3.3. Hazards resulting from atmospheric disturbances Flashcards Preview

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Define tropical storms

a large low pressure system characterised by high winds and heavy rain.

Hurricanes in Americas-Atlantic and Easter Pacific

Typhoons in SE Asia-Western Pacific

Cyclone-South Asia-Bay of Bengal and Indian Ocean


Structure of a typhoon

  • The eye: clear calm centre, typically 0-50km with low winds speed, lowest air pressure (880-970mb) and warm temperatures. In the eye, cold dry air descends from upper atmosphere producing clear skies
  • They eyewall: a ring of towering cumulonimbus (thunderstorm clouds) around the eye. Formed by intense convection and uplift. Has the strongest wind and torrential rain. Temperatures drop but air pressure rises
  • Rainbands of heavy could and rain surround the eyewall. Up to 6 cloud cells can form around the eyewall


Conditions for tropical storms

  • Sea temperatures must be 27 degrees Celsius to a depth of 60m (warm water gives off large quantities of heat when it is condensed - this is the heat that drives the tropical storm)
  • The low pressure area has to be far enough away from the equator so that the Coriolis force creates rotation in the rising air mass. If it is too close to the equator, there will be insufficient rotation => storm cannot develop
  • Air needs to be unstable so it will continue rising
  • High humidity that provides source of latent heat => keeps air unstable
  • Rapid outflow of air in upper atmosphere to allow for the continual rise of air as it draws up water vapour and hence more latent heat energy


Formation of tropical storms

  1. Warm water must be 60m deep becaue storms stir up the ocean
  2. Warm air from the ocean meets cold air. Warm air slowly rises above the cold air, forming a low pressure zone
  3. Trade winds blow in the opposite directions cause the storm to spin
  4. As the rising warm air reaches higher elevations, the liquid in it condenses and forms clouds and rain, causes pressure to decrease at higher altitudes. Condensation also releases heat which causes air to continue to rise
  5. Cool air rushes in to fill the low pressure zone, pushing more warm air up and creating a cycle that can result in high winds and storms


3 stages of a typhoon

  • Embryo cyclones: a collection of thunderstorms with no discernable eye and little rotation
  • Mature cyclones: wind speeds of at least 74mph, heavy rain and storm surges
  • Decay: when a cyclone moves over the land or cooler water and therefore dissipate its energy source


Distribution and timing

  • Found between 8 and 20 degrees North and South of the equator
  • Originate in oceans and track westwards and then north in Northern Hemisphere and South in the Southern Hemisphere
  • Move in an anticlockwise direction in the northern hemisphere and clokwise in the southern hemisphere due to the Coriolis effect
  • Occur between june and december in NH and between december and june in SH


What is the Coriolis force?

The deflection of moving objects caused by easterly (West to East) rotation of the Earth


Explanation of the Coriolis force

  • Near the equator where the Earth is widest, the outer edge of the Earth's atmosphere is moving faster than it does around the poles => atmosphere gains a more west to east spin at the equator
  • Flies upwards to the NH => to the right/east (nhin tu pole xuong equator)
  • Flies downwards to the SH => to the left/west


Why do storms spin?

  • As air flows from high to low pressure, the Coriolis force comes into effect, making the wind follow a curved path
  • In the NH, the wind turns to the right/east direction => rotates anti-clockwise
  • In the SH, the wind turns to the left/west direction => rotates clockwise


Hazards associated with hurricanes

  • Strong winds: 74-156mph
  • Torrential rain: 2 billion tons is picked up per day then dropped. Can cause large scale flooding especially if rivers break their banks
  • Storm surges: biggest - 13m in Australia, 1899.
  • Large waves at sea: can reach 15m in height


How is a tropical storm measured?

Using the Saffir-Simpso scale. Developed in 1969 by a civil engineer Herbert Saffir and Bob Simpson - director of US National Hurricane Centre


Category 1 of Saffir-Simpson scale 

  • 74-95mpgh or 119-153km/h
  • Damage primarily to trees and unanchored mobile homes
  • Some coastal flooding


Category 2 of Saffir-Simpson scale

  • Winds 154-177km/h
  • Storm surge generally 1.8-2.4m above normal
  • Some damage to roofing materials, doors and windows
  • Considerable damage to vegetation, mobile homes and piers
  • Coastal and low-lying escape routes flood 2-4 hours before arrival of the tropical storm eye
  • Small craft in unprotected anchorages break moorings


Category 3 of Saffir-Simpson scale

  • Winds 178-209km/h
  • Storm surge generally 2.7-3.6m above normal
  • Some structural damage to small residences and utility buildings
  • Mobile homes are destroyed
  • Flooding near the coast destroys smaller structures with larger structures damaged by floating debris
  • Land below 1.5m above mean sea level may be flooded inland 13km or more
  • Evacuation of low lying residences close to the shoreline may be necessary


