GEOL 110 (Ch.9 continued)
GEOL 110 (Ch.9 continued) GEOL 110
Long Beach State
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This 6 page Class Notes was uploaded by Elizabeth Rubio on Monday April 11, 2016. The Class Notes belongs to GEOL 110 at California State University Long Beach taught by Ewa Burchard in Spring 2016. Since its upload, it has received 12 views. For similar materials see Natural Disasters in Geology at California State University Long Beach.
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Date Created: 04/11/16
21116 Low and High Pressure Centers Air movement can cause changes in pressure – Convergence occurs when air flows in increasing pressure – Divergence occurs when air flows out decreasing pressure • At surface, air moves from surface high pressures (H) to low pressures (L) – Air at low rises into atmosphere and then diverges in the upper atmosphere – A surface low is often associated with a high aloft and vice versa Unstable Air Tendency of air is to remain in place – Atmospheric stability – Air parcels resist movement or return to original spot after they move • In unstable air, parcels are rising until they reach air of similar temperature and density – Air is unstable when lighter, warm/ moist air is overlain by denser cold or dry air – Some air sinks and some air rises Fronts • Boundary between cooler and warmer air masses – Air masses do not mix – Warmer air will always be lifted by the colder, denser air mass – Air masses also have different humidity levels, densities, wind patterns, and stability • Different fronts – Cold front when cold air is moving into warm air – Warm front when warm air is moving into cold air – Stationary front where boundary shows little movement – Occluded front where rapidly moving cooler air overtakes another cold air mass wedging warm air in between. Hazardous Weather and Geographic Regions at Risk Severe weather refers to : – Thunderstorms – Tornadoes – Hurricanes (Chapter 10) – Blizzards – Ice storms – Mountain windstorms – Heat waves – Dust storms • Hazardous due to the energy they release and damage they are capable of causing Thunderstorms Most occur in equatorial regions – Most common in the afternoon or evening hours in spring or summer • Three conditions necessary 1.Warm and humid air available in lower atmosphere 2.Steep vertical temperature gradient such that the rising air is warmer than the air above it • Colder air over warmer, moist air 3.Updraft must force air up to the upper atmosphere • Formation – Moist air is forced upwards, cools and water vapor condenses to form cumulus clouds Stages 1. Cumulus stage • Moisture supply and updrafts continue, clouds grow • A continuous release of latent heat from condensation warms the surrounding air causing the air to rise further • Precipitation one of two mechanisms – Expanding the cloud into colder air causes water droplets to freeze;? larger snowflakes fall and melt as raindrops – Large droplets grow until they cannot be supported by updrafts 2. Mature stage • Downdrafts and falling precipitation leave the base of the cloud • Updrafts and downdrafts are present and continues to grow until it reaches the top of unstable atmosphere (tropopause) • Storm produces heavy rain, lightning and thunder, and occasionally hail 3. Dissipating stage • Upward supply of moist air is blocked by downdrafts • Thunderstorm weakens, precipitation decreases, and the cloud dissipates Cont. Types 1. Airmassthunderstorms a. – Most individual thunderstorms b. – Last less than 1 hour and do little damage 2. Severe Thunderstorms – Under right conditions, thunderstorms can be severe • Classified as severe by National Weather Service if: – Winds > 93 km (58 mi.) per hour, or : Hailstones > 1.9 cm (0.75 in), or: generates a tornado – Necessary conditions • Large changes in vertical wind shear – Greater the wind shear, the more severe the storm • High water vapor content in lower atmosphere • Updraft of air & Dry air mass above a moist air mass Cont. Severe thunderstorm types 1. Mesoscale convective systems (MCS) • Most common type • Large clusters of selfpropagating storms in which downdrafts from one cell leads to formation of another nearby –This continued growth means the storms can last for 12 hours or more 2.Squall lines Long lines of individual storm cells common along cold fronts • Updrafts form anvilshaped clouds extending ahead of the line – Downdrafts surge forward as gust front in advance of precipitation • Can also develop along drylines – Fronts with differing moisture content 3. Supercells • Upward spiraling column of air known as a mesocyclone • Smaller than MCSs and squall lines, but more damaging • Extremely violent and spawn most tornadoes • Last from 2 to 4 hour Cont. • Lightning – Common occurrence during thunderstorms • Flashes of light produced by discharge of millions of joules of electricity • Extreme heat from discharge causes air to rapidly expand – Produces thunder – Most is cloudtocloud – Cloudtoground is less common • But 25 million ground strikes in U.S each year • Complex process (simplified here) Kills about 100 and injures more than 300 each year in the United States Cont. Hail Hard, round, irregular pieces of ice originating from thunderstorms – Contain rings due to adding coatings during