Chapter 15: Weather Systems Copyright © The McGraw-Hill Companies, Inc. Permission required for...

Chapter 15: Weather Systems

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

1. The Weather around Us

2. The Science of Weather: From

Folklore to Forecasting

3. Air Masses

5. Midlatitude Cyclones

4. Frontal Systems

and Tornadoes

6. Severe Weather: Thunderstorms

7. Severe Weather: Hurricanes

Learning Objectives• Students will explain concepts related to weather systems.• Students will recognize the role of technology in our knowledge of

weather patterns and forecasts.• Students will classify air masses based on their locations.• Students will compare and contrast warm, cold, and occluded

fronts.• Students will synthesize the development of mid-latitude cyclones

and frontal systems.• Students will describe the process of thunderstorm, tornado, and

hurricane formation.• Students will recognize when/where tornadoes are most likely to

occur.• Students will place hurricanes within the context of the Earth

system.

The Weather around Us

The Good Earth/Chapter 15: Weather Systems

Would you want to be riding in one of these vehicles?

On average, 2-3 U.S. weather disasters per year cause more than a billion dollars in damage and threaten numerous lives.

Today, more than 100 million residents make the South the most heavily populated region in the U.S.

People are migrating to Florida, Georgia, and N.C. more than any other state.

Most people are migrating from states that have low incidences of extreme weather events (New York, Illinois, New Jersey).


The Weather around UsWeather = the state of the atmosphere at any given time

and place.

Extreme weather can threaten lives, disrupt transportation, and cause billions of dollars in destruction.

Billion dollar weather disasters, 1980-2005.

The Weather around Us

• Almost 3 times as many people die from extreme temperatures than any other weather related causes.

• More than 26,000 people in Europe died in 2003 during hottest summer on record there (104°F or more).

• Extreme weather events can change these statistics (e.g. Katrina killed over 1,000 people)

• Extreme heat – “…underrated and least understood of the deadly weather phenomena.”


Weather-related fatalities.

The Weather around Us• Can extreme heat, tornadoes, hurricanes, and other weather phenomena

be stopped?

− No, but meteorologists work to provide timely warnings.

− Hurricane Katrina – most expensive disaster in U.S. history.

− Over $100 billion in damage.


Hurricane Katrina approaches the Louisiana coast, August 28, 2005.

Weather Systems Self Reflection Survey


Answer the following questions as a means of uncovering what you already know about weather systems:

1. Explain three examples of how the weather influences your daily life.




2. Describe your ideal annual weather conditions.




3. Describe the most extreme weather event you have experienced.

Go back to the Table of Contents

Go to the next section: The Science of Weather: From Folklore to Forecasting


The Science of Weather: From Folklore to Forecasting


Mankind has been trying to make sense of the weather since ancient times.

Meteorology is the study of the atmosphere and its weather.

1637 – Descartes published first scientific text on weather, but had no tools to make measurements.

By the 1700’s the technology caught up:

- 1643 first barometer

-Gabriel Fahrenheit (1714) and Anders Celsius (1742) invented first Mercury thermometers

- hygrometer (measures humidity) invented a few decades later

A paradigm shift in our understanding of weather occurred.


With the development of communications devices information on weather from multiple sources became

available, and patterns in weather data began to emerge.

-Weather systems migrate from west to east

-Fair skies are associated with high-pressure

-Rain often accompanies low-pressure systems

U.S. National Weather Service was established in 1870

By 1872 it was creating national weather maps

By 1878 daily observations were being collected at hundreds of sites and relayed cross-country by telegraph


Not until early 1900’s did Norwegian scientists identify the concepts of air masses and frontal systems


• Air masses = large volumes of air with similar temperature and pressure.

• Frontal systems = locations where air masses interact.

• Few gas molecules exist here.

These concepts allowed meteorologists to use raw temperature and pressure data to predict weather over large regions.

The recognition of these basic types of air masses and their motions made it possible to develop rules for the evolution of weather systems.

Later these rules were turned into mathematical equations that could be programmed into computers to produce forecasts


Weather Systems Conceptest


The Bergen school (the Norwegian scientist who identified the concepts of air masses and frontal systems) looked at individual weather cases and drew general conclusions about weather. Which of the following types of reasoning did they use:

a.Inductive Reasoning

b.Deductive Reasoning



Once the rules of weather evolution were established, scientists anywhere could readily apply these rules to predict future weather patterns. This is an example of:

a.Inductive Reasoning

b.Deductive Reasoning


The Science of Weather: From Folklore to ForecastingToday, all weather

information and data come from the NWS.

