9.3 Hazards resulting from atmospheric disturbances

• Distribution of Tropical Cyclones:

  -Between the Tropic of Capricorn and the Tropic of Cancer-80% of Global Cyclones form in the ITCZ Zone.-12% Atlantic-30% in the Pacific-24% in the Indian Ocean-15% in the East Pacific-Most Tropical Rainstorms occur 5-20 N/S; warm ocean water within this region.- Since 1995, the frequency of storms has increased, it has been calculated the average storm has increased by 70%, in the past 30 years.
• What is a tropical cyclone?

  A tropical cyclone is a rotating low-pressure weather system that develops over warm equatorial ocean waters, which is categorised as a tropical storm when its wind speed reaches a certain level, and as a Typhoon, Hurricane, or Cyclone when its wind speed increases further. (South Pacific and Indian Ocean)
• What is a Hurricane?


  These are tropical cyclones, that occur in the Atlantic Ocean, and the Northeast Pacific Ocean
• What are the conditions needed for the formation of a Tropical Cyclone?

  Sea Temperature and Depth:

  →The sea surface temperature must be greater than 26.5˚C, extending to a depth of at least60 m(Layer of the Ocean where most heat energy is concentrated),in order to lead to atmospheric instability and provide ample latent heat for storm development.

  →Converging Winds:The Wind's near the Ocean's surface, winds must converge and force air to rise and form storm clouds.

  -Atmospheric Instability:In an unstable atmosphere with high humidity, the saturated (SALR) is around 4 Degrees Celsius per KM.

  (This can cause the air to release latent heat through condensation which can lead to further rising and intensification of the storm)

  -Low Vertical Wind Shear:(The difference in speed, between the ocean and the atmosphere)

  →The Wind Shear should be less than 20 Knots, to allow intensification to occur.

  →This means that winds do not vary greatly with height.

  →Strong upper-level winds will prevent the eye from forming

  
  

  Low wind shear benefits storm intensification by enabling the maintenance of a vertical structure that allows warm moist air to rise and cooler drier air to sink around the storm, promoting intensification by keeping the warm core above the surface circulation centre.

  
  

  →Presence of tropical disturbances where there are large areas of unsettled weather

  - This is an elongated area of low pressure (trough)- Only 10% of tropical disturbances turn into tropical cyclones

  Coriolis Effect:

  -The Coriolis effect is the apparent deflection of air or water due to the Earth's rotation.

  It's strongest between 5-20 degrees latitude where the Earth's rotational velocity and the vertical component of the air's velocity are highest.

  
  

  The Hadley cell produces a low-pressure region called the Intertropical Convergence Zone (ITCZ) near the equator, where hurricanes form.

  -Hurricanes are formed by a combination of warm, moist air rising from the ocean, converging winds, and the Coriolis effect, which can lead to the formation of a tropical disturbance evolving into a hurricane.

  Hurricanes rarely form near the equator due to the weak Coriolis effect and near the poles due to the lack of large temperature differences to drive the Hadley cell circulation.
• hurricane formation

  Formation of a tropical storm:

  Rotation of cyclones- Anti-clockwise in the northern hemisphere- Clockwise in the southern hemisphere

  1. Warm ocean temperature causes the evaporation of water2. As the moisture rises, the air will become more humid, resulting in condensation3. Condensation releases heat, which causes air to expand and rise4. At the same time, the winds converge, forcing the air to rise5. This creates an area of low pressure6. As the air continues to rise quickly, it draws more warm, moist air up from above the ocean, leadingto the formation of stronger winds7. This causes the rapidly rising, warm air to spiral upwards, which cools and condenses to form largecumulonimbus clouds (these clouds form the eye wall of the cyclone, and the centre is the eye)8. The eyewall produces heavy rainfall, whereas the eye has calm and dry conditions (clear skies) due tothe sinking of cold air9. The tropical storm then weakens as it displaces on land, due to the lack of warm ocean water

  The highest wind speeds can be found in the eye-wall, where they can reach up to 320 km/h.

