Factors Affecting WindsWinds

Factors Affecting Winds Winds

 Horizontal movement of air

Due to horizontal differences in AP

 Air flows from areas of high AP to areas of low AP

1. Pressure Gradient 2. Coriolis Effect

3. Friction

4. Centrifugal Force

Movements in atmosphere

• Air (wind) always moves from regions of high pressure to low

• Cool dense air, higher surface pressure

• Warm less dense air, lower surface pressure

• Hypothetical

nonspinning Earth

• Air rises at equator (low pressure)

• Air sinks at poles (high pressure)

• Air flows from high to low pressure

• One convection cell or circulation cell

Movements in the Air – Non-Rotating Earth

Movements in Air on a Rotating Earth

• Coriolis effect causes deflection in moving body

• Due to Earth’s rotation to east

• Most pronounced on objects that move long distances across latitudes

• Deflection to right in Northern Hemisphere

• Deflection to left in Southern Hemisphere

• Maximum Coriolis effect at poles

• No Coriolis effect at equator

Change in Earth’s rotating velocity with latitude – 0 km/hour at poles

– > 1600 km/hour (1000 miles/hour) at equator

Movements in Air on a Rotating Earth

Atmosphere

Warm air poleward, cool air towards equator (similar to ocean currents)

3 sets of cells distribute heat: (a) polar (b) tropical (Hadley Cell) (c) mid-latitude (Ferrel Cell)

Near Equator  rising air  Equatorial Low precipitation

jungles

20°-30° N & S  upper level flow from equator cools  sinks  Subtropical High  arid conditions  deserts

Some air  back toward equator  deflected by CF  Trade Winds

Rest of air  poleward  deflected by CF  Westerlies

Meanwhile, at the poles, the Polar High is creating the Polar Easterlies

Westerlies (warm) continue poleward  encounter Polar Easterlies (cold) in a region known as the Subpolar Low

This interaction produces the stormy belt known as the Polar Front

4 Main Pressure Zones

• 2 High pressure zones

– Subtropical highs (30° N & S) – Polar highs (90° N & S)

– Clear skies

• 2 Low pressure zones

– Equatorial low (0°)

– Subpolar lows (60° N & S)

– Overcast skies with lots of precipitation

3 Global Wind Belts



Trade winds



Prevailing westerlies



Polar easterlies Boundaries

Between Belts



Doldrums or Intertropical Convergence Zone (ITCZ)



Horse latitudes



Polar fronts

Three-Cell Model of Atmospheric Circulation

• More complex in reality due to:

– Tilt of Earth’s axis and

seasons – Lower heat

capacity of

continental rock vs. seawater

– Uneven

distribution of land and ocean

January Atmospheric Pressures and Winds

Daytime air above land heats up much more intensely & expands  less force (weight) exerted on the Earth

below  low Air Pressure when

compared to water (higher Air Pressure)

Results in a Sea Breeze (water to land) in the late afternoon

Nighttime  air above land cools down more rapidly & contracts  more force (weight) exerted on the Earth

below  high Air Pressure when

compared to water (lower Air Pressure)

Results in a Land Breeze (land to water) during the night

Local Winds

Weather vs Climate

• Weather – conditions of atmosphere at particular time and place

• Climate – long-term average of weather

Ocean influences Earth’s weather and climate

patterns.

Cyclones vs Anticyclones

Low pressure centers

High pressure centers

Winds blow inward (convergence)

Winds blow outward (divergence)

Counterclockwise (N.H.)

Clockwise (N.H.)

“piling up” of air toward center

“convergence aloft”

Heavier air column

Net upward air movement

“divergence aloft” (clouds &

rain)

Descending air is compressed

No clouds, nor precipitation

Fair weather

• Cyclonic flow

– Counterclockwise around a low in Northern

Hemisphere

– Clockwise around a low in Southern Hemisphere

• Anticyclonic flow

– Clockwise around a low in Northern Hemisphere

– Counterclockwise around a low in Southern

Hemisphere

Winds

•large body of air (1000+ miles across & several miles thick)

•characterized by a homogenous temperature & moisture content at any given altitude

•can influence a large portion of a continent as it moves

Air Mass

Air Mass Classification Source Region

•area where air mass gets its characteristic properties

(temperature &

moisture)

