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EOS 170 > Lecture 18 - Hurricanes and Tornadoes > Flashcards

Lecture 18 - Hurricanes and Tornadoes Flashcards

1
Q

What are tropical cyclones?

A

They are storms with one-minute sustained winds of more than 120 km/h.

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2
Q

Where do tropical cyclones form?

A

in the tropics, normally at latitudes of 5–20° N or S

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3
Q

How are tropical cyclones subdivided?

A

According to geography: hurricanes in the north Atlantic and eastern Pacific Oceans, typhoons in the western Pacific, and cyclones in the Indian Ocean.

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4
Q

What are tropical cyclones driven by?

A

The summer heating of tropical oceans, and require warm shallow seawater (>27℃ down to at least 50 m) to promote evaporation, unstable air in order to start cyclonic rotation, and a weak vertical wind shear to provide stable conditions for the storm to develop.

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5
Q

What is the first stage of tropical cyclone formation?

A

In a tropical disturbance, low pressure draws together thunderstorms, with moderate winds of <36 km/h.

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6
Q

What is the second stage of tropical cyclone formation?

A

The thunderstorms coalesce into a tropical depression with strengthening winds of 37–63 km/h.

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7
Q

What is the third stage of tropical cyclone formation?

A

Winds start moving in a cyclonic pattern (an inward spiral), forming a tropical storm with strong winds of 64–119 km/h.

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8
Q

What is the fourth stage of tropical cyclone formation?

A

Once winds reach 120 km/h, the storm is defined as a tropical cyclone. A circular region of low pressure and calm, clear weather — the eye — forms at its centre, surrounded by a cylindrical eyewall in which strong winds spiral upwards, cooling, condensing water vapour, and releasing latent heat of condensation.

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9
Q

When do hurricane winds decline?

A

When the warm water energy source is cut off as they make landfall or reach colder waters.

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10
Q

What happens when tropical cyclones enter mid-latitudes (30-40° N)?

A

They sometimes interact with other weather systems and linger on as extratropical cyclones, impacting areas much farther north including Canada’s east and west coasts.

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11
Q

What does the Coriolis effect govern?

A

the sense of cyclonic motion

the deflection of tropical cyclone paths

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12
Q

What is the Coriolis effect?

A

As the Earth spins on its axis, a point at the Equator moves faster than a point at the North or South Pole as it has longer to travel in each revolution. This difference deflects moving objects to the right in the northern hemisphere and to the left in the southern hemisphere.

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13
Q

What impact does the Coriolis effect have on winds around low pressure centres?

A

The winds circulate in an anti-clockwise pattern in the northern hemisphere, but a clockwise pattern in the southern hemisphere.

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14
Q

Where does the path of a North Atlantic hurricane generally start?

A

Between 10–20° N with westward motion from the trade winds, then bends to the right (i.e. north) due to the Coriolis effect, and then finally curves eastward due to mid-latitude westerly winds.

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15
Q

What is the precise shape and length of the path taken by a North Atlantic hurricane dictated by?

A

The size and shape of the Atlantic high pressure zone known as the Suptropical Ridge.

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16
Q

On what scale is tropical cyclone strength measured?

A

the Saffir-Simpson Hurricane Wind Scale

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17
Q

What are the categories of the Saffir-Simpson Hurricane Wind Scale determined by?

A

wind speed
barometric pressure
storm damage

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18
Q

What is the speed of a Category 1 hurricane?

A

120 km/h

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19
Q

What is the speed of a Category 5 hurricane?

A

> 250 km/h

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20
Q

What is barometric pressure?

A

the intensity of the low pressure eye

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21
Q

What has accounted for the majority of fatalities in the deadliest tropical cyclones?

A

coastal flooding from storm surges

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22
Q

Why do tropical cyclones not form in the south Atlantic?

A

due to high winds

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23
Q

Why do tropical cyclones not form in the south east Pacific?

A

due to insufficient water temperature

24
Q

Why do tropical cyclones not form in the Red Sea and Persian Gulf?

A

due to insufficient surface area

25
Q

Why do tropical cyclones not form at the equator?

A

due to insufficient Coriolis effect

26
Q

How do hurricanes start?

A

summer heating of tropical oceans with; warm shallow water to enhance evaporation, unstable air for thunderclouds to form, and weak vertical wind shear to stop the storm from breaking up.

27
Q

What is the structure of a hurricane?

A

Air moves towards the low pressure eye over the warm sea, evaporating water.
Humid air spirals up the eyeball. Rising water vapour cools and condenses, forming clouds. This releases latent heat of condensation, intensifying the updraft.
Cold air is sucked down the eye.

[see diagram and learn]

28
Q

How does storm motion speed affect ground wind speed?

A

Storm motion speed along its track is added or subtracted from the storm wind speed to give the actual wind speed experienced on the ground.
Example – a hurricane with the internal wind speed of 160 km/h is moving towards the NW at 30 km/h.
• Ground wind speed to the NW = 190 km/h.
• Ground wind speed to the SE = 130 km/h.

29
Q

How is a storm’s energy, expressed in terajoules (1 TJ = 1 000 000 000 000 J), calculated?

A

based on its wind speed and the size of its wind field

30
Q

What would be the impact of a category 1 hurricane?

A

Category 1: Minimal damage.

Winds damage trees and unanchored mobile homes

31
Q

What would be the impact of a category 2 hurricane?

A

Category 2: Moderate damage.
Some trees blown down
Major damage to mobile homes and some roofs

32
Q

What would be the impact of a category 3 hurricane?

