Unit 9 Atmospheric Circulation

and Hurricanes

What causes the oceans to move?

• Wind

• Gravity (tides)

• Earth’s rotation

• Salinity

• Temperature

9.1 Air Circulation

• We can’t understand the oceans without understanding the atmosphere.

• Both the oceans and atmosphere are fluids that constantly interact

• Currents, wind, and waves all result from the interaction of ocean and atmosphere.

What do you remember from Physics about convection?

• Convection is the movement of heat through fluid movement (gases and liquids)

• Convection currents move in air and water because of differences in density

• Warm fluids rise because they are less dense

• Cool fluids sink because they are more dense

Convection is what causes wind

• Winds are currents of air

• Wind develops whenever two adjacent bodies of air have different densities (measured in air pressure)

• Wind always blows from areas of high pressure to areas of low pressure

Convection is how the earth moves heat from the equator to the poles

• Why is the equator hotter than the poles?

• Not because it’s closer to the sun, but because of its angle to the sun.

• Each ray of sun is more intense at the equator because it heats a smaller area of land and because it has less atmosphere to travel through

All that heat at the equator must go somewhere or the oceans would boil!

• In a simplistic model of the earth’s circulation…

• hot air rises at the equator and drifts towards the poles

• cold air from the poles drifts towards the equator to replace the hot rising air

• Two simple convection cells are, formed, one for each hemisphere

But we also have to account for the spinning earth!

• The speed at which the earth is spinning depends on where you are between the equator and poles

• Here in Waco we are moving at 1400 km/hr

Imagine a missile shot from the pole or equator

• Objects moving towards the equator deflect left

• Objects moving towards the poles deflect right

Why does the missile curve?

• From your perspective the missile shot from the pole appears to curve to the left

• In reality the missile traveled in a straight line and the earth moved while it was in flight

• The missile shot from the equator was already moving to the right at 1700 km/hr

• As it travels north it continues to move to the right at 1700 km/hr while the earth under it is moving slower as it moves toward the pole so it appears to curve to the right.

How does the earth’s rotation affect the winds?

• Just like the missile…

• Air moving towards the poles deflects to the east

• And air moving towards the equator deflects to the west

Three convection cells form in each hemisphere – The Hadley Cell

• At the equator (also called the intertropical convergence zone) hot humid air rises, drops its moisture as rain, and drifts towards the poles at high altitude.

• It gradually becomes cooler and more dense and finally sinks at about 30° latitude (called the horse latitudes)

• The cool air then moves back down along the surface towards the equator.

• The air moving towards the equator deflects west creating the trade winds

• Sailors call the equator “the doldrums” because the rising air produces weak, light, shifting winds

The Polar Cell

• The Polar Cells are located between 60° and 90° N and S latitudes.

• At 60° the air is still warm and moist enough to rise and drift towards the pole

• Like the Hadley Cell, the dense polar air drifts down along the surface and deflects west

• This forms the winds called Polar Easterlies

• Winds are named for the direction they are coming from

The Ferrel Cell

• Between the Hadley and Polar Cells the cycle reverses.

• Some descending air at 30 ° from the Hadley Cell shifts to the poles, and some of the rising air at 60 ° from the Polar Cell shifts to the equator forming the Ferrel Cell

• Because the surface air is moving in the opposite direction as the Hadley and Polar Cells, it deflects towards the east

• This pattern results in the winds called “Prevailing Westerlies” (moving from west to east )

All the Cells in Action

Video Clips

• Atmospheric Circulation

• Bill Nye on Wind

• Coriolis Effect

• Coriolis Effect Explained

• Magic School Bus on Wind

9.2 Hurricanes – Natures Fury!!!

Check these out…

You’d probably want to

evacuate at this point…

What kind of damage are we talkin’ about here?

Might want to take the detour…

Ok, So What Exactly Is a Hurricane?

A hurricane can best be

described as a huge

tropical storm (up to 600

miles in diameter)!

Winds can be up to 200

mph!

Storm usually doesn’t last

for more than 7-10 days.

It moves across the ocean

at around 10-20 mph…not

too fast really!

The winds are the strongest around the

eye wall. The eye of the storm is usually

about 20 miles in diameter. Within the eye,

winds are calm and the weather is great.

Strongest winds are on the right side,

heaviest rain is usually on the left side.

Arrows indicate

“feeder bands”

or “rain bands”

Hurricane Katrina

• FACT: The energy contained in a single large hurricane is greater than that generated by all energy sources in the United States over the past 20 years.