  Rainfall in the interiorcan be as high as 250mm/hr.Hurricanes can have adiameter of up to 550 kmand are fuelled by the release of latent heat during the process of evaporation and condensation.
• How does Atmospheric Instability Grow?

  1. Condensation releases heat, which causes air to expand and rise

  2. This leads to a larger low-pressure zone

  3. As air moves from high pressure to low pressure, it'll create a constant supply of moist air into the low-pressure system
• What might cause a Hurricane to change its path?

  -Wind Shear:Wind shear alters a hurricane's forecasted path by causing a change in wind speed or direction with height. (Only, if the upper-level winds are strong and coming from a different direction than lower-level winds)

  -Steering Currents

  -Land Interaction:When it interacts with a mountain, it can change the direction of a hurricane and disrupt the flow.

  -Interaction with cooler water:It can weaken and change the direction of a Hurricane.

  -Coriolis Effect:
• Hazards from Hurricanes/Cyclones/Typhoons: (Large-Scale)

  Inland Flooding

  Heavy rainfall and inland flooding:

  →Tropical cyclones often produce widespread, torrential rains, which will cause destruction:

  →The seepage of water into buildings can cause structural damage. Steady and persistent rain can cause structures to collapse due to the weight of the absorbed water.

  →Inland flooding can lead to more deadly and destructive floods. Intense rainfall can cause flash flooding, which is a rapid rise in water levels.

  →Slow-moving and larger storms produce more rainfall

  →The tropical cyclone can pickup two billion tonnes of moisture per day and release it as rain

  → Mountainous terrain can enhance rainfall from a tropical cyclone

  →Intense rainfall can also cause coastal flooding, damaging beaches or other coastal lands
• Storm Surge: (Large Scale)

  →Heavy winds produced by hurricanes push the ocean in front of them. As this water gets pushed into the shallow zones along the coastline sea level rises.

  →The storm surge is driven by winds, the height of the rise in sea level is related to the velocity of the wind. For a moving storm, greater winds occur on the right-hand side of the storm (in the northern hemisphere).

  →Sea level also rises beneath the eye of the storm due to the low pressure in the eye. But the surge generated by this low pressure is usually much less than the wind-driven surge.

  → Category 5 tropical cyclones can produce storm surges of more than 6m (20 feet).

  →The storm surge occurs ahead of the eye of the storm, the surge will reach coastal areas long before the hurricane makes landfall. This is an important point to remember because flooding caused by the surge can destroy roads.

  → After a storm, a flood surge on land must drain off, and winds can push water in the opposite direction, causing damage from theebb surge.
• Hurricane Tornados (Large Scale)


  -Tornadoes are a primary hazard of hurricanes because they can form in the outer bands of the hurricane as it approaches land or in the rain bands that spiral into the storm's centre.

  
  

  Friction between a hurricane's outer bands and land can generate intense vortices that rapidly develop into tornadoes. When a hurricane makes landfall, wind shear can cause a rotation of air, leading to the formation of weak tornadoes.
• High Winds (Large Scale)


  Tropical cyclones can cause strong andfast-moving winds, which can reach over 75 mph.
• Mass Movement (Large Scale)


  Mass movements:

  
  

  Flooding can cause soil erosion, which will lead to a decrease in vegetation cover. As flooding continues, the slope will be more prone to mass movements, thus landslides occur
• Tornadoes/Mesocyclone:

  Tornadoes=Small and Short-Lived, but highly destructive storms

  Mesocyclone:A rotating column of air (usually updraft)
• Small-Scale Tropical Disturbances (Tornadoes)


  There is more atmospheric instability during the summer months compared to the earlier months,

  
  

  There is far cooler air in the summer months from the Rockies as the snow melts, the mixture of warm and cold air from the Rockies mixture with the warm air from the Gulf of Mexico.
• What is the global distribution of Tornadoes?

  →The United States has the most tornadoes of any country, many of them form in Central US, known as Tornado Alley. This extends in Canada, particularly in the prairie provinces and Ontario.