A

Category 3: Extensive damage.
Large trees blown down, foliage striped
Mobile homes destroyed
Structural damage to small buildings

33
Q

What would be the impact of a category 4 hurricane?

A 
Category 4: Extreme damage.
All signs blown down
Heavy damage to windows, doors, roofs
Coastal buildings suffer major damage
Flooding kilometres inland
34
Q

What would be the impact of a category 5 hurricane?

A

Category 5: Catastrophic damage.
Severe damage to windows, doors, roofs
Small buildings overturned and blown away
Major damage to all buildings less than 500 m from the shoreline and 5 m above sea level.

35
Q

When is storm season in the North Atlantic?

A

from 1 Jun to 30 Nov (peak mid-Aug to late Oct)

36
Q

What is the typical diameter of a North Atlantic hurricane?

A

200–1300 km

37
Q

What is the typical diameter of the eye of a North Atlantic hurricane?

A

16–70 km

38
Q

What is the typical lifespan of a North Atlantic hurricane?

A

1–30 days

39
Q

What are some methods of hurricane mitigation?

A

Land Use Planning:
• Avoid population centres in low-lying coastal lands.
Building Codes:
• Secure roofs and eliminate or strengthen any eaves
• Strap the roof to walls inside attic
• No thin asphalt roofs stapled to plywood
• Stronger building codes for mobile homes

40
Q

Why are tornadoes the most intense of all atmospheric phenomena?

A

They have winds of up to 500 km/h.

They can severely damage or even demolish buildings, and can pick people up and throw them about.

41
Q

What was the deadliest tornado on record?

A

The Daulatpur–Saturia tornado, Bangladesh which killed ∼1,300 in 1989.

42
Q

Where are tornadoes most prevalent?

A

In the American Midwest (tornado alley, which stretches from Texas to the plains of south-central Canada).

43
Q

Why are tornadoes most prevalent in the American Midwest?

A

At least three different air masses meet here:
high altitude, fast jet stream winds from the west
mid altitude winds of cool dry air from the Rocky Mountains
low altitude flow of warm, moist air from the Gulf of Mexico
(Warm, dry air from the southwest sometimes provides a fourth distinct air mass.)
The converging air masses cause atmospheric instabilities with rapid rotations.

44
Q

Why can supercell thunderstorms form tornadoes?

A

Wind shear shifts the updraft, tilting the thundercloud into an anvil shape
This separates the warm updraft from the cool downdraft.
This allows for more intense thunder- storms and a higher chance of a tornado forming.

45
Q

How is the strength of a tornado described?

A

by the six-point Fujita scale (F0–F5)

46
Q

What is the Fujita scale based on?

A

wind speed
path length and width
the level of storm damage

47
Q

Why can Single-cell thunderstorms not form tornadoes?

A

Warm, moist air rises vertically in a single updraft.
Moisture condenses from cooling air.
Falling rain/hail causes a cool downdraft, surpassing the updraft.
This limits thunder-storm intensity and reduces the chance of a tornado.

48
Q

On average, how many tornadoes are there in the US each year?

A

~1200 per year

49
Q

On average, how many tornadoes are there in Canada each year?

A

~100 per year

50
Q

What would be the impact of a F0 tornado?

A

Light damage:
Winds of 64–116 km/h.
More than half of all tornadoes.
Path length 0–1.5 km.
Path width up to 15 m.
Damage to roof shingles, signs, billboards, windows and chimneys.
Break twigs and tree branches, pushes over shallow-rooted bushes.

51
Q

What would be the impact of a F1 tornado?

A

Moderate damage:
Winds 117–180 km/h.
Around one quarter of all tornadoes.
Path length 1.5–5 km.
Path width 16–50 m.
Damage to section of roofs; demolishes sheds, garages and weak outbuildings.
Overturns mobile homes; pushes cars of roads; damages crops and uproots trees.

52
Q

What would be the impact of a F2 tornado?

A 
Considerable damage:
Winds 181–253 km/h.
Around one in six of all tornadoes.
Path length 5–16 km.
Path width 50–160 m.
Roofs and chimneys torn off homes; frame homes lifted and moved; mobile homes destroyed.
Rail cars tripped over and vehicles lifted off ground.
Large trees are snapped or uprooted.
53
Q

What would be the impact of a F3 tornado?

A 
Severe damage:
Winds 254–332 km/h.
<5% of all tornadoes.
Path length 16–50 km.
Path width 160–500 m.
Exterior walls collapse and entire roofs blown off well made houses.
Levels or uproots trees in forests, woodlots flattened.
Heavy vehicles lifted.
Metal buildings collapse.
Ground furrowed to ~1 m.
54
Q

What would be the impact of a F4 tornado?

A

Devastating damage:
Winds 333–418 km/h.
<1% of all tornadoes.
Path length 50–160 km.
Path width 500–1500 m.
Well-built buildings (e.g. brick buildings) levelled.
Steel/concrete buildings lose roofs and walls.
Cars tossed over 100 m; locomotives overturned.
Bark stripped from trees; trees tossed like javelins.

55
Q

What would be the impact of a F5 tornado?

A

Incredible damage:
Winds >419 km/h.
Extremely rare.
Path length >160 km.
Path width 1500–4000 m.
Lasts for only a few seconds but exhibits terrifying wind strength.
Well-built homes disintegrated and rubble carried great distances.
Heavy vehicles carried through the air over 2 km.

EOS 170 (22 decks)

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