Scary…so when do I need to watch out for these things??

Since we’re in the USA, we’ll focus on what meteorologists call the “Atlantic Basin”.

The Atlantic basin includes the Atlantic Ocean, Caribbean Sea, and the Gulf of

Mexico.

The “hurricane season” is just a name for the time period when we expect to see tropical systems develop. The hurricane season for the Atlantic runs from June 1st – November 30th.

Peak season is the middle part of September…why

do you think this is?

All Hurricanes form in the Inter-Tropical Convergence Zone

The ITCZ is an

area of low

pressure located

roughly 5 degrees

North and South

of the Equator. It

is a place where

air converges,

rises, and

condenses

(forming clouds).

It is the rainiest

place on Earth!

Tropical Cyclones have different names depending on location • Hurricanes are found in the Atlantic • Cyclones are found in the Indian Ocean • Typhoons are found in the Western Pacific

So where are our “Atlantic Basin” Hurricanes form????

Storms of the Atlantic basin will begin

forming in one of three places:

• Off the coast of Africa

• In the Caribbean Sea

• In the Gulf of Mexico

So What Does a Hurricane Need in Order to Develop?

So a hurricane needs warm water, time to grow, and favorable upper

level winds in the troposphere. If the winds are too strong, they will

blow the hurricane apart – we call that wind shear!

Evolution of Hurricane Development -- Stages

Stage 1 – Tropical Depression

Usually has begun its life off of the west coast of Africa in the ITCZ.

Tropical depressions have winds of less than 39mph, and are not given a

name.

Lacks structure – no

well developed

feeder bands or eye

Evolution of Hurricane Development -- Stages

Stage 2 – Tropical Storm

At this point, the tropical storm is beginning to develop some serious

structure. Winds range from 40-73 mph. In the Atlantic, storms are given

a name when they reach this stage.

Feeder bands are

beginning to develop

– you can see the

center of the low

pressure system

more easily. Eye and

eye wall still not well

formed.

Evolution of Hurricane Development -- Stages

Stage 3 – Category 1 Hurricane

Winds range from 74-94 mph.

Well developed

feeder bands. An

eye begins to form

(although it is

covered in clouds).

Storm is tightening

around center.

Evolution of Hurricane Development -- Stages

Stage 4 – Category 2 Hurricane

Winds range from 95-110 mph.

Well developed

feeder bands. An

eye and eye wall are

usually very well

formed. Storm

continues to tighten

around center.

Evolution of Hurricane Development -- Stages

Stage 5 – Category 3 Hurricane

Winds range from 111-130 mph.

Now considered a

“Major Storm”.

Intense flooding and

building damage will

occur to most areas

on the coast. Further

inland, the damage

will still be

substantial.

Evolution of Hurricane Development -- Stages

Stage 6 – Category 4 Hurricane

Winds range from 131-155 mph.

All shrubs, signs and

trees blown down.

COMPLETE

DESTRUCTION of

mobile homes.

Extensive damage to

doors and windows.

Major damage to lower

floors of structures near

the coast.

Evolution of Hurricane Development -- Stages

Stage 7 – Category 5 Hurricane

Winds greater than 156 mph!

Complete roof failure on

many residential and

industrial buildings.

Some complete building

failures with small

utility buildings blown

over or away.

Massive evacuation of

residential areas on low

ground within 5-10

miles of the coastline

maybe required.

Saffir-Simpson Scale of Hurricane Intensity

Storm Surge: sea level rises because of the wind and low

atmospheric pressure system

Most hurricane damage is due to storm surge not winds!

Hurricane Naming System – Why do they have names anyway?

•In 1953 the National Weather

Service began naming storms after

women.

•In 1979, men’s names were added

to the list of hurricane names.

• In the Atlantic, there are 6 lists of

names that are recycled.

• In the case of storms so devastating

that reusing the name is

inappropriate, the name is taken off

the list and another name is used to

replace it. The name “Katrina” is

one that was removed from the list.

Alberto

Beryl

Chris

Debby

Ernesto

Florence

Gordon

Helene

Isaac

Joyce

Kirk

Leslie

Michael

Nadine

Oscar

Patty

Rafael

Sandy

Tony

Valerie

William

This year’s list

More cool stuff to check out…

Aftermath…

Aftermath…

Aftermath…

Aftermath…

Tropical cyclone have an eye of anticyclonic circulation!