Chapter 6—Weather Systems

General Introduction

• What is a weather system?– An atmospheric circulation pattern and the

weather associated with it

• Spatial scale: ~ 1km ~ 2000km

• Temporal scale: hour(s) weeks

• Tornado/tornadic thunderstorms are at smallest spatial and temporal scales, while cyclones, anticyclones, hurricanes are at largest spatial and temporal scales

Scales of Weather/Climate Phenomena

Air Masses• Definition: a large body of air that has fairly uniform

weather attributes, mainly with respect to temperature and moisture

• Horizontal and vertical extents vary from one air mass to another

• Vertical temperature profile also differ between air masses

• Location, season, and upper-air flow play roles in determining type of air masses affecting a region (think of San Diego vs. Tucson, or just Tucson in May vs. Tucson in July and August)

Air Masses (cont’d)

• Names depend on temperature and moisture characteristics, which are determined by the source region (area where air mass develops)– Moisture: maritime (m) vs. continental (c)

– Temperature: Arctic (A), Antarctic (AA), Polar (P), Tropical (T), Equatorial (E)

• Combine moisture and temp letters to get various types• Most common air mass types to affect U.S.: mP, mT,

cP, cT, and cA

Air Masses Affecting North America During Winter

Air Masses Affecting North America During Summer

Movement of Air Masses

• Upper-level wind patterns move surface weather systems and air masses

• Air masses modify from original state as change location due to:– Distance from source region– Characteristics of new location– Advection of other temp and moisture qualities

** Stability is affected by movement **

Fronts

• Front: leading edge of an air mass or the boundary between different air masses

• There are four main types of fronts in terms of temperature and and moisture characteristics:1. Cold 3. Occluded2. Warm 4. Stationary

• Other types of fronts exist, but usually only occur on small scales (gust front) or in specific regions (dryline)

Fronts and Air Masses

Cold Front

• Leading edge of cool/ cold air

• Designated as blue line with triangles on side of front in direction of movement

• Cooler, denser air forces warm air up

• Generally associated with showers and t-storms

Interaction of Cold and Warm Air

cold air mass warm air mass

front

Cold Front

Warm Front

• Leading edge of warmer air

• Designated as red line with half circles on side of front in direction of movement

• Warmer air rides up and over cooler air

• Generally associated with steady precipitation, but depends on stability

Warm Front

Occluded Front

• Occur when cold front catches warm front

• Designated as purple line with alternating triangles and half circles that indicate direction of movement

• Different types• Can see steady or intense

precip, depending on stability

Occluded Fronts

Cold air

Warm air

In an occluded front, a warm front is overtaken by a cold front.the warm air is pushed aloft and it is not longer in contact with the ground

Cool air

Stationary Front

• Occur when boundary between air masses is not moving

• Designated as blue triangles pointing into warmer air and half circles “pointing” into cooler air

• If any precip is falling, it tends to be light and steady

Cyclones and AnticyclonesReview from yesterday

• Cyclone– Associated with low pressure– In N.H. counterclockwise motion– Convergence at surface: air spirals toward center and

“piles up” (like pouring water into clogged funnel)– In general, can expect cloudy weather and precipitation

• Anticyclone– Associated with high pressure– In N.H. clockwise motion– Divergence at surface: air flows away from center (like

pouring water on an up-side down bowl)– In general, can expect fair weather

Types of Cyclones

• Wave cyclone or mid-latitude cyclone– Large spiral of air that repeatedly forms,

intensifies, and dissolves along polar front

• Tropical cyclone– Smaller than wave cyclone, forms between 8-

15°N and between 8-15°S

• Tornado– Small area of intensely spiraling air

Wave or Mid-latitude Cyclone

• Large spiral of air that repeatedly forms, intensifies, and dissolves along polar front

• Life cycle:1. Early: wave forms along boundary between warm and

cold air (polar front); cold air moving southward, warm air moving northward

2. Open: wave “deepens,” cold front and warm fronts develop; precip strongly developed

3. Occluded: cold front has caught warm front; warm air forced aloft at center of L; precip intensifies

4. Dissolving: polar front distinguishable; still warm air aloft; precip slowly ends; clouds go bye-bye

Early Stage

• Wave forms along boundary between warm and cold air (polar front)

• Cold air moving southward, warm air moving northward

Open Stage

• Wave “deepens,” cold front and warm fronts develop

• Precip strongly developed

Occluded Stage

• Cold front has caught warm front

• Warm air forced aloft at center of L

• Precip intensifies

Dissolving Stage

• Polar front distinguishable again

• Still warm air aloft• Precip slowly ends• Clouds go away

Variation of Weather Within a Wave/Mid-latitude Cyclone

• If you stop a wave cyclone and walk/drive through it from behind the cold front to past the warm front, you would experience:

1. Behind cold front: NW winds, cold temps, low humidity

2. At cold front: stronger winds, precip (usually intense)3. Between cold and warm fronts: SW or S wind,

warmer temps, higher humidity4. As approach warm front: still warm temps and high

humidity, sometimes precip (usually light)5. Past warm front: SE or E winds, cooler temps, lower

humidity (but higher than behind cold front), precipAs cyclone passes over a region you would experience

these in reverse order

Tornado

• Small area of intensely spiraling/rotating air

• May only last short time (20-30 minutes), but are very destructive

• Can occur just about anywhere when conditions are appropriate (topography does not necessarily preclude development)