updrafts • Hail moves up and down in lower part of the storm adding layers of liquid water which then freezes – Cause mostly property damage • Averages 41 billion per year in the United States – Most common locations • North America: Great Plains in United States, Calgary region of Alberta, Canada • Other regions: Northcentral India, Bangladesh, Kenya, and Australia – Deaths not uncommon in Bangladesh and India because of poorly constructed dwellings Tornadoes Usually spawned by severe thunderstorms – One of nature’s most violent natural processes – 1992 to 2002, killed average of 62 people per year – Variety of shapes • Rope • Funnel • Cylinder • Wedge • Defined by vortex extending downward from the cloud and touching the ground – Called funnel clouds when it does not touch the ground • Form where there are large differences in atmospheric pressure over short distances Cont. 1. Organizational stage – Vertical wind shear causes rotation to develop within the storm – Strong updrafts in advance of the front tilt the horizontally rotating air vertically – Wall cloud rotates and funnel descends 2. Mature stage – Visible condensation funnel extends to ground – Moist air drawn upward – In stronger tornadoes, smaller whirls may develop within tornado • Suction vortices • Responsible for the greatest damage 3. Shrinking stage – Supply of warm air is reduced and tornado begins to thin • More dangerous because wind speeds increase as diameter decreases 4. Rope stage – Downdrafts cause tornado to move erratically and disappear Cont. Classification of tornadoes – Classified according to most intense damage that they produce – Assigned value on Enhanced Fujita (EF) Scale • Survey determines levels of damage experienced by 26 types of buildings, towers, and poles and hardwood and softwood trees Waterspouts • Tornadoes that form over water • Develop beneath fair weather cumulus clouds as a result of wind shear Cont. • Occurrence of tornadoes – Found throughout the world, but much more common in the United States • Has the just the right combination of weather, topography, and geographic location – Most U.S. tornadoes occur in midwestern states between Rocky Mountains and Appalachians • Spring and summer in late afternoon and evening are most common times • Highest risk is in “Tornado Alley” – stretches from north to south through the Great Plains states – Several tornado prone areas in Canada • Include Alberta, southern Ontario, and southeastern Quebec – Waterspouts • Most take place in tropical and subtropical waters Blizzards and Ice Storms • Blizzards – Severe winter storms with large amounts of falling or blowing snow, high winds, low visibilities for extended period of time • Whiteout– Extremely low visibility – Official thresholds differ • In United States: winds > 56 km (35 mi.) per hour, visibilities < 0.4 km (0.25 mi.) for at least 3 hours • In Canada: winds > 40 km (25 mi.) per hour, visibilities < 1 km (1.6 mi.) for at least 4 hours – Ground blizzard • High winds picking up previously fallen snow • Develop numerous times each winter in Antarctica, Alaska, parts of Canada, and Great Plains states Cont. Causes of blizzards – Interaction between upperlevel low pressure trough and surface low pressure – Colorado and coastal storms derived from moist ocean air – Alberta Clippers are drier with less snow and cold temperatures – Nor’easters on East Coast have hurricane force winds, heavy snows, intense precipitation, and high waves • Wind chill – Wind cools skin, evaporates moisture, reduces time it takes for frostbite to form – A reason blizzards are generally more dangerous than other snowstorms Cont. Ice storms – Prolonged periods of freezing rain • Upon contact with cold objects, rain immediately freezes to form a coating of ice – Develop during winter on the north side of a stationary or warm front – Three conditions for freezing rain 1.Ample source of moisture 2.Warm air over shallow layer of cold air 3.Objects on land close to or at freezing Fog A cloud in contact with ground Air cooling to condensation or adding water to cooled air through evaporation • Cooling – At night heat radiates from land – Warm air blows over cold water – Humid air rises up a mountain side • Evaporation – Cold air flows over warm body of water – Warm rain falls through cool air Drought Extended period of low precipitation Makes a shortage of water, food, and power Affects more people than any other natural hazards. Mountain Windstorms Develop seasonally on the downwind side of mountain ranges or glacial ice fields • Mountains block prevailing winds and can, under specific conditions, cause winds to move quickly down slopes – Chinooks, east of Rocky Mountains – Santa Ana, in Southern California • Can cause roof and tree damage, blow cars off highways, contribute to large wildfires Dust storms – Strong windstorms in which dust reduces visibility for significant amount of time – Can be several hundred kilometers in diameter and carry 100 million tons of dust – Safety hazard for travel – Affect climate and human health • Sandstorms – Desert phenomenon where sand transported in a cloud – Rarely extends > 2m (6.5 ft.) above land – Along with dust storms, occur mostly in midlatitude, semiarid, and arid regions
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