Category 4 of Saffir-Simpson scale

  • Winds: 210-249km/h
  • Storm suge generally 3.9-5.5m above normal
  • Some complete roof structure failures on small residences. 
  • Complete destruction of mobile homes
  • Extensive damage to doors and windows
  • Land below 3m above sea level may be flooded, requiring massive evacuation of residential areas as far inland as 10km


Category 5 of Saffir-Simpson scale 

  • Category 5
    • Winds greater than 249km/h or 155mph
    • Complete roof failure
    • Some building failures
    • Massive evacuation
    • Flooding causes major damage to lower floors of shoreline buildings


Social effects

  • death and injuries
  • contaminated water caused diseases
  • crop loss -> famine
  • social unrest and crime
  • homelessness


Economic effects

  • building and infrastructure damage
  • local economic recession
  • slows national economy
  • increased insurance premiums
  • reduces future investment


Physical/environmental effects

  • triggers tornadoes and temperate storms
  • flooding, mudflows, landslides
  • salinisation of water table and soil = poor soil and water quality
  • damage to trees, vegetation and wildlife
  • damage to coral reefs


Advantages of tropical storms

  • Malthusian theory - a natural check on population
  • Help evolution "survival of the fittest" as the poorest and least educated are most affected
  • Heat energy transfers:
    • Vertical - they help excess heat energy rise through the atmosphere
    • Horizontal - they move large amounts of heat energy away from the equator towards Poles
    • This helps cool the constantly heated tropical region
    • While warming the cooler temperate and polar areas


Define tornado

A violent windstorm associated with severe thunderstorms or tropical storms

Characterised by a narrow, twisitng funnel like column of cloud that reaches the ground from a towering cumulonimbus cloud


Conditions for tornadoes to form

  • Moist air which is unstable
  • Two different air masses – one warm and one cold
  • intense heat causing intense convection
  • Cumulonimbus clouds to continue updraft
  • Coriolis effect
  • Strong vertical wind shear
  • Jets streams to help maintain tornadoes as they remove air at high levels which enables fast inflow at ground levels 


Formation of tornado

  1. Warm moist air from Gulf of Mexico meets cold air from Arctic over land
  2. Cold air undercuts the warm air at a cold front forcing warm air to rise rapidly making the air unstable.
  3. Extremely powerful up-draughts of warm air (convection) form a small low-pressure system called a mesocyclone. The rising air reaches the top of the cloud and pushes above the normal cloud ceiling to form cumulo-nimbus thunder clouds.
  4. These develop into severe thunderstorms known as ‘Supercells'
  5. Converging air is deflected by the coriolis force into a circular path and together with the high vertical wind shears (created from the different wind speeds and directions of the warm and cold air masses) results in air spinning upwards.
  6. The cork screw motion develops further as warm air rises and the cool air descends on the outer edge. The jet stream helps to maintain the updraught by removing air at high levels.
  7. This rapid rotation extends downwards towards the surface, initially as a funnel cloud and if it touches the surface, a tornado forms.


Distribution of tornadoes

  • Concentrated between latitudes 20o and 60oN and S (contrasting masses meet between these latitudes with one of them being warm and Coriolis effect is strong)
  • Equatorial regions don't experience tornadoes as they lack the contrasting air masses for their development and insufficient Coriolis force for a circulation to develop
  • Polar regions lack the warm moist air necessary for their formation
  • Occur most in the USA
  • Large numbers of weak tornadoes occur from Australia to the UK
  • Other countries include other European countries, India, Bangladesh, Uruguay, China and Japan.
  • Tornados could form anywhere with the appropriate atmospheric conditions.


Fujita scale

  • F0: 

    • Light damage (<73 m.p.h.) 

    • Some damage to chimneys; branches broken off trees; shallow-rooted trees pushed over; sign boards damaged.

  • F1: 

    • Moderate damage (73-112 m.p.h.) 

    • Peels surface off roofs; mobile homes pushed off foundations or overturned; moving autos blown off road.

  • F2:  

    • Considerable damage (113-157 m.p.h.)

    •  Roofs torn off frame houses; mobile homes demolished; boxcars overturned; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground.

  • F3: 

    • Severe damage (158-206 m.p.h.) 

    • Roofs and some walls torn off well-constructed houses, trains overturned; most trees in forest uprooted; heavy cars lifted off ground and thrown.

  • F4: 

    • Devastating damage (207-260 m.p.h.)

    • Well-constructed houses levelled; structure with weak foundations blown off some distance; cars thrown and large missiles generated.

  • F5: 

    • Incredible damage (261-318 m.p.h.) 

    • Strong frame houses lifted off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 metres (109 yards); trees debarked; incredible phenomena will occur