  →There are around1000 tornadoes a year,resulting in an average of around80 deaths and 1500 injuries.(1/10 of a century ago due to improved monitoring/predicting)

  →Tornadoes occur on every continent, but they are most common in an area of the central U.S. known as Tornado Alley. Tornado Alley includes parts of Texas, Oklahoma(Great Risk- Population density is high accounting for 1 million people ),Kansas, and Nebraska. This region sees the most tornadoes because of a combination of factors including warm, moist air from the Gulf of Mexico clashing with dry, cold air coming down from the Rocky Mountains and Canada.

  →The U.S. averages over1,200 tornadoes each year,more than any other country.Texas is the most tornado-prone state, with about 10% of all U.S. tornadoes. Oklahoma and Kansas also frequently see tornadoes, with Oklahoma ranking second in the U.S. for the number of tornadoes that occur each year.
• Formation of a Tornado:

  A surface low-pressure system develops to the East of the Rocky Mountains, causing warm, moist air to be dragged up from the Gulf of Mexico, warm/hot dry air from the desert states, and cold dry air down from the Rocky Mountains. These three air masses converge over the Southern States, causing instability in the atmosphere to develop.

  1)Horizontal rotating column:The process begins with the development of a horizontal rotating column of air, often caused by strong vertical wind shear. Fast winds at different altitudes create a shearing effect that causesthe air to rotate horizontally.

  
  

  2)Atmospheric instability:The atmosphere becomes unstable when warm, moist air is present. This instability can be triggered by factors such as temperature differences or the presence of a frontal boundary. The warm air becomes buoyant and tends to rise.

  
  

  3)Strengthening updrafts:As the warm, moist air rises due to atmospheric instability, the updrafts within the storm intensify. Continuous convection heating transfers heat from the Earth's surface to the rising air, further fueling its ascent, forming our culumbinous clouds.

  Flank line:The warm, moist inflow along the flank line contributes additional energy and moisture to the storm, enhancing the updraft and providing favorable conditions for tornado formation.

  4)Folding and flipping: The rising air within the storm begins to fold the horizontal rotating column of air, flipping it from the horizontal (x-axis) to a vertical (y-axis) position. This process occurs due to the interaction between the updraft and the rotating column.

  
  

  5)Mesocyclone formation:The flipped and folded rotating column of air becomes a mesocyclone, which is a rising and rotating column of air with a vertical axis. The mesocyclone provides the necessary conditions for tornado formation.Coriolis effect and rotation: As the warm, moist air rises into the mesocyclone, it begins to rotate due to the Coriolis effect caused by the Earth's rotation. This rotation can become more intense and focused as the mesocyclone tightens, enhancing the potential for tornado formation within the rotating column of air.

  
  

  6)Downdraft formation:As the rising air within the mesocyclone cools, it becomes denser than its surroundings and starts to sink. This sinking motion, along with precipitation such as rain or hail, leads to the formation of a downdraft. The downdraft, affected by wind shear, can become tilted and move away from the updraft. We now have a super-cell thunderstorm.

  
  

  7)Stretching and rapid rotation:The mesocyclone can become stretched and rotate more quickly due to the conservation of angular momentum. As the mesocyclone spins faster, it gains size and rotational speed.

  
  

  8)Wall cloud formation:The stretched mesocyclone begins to descend towards the ground, with falling air pressure causing water vapour to cool, condense, and become visible, forming a cloud known as a wall cloud. A funnel cloud is formed.

  9)Tornado formation:Once the funnel cloud touches the surface, a tornado is formed.

  
• Primary Hazards of Tornadoes : (Hail)

  Warm, moist air rises:A thunderstorm begins with warm, moist air rising from the Earth's surface. This warm air rises due to convection, which is the transfer of heat through fluids like air or water.

  Updrafts carry water droplets:As the warm air rises, it cools and condenses into water droplets. Updrafts, which are currents of rising air, carry these droplets higher into the atmosphere.