• All states in US have tornadoes

Tornadoes• Small vortex with very intense winds that

extends from base of thunderstorm to the Earth’s surface

• Visible due to dust, debris, and condensed moisture

• Diameter: 100s of meters to > 2 km at base• Usually short-lived (20-30 minutes)• Most common in US, especially during

spring and summer, due to appropriate ingredients

• Tornado Alley – region from west-central Texas to western Great Lakes

Fujita Scale (or F-scale)

Rating Category Estimated windmph km/hr

F0 Weak 40-73 65-118F1 74-112 119-181F2 Strong 113-157 182-253F3 158-206 254-332F4 Violent 207-260 333-419F5 261-318 420-513F6* Catastrophic > 319 > 513

* hypothetical

Tornado Alley

A Few Examples• Tri-State Outbreak (March

18, 1925)– Missouri, Illinois, and Indiana– 7 tornadoes over ~ 440 mi

(700km), 695 deaths

• Palm Sunday Outbreak (1965)– Midwest US– 30 tornadoes in 5 states, 256

deaths

• Topeka, KS (June 8, 1966)– 16 deaths, hundreds of houses

destroyed or damaged, Washburn University badly damaged

A Few More Examples

• Lubbock, TX (May 11, 1970)– 26 deaths, > $100 million in damage– Weather observer records

• April 3-4 Outbreak (1974)– 13 states (central AL to s. MI, central IL to central VA)– 16 hrs, 148 tornadoes, 307 deaths, > 6000 injuries,

approximately $600 million in damage

• Central Oklahoma Outbreak (May 3, 1999)– ~ 74 tornadoes through Oklahoma– In OKC, estimated$1.2 billion in damage, 41 deaths, 675+

injuries (winds estimated at 300 mph)

Cyclone Tracks• Wave cyclones

tend to form in certain areas and travel common paths

• Mid-latitude wave cyclones tend to travel eastward

• Tropical cyclones tend to move westward

Tropical Cyclones

• Development involves 3 stages– Tropical Disturbance (Easterly Wave)– Tropical Depression– Hurricane

• Actually have different names in different region during stage 3– Hurricane: Atlantic and eastern Pacific Oceans– Typhoon: western Pacific– Cyclone: Indian Ocean

Storm Life Cycle• Tropical Cyclones or Tropical Disturbance

(Easterly waves) are troughs of low pressure moving eastward with a mass of thunderstorms and counterclockwise flow

• Tropical Depression: winds up to 38 mph (33 knots)– Referred to as TD and Number

• Tropical Storm: wind range from 39-73 mph (34-63 knots), isobars “packed,” – Referred to with Name

• Hurricane: wind exceeds 74 mps (64 knots) then ranked based on central pressure, storm surge height, wind speed using Saffir-Simpson scale

Rare Satellite Image

Hurricanes

•the most powerful and destructive tropical cyclone in the western hemisphere •Hurricane=Tropical Cyclone

Hurricanes• Intense tropical cyclone containing strong winds

and excessive precipitation• Energy from latent heat release, loses energy over

land or cooler water• Diameter—Average approximately 300 miles

wide• Hurricane force winds extend 25 miles (small) to

150 miles (large) and tropical storm force winds (300 miles)

• Associated problems include: strong winds, tornadoes, heavy rain, storm surge (most devastating aspect), death, water and food supply

Hurricanes

characteristic central “eye” (clear skies and calm winds)air descends from high altitudes, warming wind speeds are highest at the “eye wall” winds spiral outward creating high wind speeds

Ingredients for a Hurricane

• 1. Pre-existing disturbance with thunderstorms (Easterly wave)

• 2. Warm (80ºF) ocean temperatures (150 ft)

• 3. Light upper level winds (which provide little wind shear)

• 4. Convergence of moisture and heat

Hurricanes (cont’d)

• Different names throughout globe– Hurricane in Atlantic and eastern Pacific

– Typhoon in western Pacific and Philippines

– Cyclone in India and Australia

• Official hurricane seasons– N. Atlantic Ocean: June 1 – November 30 (peak

occurrence from mid-August to late October)

– N. Pacific Ocean: May 15 – November 30 (peak occurrence from July through October)

Simpson-Saffir Scale of Tropical Cyclone Intensity

Categories 1 to 5 (5 is the most intense and devastating)

categorized by central pressure, storm surge wave height and mean wind speed

Simpson-Saffir Scale of Tropical Cyclone Intensity

Category Central Storm Mean

Pressure Surge Wind

(mb) (ft) (mph)

1 (weak) > 980 4-5 74-95

2 (moderate) 965-979 6-8 96-110

3 (strong) 945-964 9-12 111-130

4 (very strong) 920-944 13-18 131-155

5 (devastating) < 920 >18 > 155

Naming Hurricanes• Before 1953: used latitude and longitude (or location of

damage)• 1953 – 1977: used only female names (in alphabetical

order)• 1978: alternate between male and female names for

eastern Pacific Ocean• 1979: alternate between male and female names for

Atlantic Ocean• Also:

– In Atlantic, begin at start of alphabet each year– In Pacific, continue where left off previous year– Names retired if causes much devastation (life, property)– See http://www.aoml.noaa.gov/hrd/tcfaq/B2.html for recent

and upcoming names

MH Katrina- Hit as Cat. 4 (145 mph winds) August 29th 2005

Hurricane winds extend 125 miles from centerNew Orleans and Biloxi both in Eye Wall

Eye is 30 miles across

New Orleans

Biloxi

Atlantic Sea Surface Temperature Departures

Strong multi-decadal fluctuations in Atlantic temperatures dating back to the 1870’s.

1970 19951870 1900 1930

Florida Coastal Population

*87M Atlantic/Gulf Residents

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Questions