NWS processes over 1 million surface, air, and satellite weather observation every day.

Two GEOS (geostationary operational environmental satellite) satellites in orbit provide weather coverage for 60% of the planet’s surface.

Over 100 Doppler radar sites nationwide used to track changes in regional storms.

Doppler can detect dangerous swirling winds.

Satellite weather image for North America

Weather Systems Checkpoint 15.3

How are the following four key principles of science evident in the brief history of meteorology in this section?

1) Phenomena can be explained by natural causes.

2) Explanations are tentative.

3) Science is based on empirical observations.

4) Explanations should be testable.



Go to the next section: Air Masses


Air Masses

Air Mass = a large region of the lower troposphere that has relatively uniform temperature and moisture

content.


Their nature depends on their source area and changes that occur as they travel.

Air masses are identified by their temperature and the moisture characteristics of the underlying surface.

-Polar air masses found at high latitudes

-Tropical air masses found near equator

-Maritime air masses develop above oceans and are wetter

-Dry air masses form over continents

No clear boundaries between these air masses – they can migrate

Air Masses


Locations of air masses. cA = continental Arctic/Antarctic; cP = continental polar; cT = continental tropical; mP = maritime polar; mT = maritime tropical.

Northern Hemisphere Southern Hemisphere

Air Masses


Types of Air Masses:

• cA – forms over permanent snow or sea ice. Extremely cold, dry air that may move south.

• cP – forms over northernmost portions of N. America, Europe, and Asia. Cold and dry but not as cold as cA masses.

• cT – forms over continental interiors. Hot, dry air that is modified as it moves east or north.

• mP – form in the N. Atlantic and Pacific and Southern Oceans. Cool, moist air that affects NE and NW states in the U.S. Warmer than cP air. Bring rains to the coasts of Washington, Oregon, and snows to the inland mountains.

• mT – move inland from the tropical Pacific Ocean, Gulf of Mexico, and tropical Atlantic Ocean. Hot and humid. Brings hot humid summers to SE states.

Air Masses


Air masses of North America

Circles show source areas and arrows show where the air mass moves after it forms.

Source areas and paths can influence weather patterns across North America.



Of the five most common types of air masses, which ones most directly affect the area where you live?

Air Masses


Air masses can gain or lose heat and/or moisture as they move from one location to another.

Air masses are principally modified by:

- Temperature of the underlying surface

- Topography of the underlying surface

Examples:Air heated by warmer land surface will begin to rise. It is unstable and will rise until its temperature matches the surrounding atmosphere.

Air that is forced up over topography will cool and moisture will condense. Condensation removes moisture leaving dry air (rainshadow effect).

Air MassesLake effect is

common in states south and east (downwind) of the great lakes.

Dry cP air masses pick up moisture as they cross the warmer waters of the lakes. When the air mass arrives at the southern lake shore it cools, saturates, and precipitates.




Imagine that you had a device that would constantly monitor the characteristics of an air mass as it moved from its source region to another location. Draw some idealized graphs illustrating how some key characteristics of the mT and CP air masses would change.


Go to the next section: Frontal Systems


Frontal Systems


The boundary between one air mass and another is a front.

Frontal systems – pairs of relatively narrow, long, slightly curved regions where air masses interact.

Frontal systems bring clouds and precipitation and changes in moisture, temperature, pressure, and wind direction.

Weather patterns typically associated with a cyclone (low-pressure system) over the central U.S.

When warm and cold fronts merge, they form an occluded front.

Frontal Systems


Cold fronts are represented by blue triangles, warm fronts by red semicircles.

Both symbols “point” in the direction of air movement.

Cloud cover occurs in advance of the cold front, adjacent to the warm front, and around the occluded front.

Warm maritime tropical air from the gulf of Mexico lies in between the two fronts.

Which direction is this frontal system moving?

West to east.

Frontal Systems

How are wind direction, temperature, clouds, and precipitation affected by a passing frontal system?