  Supercooling:As the water droplets rise higher, the air around them becomes colder. If the temperature drops below freezing(0°C or 32°F), the water droplets turn into supercooled water, which means that they are still in liquid form but are below their freezing point.

  Coalescence:The supercooled water droplets collide and stick together, forming larger water droplets. This process is called coalescence. The larger droplets are too heavy to be carried by the updrafts, so they begin to fall towards the ground.

  Coalescence occurs because water molecules are attracted to each other due to a property called surface tension. This causes the two droplets to merge into one larger droplet.

  Refreezing and growth:As the larger droplets fall, they pass through a layer of air that is below freezing. This causes the droplets to freeze into small hailstones. These hailstones are then carried back up into the storm by strong updrafts, where they collide with more supercooled water droplets and grow larger.

  Repeat:This process of falling and rising can repeat several times, with the hailstones growing larger and larger with each cycle, until they become too heavy to be carried by the updrafts.

  Hail falls to the ground: When the hailstones exceed the updraft speed of the storm(usually around 35mph), they fall to the ground, causing damage to buildings, vehicles, and crops.
• Primary Impact: Heavy Rain (Super-cell Thunderstorm)


  Heavy rain can form in and around tornadoes through a process known as a supercell thunderstorm.

  
  

  Ø Supercell thunderstorms are characterized by strong, rotating updrafts that can produce both tornadoes and heavy rain. As the warm and moist air rises, it cools and condenses into water droplets and moist rises, it cools and condenses into water-droplet and ice, forming the cloud.

  
  

  As the updraft continues to feed the cloud, the water droplets and ice-crystal become larger and heavier, the weight exceeds the updraft and begins to fall downwards, forming rapid downdrafts (downdrafts- linked to strong winds/heavy rain).

  
  

  Downburst is a strongly localised downdraft that can produce non-tornadic winds of up to 168mph
• Primary Impact: Lightning (Super-Cell Thunderstorm)

  →Hail and smaller ice particles collide in a cumulonimbus storm cloud, causing water droplets to be propelled upwards by updrafts where they turn to ice.

  →Hail becomes negatively charged and ice particles become positively charged during collisions, creating an electric field within the cloud.

  →The top of the cloud becomes positively charged, and the bottom becomes negatively charged, creating an electrical potential difference.

  →When the attraction becomes too strong, positive and negative charges come together to balance the difference, resulting in lightning.

  →Supercell storms can produce large lightning strikes due to powerful updrafts and downdrafts creating a separation of electrical charge within the cloud.

  →Large hailstones within supercells can generate a strong electrical field, further increasing the likelihood of lightning strikes.
• Primary Impact: High Winds Speed (Downdraft)

  High winds:

  The rotating column of air between the storm cloud and the earth’s surface forms winds.

  Strong rotation of air combined with a strong uplift can cause debris to be carried up into the cloud
• Primary Impact: Multiple Vortex Tornados

  →Multiple vortex tornadoes are formed when a parent tornado contains smaller whirls or sub-vortices. The sub-vortices are rapidly spinning, smaller whirls that form inside the larger parent tornado.

  →The sub-vortices are created when there are variations in wind speed and direction within the tornado. The parent tornado can generate several sub-vortices simultaneously or sequentially.

  →The suction vortices can add over 100 mph to the ground-relative wind speed in a tornado circulation.They can also cause narrow arcs of extreme destruction right next to weak damage within the tornado paths.

  →Subvortices usuallyoccur in groups of 2 to 5 at once and usually last less than a minute each.
• Primary Impact Pressure Imbalance:

  When tornadoes occur, the rotating winds create a sudden and significant change in atmospheric pressure.This results in an intense low-pressure zone at the center of the tornado, causing a strong pressure gradient that can pull objects into the tornado and make buildings explode outward.

  
  

  As the tornado approaches a building, the external pressure drops rapidly,creating a large pressure differentialwith the relatively high internal pressure of the building, which can cause the walls and roof to explode outward.