FRONTS

Norwegian meteorologists (WWI)--analagous to battle lines

warmer, less dense air is always displaced upward (forced aloft) over cooler, denser air (acts as a wedge)

Warm Front Cold Front

Warm air overtakes cold

Cold air overtakes warm

Advances slowly Advances rapidly

Average slope = 1:200

Average slope = 1:100

Long periods of moderate precipitation

Short periods of intense precipitation

Nimbostratus clouds

Cumulonimbus clouds

•boundary between 2 adjacent air masses with different densities

•marks a major change in weather

Warm Front

Cold Front

• Disturbances with strong winds and precipitation

• Storms typically develop at fronts.

• Jet Stream – narrow, fast-moving, easterly air flow – At middle latitudes just below top of troposphere – May cause unusual weather by steering air masses

Storms

Tropical Cyclones (Hurricanes)

• Large rotating masses of low pressure

• Strong winds, torrential rain

• Classified by maximum sustained wind speed

Hurricanes

About 100 worldwide per year

Whirling, tropical storms with wind speeds up to 185 mph

50-ft. waves  flooding (storm surges)  property damage

Form in warm, tropical waters (>25°C) between 5° and 20° latitude

Hurricanes = Typhoons (N. Pacific) = Cyclones (Indian)

Tropical storm w/winds >74mph + rotary motion  hurricane

Usually develop late in summer (Aug.-Sept.) water temperatures are high enough to provide heat & moisture to air

A steep pressure gradient generates the rapid inward-spiraling winds which are fueled by the latent heat (of condensation) of huge amounts of water vapor

Hurricanes lose their intensity when their supply of moist, tropical air is cut off (Ex. move onto land)

• Tropical Depression

– Winds less than 61 km/hour (38 miles/hour)

• Tropical Storm

– Winds 61–120 km/hour (38–74 miles/hour)

• Hurricane or tropical cyclone

– Winds above 120 km/hour (74 miles/hour)

Hurricane Development

Saffir-Simpson Scale of Hurricane Intensity

Historical Storm Tracks

• Diameter typically less than 200 km (124 miles)

– Larger hurricanes can be 800 km (500 miles)

• Eye of the hurricane

– Low pressure center

• Spiral rain bands

with intense rainfall and thunderstorms

Hurricane Anatomy

Hurricane Movement

Hurricane Destruction

• Fast winds

• Flooding from torrential rains

• Storm surge most damaging Historical examples:

Hurricane Ike, 2008

Hurricane Irene, 2011

Hurricane Sandy, 2012

Galveston, Texas - 1900

• Most active season on record

– 27 named storms

– 15 became hurricanes

• Season extended into January 2006

• Five category 4 or 5 storms

– Dennis, Emily, Katrina, Rita, Wilma

2005 Atlantic Hurricane Season

• Costliest and deadliest U.S. hurricane

• Category 3 at landfall in Louisiana

– Largest hurricane of its

strength to make landfall in U.S. history

• Flooded New Orleans

Hurricane Katrina - 2005

• Rita – September 2005

– Most intense Gulf of Mexico tropical cyclone – Extensive damage in Texas

and Louisiana

Hurricanes Rita and Wilma - 2005

• Wilma – October 2005

– Most intense hurricane ever in Atlantic basin – Multiple landfalls

– Affected 11 countries

• Category 1

• Largest Atlantic hurricane on record

• Storm surge coincided with peak high tides in heavily populated New York and New Jersey.

• Severe coastal erosion

• Extreme flooding

• 233 deaths, more than

$68 billion in damages.

– Second costliest hurricane after Katrina

Hurricane Sandy, 2012

• Open ocean’s climate regions are parallel to latitude lines.

• These regions may be modified by surface ocean currents.

• Question: Are the warmer water brought about by climate change (global warming) responsible for the increase in the frequency of hurricanes?

Ocean’s Climate Patterns

Equatorial regions – warm, lots of rain

Tropical regions – warm, less rain, trade winds

Subtropical regions – rather warm, high rate of evaporation, weak winds

Temperate regions – strong westerlies

Subpolar regions – cool, winter sea ice, lots of snow Polar regions – cold, sea ice, polar high pressure