HURRICANE

– Strong Center of Low P
– Sustained winds > 74 mph, To be considered a Category 1 Hurricane
– On average there are 45 hurricanes/yr
– Size ranges from 60 miles-900 miles, with a typical size of about 400 miles
– Can last from days to weeks

What do they need to form?
– Consistent heating of the surface
– High Humidities
– Cumulus & Cumulonimbus Clouds

Formation Regions Hurricanes are called by other names (depending on where they occur)
1. Hurricanesin the Atlantic & East Pacific
2. Cyclonesin Indian Ocean near Australia
3. Typhoons off coast of China & Indonesia

Tropical Cyclone Life Cycle
1. Tropical Depression surface wind<39mph
2. Tropical Storm 39≤surface wind≤ 74mph
3. Hurricane surface winds > 74 mph

Tropical Disturbance

The majority of tropical storms & hurricanes start out as tropical disturbances

unorganized masses of thunderstorms with very little organized wind circulation

During the hurricane season in the Atlantic Ocean & Gulf of Mexico, tropical disturbances often grow from a pattern of stormy weather, called an African easterly wave

These waves typically emerge every 3-4 days off the W coast of Africa & then drift W within the trade winds into the Atlantic Ocean

If weather & ocean conditions continue to be favorable, the system may then strengthen

Tropical Depression

As a system continues to become organized & winds begin to circulate, it become tropical depression, weakest form of tropical cyclone

It is called a “depression” because it has low, or depressed, air p at its center

As the system develops, winds converge towards the center P near the center drops

During this transition, the disturbance begins to obtain its energy from the ocean not from horizontal T gradients in atm & env.wind

Tropical Storm

As bands of thunderstorms continue to develop, the depression may intensify into a tropical storm with max sustained wind speeds of 63‐117.5 km/hr

A tropical storm usually forms in this manner (from intensifying tropical depression)

Once a system is classified as a tropical storm, it is given a name

Approx. 100 tropical cyclones / yr

Many of tropical cyclones die out before they can grow stronger, with only approximately half of them (50) eventually strengthening into a mature hurricane (or typhoon)

Hurricanes

If a tropical cyclone obtains a maximum sustained wind speed > 119 km/hr, it classified as a hurricane (Typhoon or Cyclone)

At this point, the recognizable, cloud‐free eye of a hurricane typically forms

In the Atlantic, Central oe E-Pacific, hurricane intensity classified based on max surface wind speed using the 5 categories of the Saffir‐Simpson Hurricane Wind Scale

Ingredients for Hurricane Formation

Warm ocean T > 27°C, deep 200m
– need lots of evaporation
– winds churn up water → WARM water
– Both only happen equatorward of 20°N-S

Coriolis
– needed to initiate the spinning
– Need to be between > 5°N-S

Low values of vertical wind sheer
– Necessary for the storm to organize

Hurricane Structure

As air rushes inward in spiral pattern bands of clouds & thunderstorms are formed

Closer to the “eye” you get:
– The most severe thunderstorms
– The most intense rain & wind
– The eye wall has the highest wind & rain

There can be more than three sets of bands

Hurricane Intensification

The storm can intensify if there is:
Weak vertical sheer: allows condensation energy to be concentrated over a small area

Moist humid air in the upper troposphere: lots of condensation

Cold upper atm conditions: leads to a large PGF

Hurricane Damage

Wind Damages
– Hurricanes move at 45 mi/hr
– converging air moves at a faster velocity
– The velocity of the winds you experience depends on your location within hurricane
ν.motion + ν.rotation (100 + 45) m/h = 145
ν.motion – ν.rotation (100 – 45) m/h = 65

Storm Surges
– abnormal rise of sea along shore
– result of strong winds
– Can be as high as 7 m, Typically 1-2 m
– waves around 6-15 m can occur

Flooding
– Caused by intense rains that accumulate
– Rain accumulation can be up to 10 ft/day
– Typical numbers are 1-3 ft/day

Hurricane Decay

When conditions become unfavorable the hurricane starts to decay

Colder sea surface T: < 80F will cause the storm to weaken or even dissipate

Land friction caused by large land mass, & their terrain cuts off the hurricane’s circulation, & squeezes out storm’s moisture

Shearing winds aloft
– Tropical storms & hurricanes are vertically stacked systems
– Hostile upper level wind conditions produce shearing, which blow off the high cloud tops of these storms, & causes them to become disorganized

Sinking air: subsidence from high P such as subtropical ridge can inhibit development.
– Sinking air from high P hinders thunderstorm development, which is a critical element in hurricane strengthening

In general

Hurricane Watch
– if poses a direct threat to an area
– Issued 24‐48 hr before storm arrives
– Issued by the National Hurricane Center

Hurricane Warning
– if it appears that storm will strike an area
– The warning is accompanied by a percent chance of the hurricane center passing with in 65mi of a particular community
– Typically within 36 hours

Hurricane Impact

Flooding
Storm Surge
Wind Damage
Energy of winds = wind V squared: E = V²

A doubling of the wind speed
results in 4 timesthe destructive energy

Naming Hurricanes

If reach tropical storm strength (40‐74 mph)

Used to be all women’s names
Now they alternate male‐female

Names “Retired” if cause enough damage

CENTRAL PACIFIC Has their OWN List that  used for Hawaii

HURRICANE GROUP ACTIVITIY
GROUP 1 : Super Typhoon Haiyan
GROUP 2: Hurricane Katrina
GROUP 3: Superstorm Sandy
GROUP 4: Hurricane Patricia
GROUP 5: Hurricane Rita
GROUP 6: Hurricane Andrew
Group 7: Galveston Hurricane
Group 8: Hurricane Floyd
Group 9: Hurricane Camille
Group 10: Hurricane Wilma

The End