Hazards from Atmospheric Disturbances 9.3 Flashcards
What are Tropical Storms?
Large, low pressure systems with winds at least 40mph for a storm and at least 75mph for a cyclone.
Cyclone = Hurricane = Typhoon
What are tornadoes?
Rapidly rotating, violent, fast moving, narrow, funnel shaped cloud, found commonly in USA Tornado Alley
Most tornadoes have high winds (upwards of 200 mph) lasts for 5 mins and maintains contact with the ground for 4km
Where do Tropical Storms occur?
Location: primarily form along tropics between 5-30 degrees north/south, and none on equator
Density: greater in northern hemisphere
Intensity: greater in northern hemisphere, over Asian continent and East coast of North America. Few storms of intensity 5 are in southern hemisphere
Direction: northern hemisphere = deflect to right with anti-clockwise rotation
southern hemisphere deflect to left (clockwise). Storms travel east to west
Timing: northern hemisphere = June to November, southern hemisphere = November to April
Where do tornadoes occur?
Location: occur in every continent other than Antarctica. Between 20-60°N/S, but most commonly in the United States in ‘Tornado Alley’
Density: UK has the most per land size (33 per year), but the US has the most in total (>1000 per year)
Intensity: most intense in the US. 10-20 violent tornadoes per year, don’t occur outside the US
Direction: unpredictable and can travel in any direction. Most travel from southwest to northeast
Timing: typically between spring and summer due to the collision of cold polar and warm tropical air
What do tropical storms require?
Heat, Moisture, Winds and rotation
Develop in intense low-pressure zones over tropical oceans in the easterly wave
Warm sea temperature over 26.5 degrees C
How do Tropical storms form?
Quite wordy
Latent heat powers convection.
Convectional processes draw air upwards, pressure gradient supplies new air to the warm sea
Moist, warm air cools and condenses, forming large anvil-headed cumulonimbus clouds
Storm begins to rotate (anticlockwise in northern hemisphere) due to the Coriolis force deflecting the rising air and winds (to the right in the northern hemisphere). Low pressure must be far enough away from the equator for Coriolis force to be significant
The eye forms when a small amount of built-up air flows inwards to the centre of the storm instead of outwards. Pressure builds, and weight of air counteracts updraft force, air descends – no clouds
Self-sustaining (creates a positive feedback loop) and the storm generates heat, until it runs on land – dying quickly as there is now no warm, moist air to fuel it
What do tornadoes require?
Moisture, Instability, lift and windshear
How do Tornadoes form?
Cold front meets a warm front, so warm air rises
Insolation heats ground, moist air rises and rapidly condenses in the presence of condensation nuclei
Atmospheric instability allows shallow cumulus cloud rise, forming a towering cumulonimbus cloud
Intense low pressure at surface from updrafts
Cyclonic motion (Mesocyclone) develops
Wind shear (as a result of gradient between wind speeds) causes air to overtake other air, causing rotation. Horizontal rotation flicked vertical by updrafts. Air spins rapidly relative to storm rotation
Spiralling air moving upwards due to Coriolis effect adds rotational momentum to system
Heavy rainfall due to high potential energy and moisture drags down a column of rapidly descending cold air – dragging mesocyclone closer to surface
Mesocyclone feeds off moisture pulled from the ground and warm saturated base of cumulonimbus cloud
Moisture concentration forms a rotating wall cloud – tornado is 10-20 minutes away
Mesocyclone spins increasingly fast, and Rear Flank Downdraft narrows base and pulls it closer to ground
Intensity continues to increase as long as warm moist air can provide latent heat and as long as RFD does not cut off moisture supply at base of tornado
What Saffir-Simpson scale?
Category 1 - Winds of 75-95 mph Category 2 - Winds of 96-110 mph Category 3 - Winds of 111-129 mph Category 4 - Winds of 130-156 mph Category 5 - Winds of 157+ mph
What Hazards are caused by Large-scale atmospheric disturbances?
Storm surges - An abnormal rise in sea water, strong winds in contact with the surface by friction cause water to pile up and rise at the coast, it is worse with strong winds
Intense rainfall - most intense either side of the storm eye where the thickest and tallest clouds are.
High winds - Worse when the storm is further away from the equator as stronger impact from the Coriolis force
Mass movements - mudflows and landslides
Air pressure imbalances - can unsettling the local weather system
