is retired from the six - year cycle Ex: Alicia, 1983; Hugo, 1989 wave cyclones = large storms that develops along cold or stationary fronts 1. low-pressure centers 2. wind spiral in toward central region 3. usually lasts several days 4. in North America move in easterly direction about 32 to 64 km/hr 5. spin counterclockwise anticyclone = storm that spirals outward from high pressure center 1. spins clockwise in Northern Hemisphere 2. bring dry weather 3. if stalls over area can cause droughts and air pollution problems RECURRING WEATHER: drought = extended periods of well-below-normal rainfall 1. result of shifts in global wind patterns that allow large high pressure systems to last for weeks or months over continental areas 2. under dome of high pressure, air sinks and will resist any attempt to lift moisture through it 3. will last until global patterns shift enough to move high pressure system out of way heat wave = extended periods of above-normal temperatures 1. formed by same high pressure systems that cause droughts 2. air warms by compression and causes above normal temperatures 3. high pressure system also blocks cooler air masses from moving into an area 4. can lead to serious health problems 5. heat index = assesses effect of body’s increasing difficulty in regulating its internal temperature as relative humidity rises a. routinely reported in oF b. issued by National Weather Service cold wave = extended period of below-normal temperatures 1. formed by large, high pressure systems 2. caused bys systems of continental polar or arctic origin 3. movement influenced by jet streams 4. wind-chill factor = estimates heat loss from human skin caused by combination of cold air and wind 5. wind chill index = estimates how cold air actually feels to human body National Weather Service = issue advisories when severe weather is observed or conditions exist 1. watch = when expect severe weather over area for next few hours a. issued for severe thunderstorms, tornadoes, floods, blizzards, and hurricanes b. covers area of 100 km by 200 km or larger (60-120 miles) c. during: 1) stay updated on weather reports 2) be ready to take shelter 3) plan where to go when warning is issued 2. warning = severe weather conditions exist a. tornado spotted, severe thunderstorm reported, hurricane approaching an area b. will give location of storm when detected, area into which it is likely to move, and period of time during which storm could hit c. during: 1) take immediate action - evacuate if necessary 2) always have flashlight, batteries, and blankets ready if an emergency should happen 3. type of severe weather determines where advisories are issued from a. National Severe Storms Forecast Center 1) located in Kansas City, Missouri 2) severe thunderstorms and tornadoes b. National Hurricane Center 1) located in Miami, Florida 2) issue hurricane warnings when conditions are expected within 24 hours c. local weather stations can issue severe thunderstorm or tornado warnings whenever necessary