1. Hurricanes and Nor'easters (Chapter 14)

Tropical cyclone: Large, powerful, warm core low pressure systems over tropical or subtropical water.

Tropical cyclones are given different names in different parts of the world:

• Atlantic, Caribbean and western Pacific: Hurricane.

• Eastern Pacific: Typhoon.

• Indian Ocean: Cyclone.

Formation of hurricanes:

1. Hurricanes form over warm tropical waters between the equatorial convergence zone and the subtropical high, e.g. In the Hadley cell.

2. Few frontal storms affect this zone, and easterly winds are generally light.

3. Without strong winds, warm are can rise to form towering convection columns.

4. If there are no upper level winds, the storm can continue to build and winds start blowing into the low pressure system. Because of the Coriolis effect, the low starts rotating in a counter-clockwise direction.

5. If the storm remains over warm water, warm air convection becomes stronger, wind speeds increase and the rate of rotation increases.

6. When cool air starts sinking in the center of the storm to replace some of the rising warm air, an eye starts to develop. This tropical storm becomes a hurricane when wind speeds reach 74 mph.

Wind flow in hurricanes: Animation 1 Animation 2

Easterly winds in the Hadley cells steer hurricanes from east to west. Because of this, in North America, the Caribbean, the Gulf coast, Florida and east coast states are most at risk for hurricanes.

Hurricane strength is categorized using the Saffer-Simpson scale. This scale is based on wind speed and pressures in the eye of the storm.

The Saffer-Simpson scale ranges from category 1 (119-153 kph (74-95 mph) winds, and maximum 980 mbar (28.94 in) pressures. In category 5 storms, the highest in the Saffer-Simpson scale, winds exceed 249 kph (155 mph) and pressures are less than 920 mbar (27.17 in).

Weaker storms tend to move slower, and cover a wider area. These storms present a high risk of heavy, long term rains and inland flooding.

Stronger storms tend to move rapidly, and cover a smaller area. Storm surge, coastal flooding, waves and wind are the greatest hazards.

II. Nor'easters / Extratropical cyclones

1. Cold core storms that form along fronts where there is a large difference in temperatures (very cold air meets warm air).

2. Most common from October to April (fall, winter and spring storms).

3. These storms can form in

• East slope of the Rock Mountains (Colorado)

• Gulf of Mexico

• Bahamas and east Florida

• Atlantic coast

4. Lack a distinct eye and are not circular. Often coma shaped

The strength of nor'easters is measured using the Dolan-Davis scale. This shale is based on wave heights (deep water waves) and the duration of the storm.

III. Hazards of Hurricanes and Nor'easters

Hurricanes and nor'easters present similar hazards.

	Hurricanes	Nor'easters
Storm surge	X	X
Waves	X	X
Heavy rain	X	X
Heavy snow		X
Wind	X	X
Coastal flooding	X	X
Inland flooding	X

Storm surge: Elevated bulge of seawater that inundates and floods coastal areas.

• Low pressure centers in hurricanes (the eye) and nor'easters pull (suction) water upward forming a bulge.

• High winds push and pile up seawater.

• Winds are strongest in the northeastern quadrant of these storms, so land impacted by this part of a storm experiences the highest storm surge and the greatest coastal flooding.

Storm surge will float and carry objects inland (i.e. Beach houses, boats, etc.), and will cause coastal flooding.

Waves: High winds push large waves. Waves erode shoreline and can dramatical change beaches through erosion, and can damage ocean front structures.

Wind: In hurricanes, wind often causes the majority of damage. Wind can collapse walls, lift roofs off of buildings, topple trees, damage power lines, and make projectiles out of loose debris.

The greatest damage caused by wind, storm surge, and waves in hurricanes and nor'easters generally occurs in the northeastern sector of the storm. In both cases, it is in the northeastern sector that wind blows over the largest expanse of open water (greatest fetch).

Other Damaging Effects of Hurricanes and Nor'easters.

Rainfall, flooding and landslides: Tropical storms and minimal hurricanes tend to move slowly and to cover wide areas. This results in heavy, long duration rains covering wide areas. This in turn can lead to extensive inland flooding. In mountainous areas, such storms can result in devastating landslides.

Heavy snow and ice: Nor'easters, because they pull in very cold air from the north, can produce very heavy snowfalls. Especially in the southern United States, this can down trees, knock out power over wide regions, close highways, and damage buildings.