Use this map to answer the following questions:

1. The map illustrates the relative positions of a warm front and a cold front. Where is the warm front located?

a) Between A and B

b) Between C and D

c) At E




2. Where is it raining?

a) A and B

b) B and C

c) C and D

d) B and D




3. Which location is in a maritime tropical air mass?

a) A

b) G

c) E

d) H




4. Which location will become warmer in the next 12 hours?

a) A

b) B

c) C

d) D

Frontal Systems


Advancing Cold Fronts:

• Warm air is displaced up and over an advancing cold front

- The warm air is less dense and therefore lighter than the cold air

- Cold fronts slope steeply

- The rising air undergoes rapid cooling and condensation in a narrow region above the cold front

- Condensing water vapor fuels formation of tall cumulonimbus clouds that usually produce heavy but short-lived rains

A squall line is a linear pattern of thunderclouds that may accompany a

rapidly advancing cold front.

Above: Squall line in the Gulf of Mexico.

Frontal Systems


Advancing Warm Fronts:

• Weather changes are not as severe

- The warm air does not rise as rapidly

- Warm fronts slope gently

- Warm fronts move more slowly and the associated rain may last longer than with a cold front

- Warm fronts extend over a larger area

- Temperatures and humidity rise

- Winds change direction

Occluded front. A cold air mass overtakes a warm air mass.

Nimbostratus clouds generate precipitation along an occluded

front.



Use the Venn diagram provided here to compare and contrast the characteristics of warm fronts and cold fronts. Identify at least 10 features, write them in the table, and put each of their corresponding numbers in the correct location on the diagram.

Cold front Warm front1.

2.

3.

4.

5.

6.

7.

8.

9.

10.


Go to the next section: Midlatitude Cyclones


Midlatitude Cyclones


Midlatitude cyclones are regional-scale low-pressure systems formed between 30° and 60° N or S.

• This is where continental polar and maritime tropical air masses collide to form frontal systems

• This collision zone migrates south during winter and north in the summer

• Can be 1-2 km (621 – 1242 miles) across

• Can affect much of the continent for up to a week

• Midlatitude systems tend to dominate weather patterns in the U.S.

• Midlatitude cyclones develop where surface irregularities (mountains or water boundaries) cause a local sideways-acting force to distort a front

• The front is transformed as warm air pushes northward and cool air pushes south, generating a counterclockwise rotation



A midlatitude cyclone

(a)At the point where a low-pressure system (cyclone) interacts with neighboring high-pressure systems, a midlatitude cyclone forms. It is centered over the low-pressure system and is characterized by warm (red) and cold (blue) fronts. Cloud cover concentrates over the fronts and low-pressure center.

(b)Classic comma-shaped cloud pattern associated with midlatitude cyclones. Central U.S., Christmas Eve, 1997.



a. A cyclone forms.

Three stages in midlatitude cyclone development over the U.S.

b. The cyclone reaches mature stage, with well-developed warm and cold fronts.

c. The cyclone begins to weaken as warm and cold fronts merge to form an occluded front.



Examine the image at right. Where is the low-pressure system at the center of the cyclone? What type of front is represented by the line of clouds extending toward the Gulf of Mexico?


Go to the next section: Severe Weather: Thunderstorms and Tornadoes


Severe Weather: Thunderstorms and Tornadoes


Severe weather (as defined by NWS) has one or more of the following elements:

A tornado

Damaging wind speeds (more than 58 mph)

Penny sized or larger hail

Only 10% of all the 100,000 thunderstorms that form over the U.S. each year have these conditions.

Most thunderstorms are around 16 miles across and last less than 30 minutes.



Moisture in the atmosphere + warm air + a lifting mechanism = a thunderstorm!

The life cycle of a thunderstorm.



Lightening! Lightening joins two centers of opposite charge associated with a thunderstorm. Cloud to ground lightening – negative charges in a cloud are connected to positive charges on the ground. Cloud to cloud

lightening – opposite charges in one or more cloud are connected.



Thunderstorms can occur anywhere in the U.S. but their causes vary by region:

Central or eastern portion of the states – isolated afternoon thunderstorms in warm summer months when moist air rises. They are typically brief and associated with a single large cloud called a cell. Warm land surface heats the overlying air, causing it to rise, condense, and generate a thunderstorm.

Eastern U.S. – thunderstorms associated with midlatitude cyclones. Severe storms from a series of cumulonimbus clouds called supercells. They are associated with frontal lifting along a cold front between cP and mT air masses. Most common during spring and early summer.

Rocky Mountains – related to orographic lifting.

Florida – warm maritime air rises due to several lifting mechanisms.

Thunderstorms are rare in Pacific coast states because weather is influenced by cool ocean more stable air.


Updrafts responsible for the formation of thunderstorm clouds are most likely to occur with which combination of conditions?


a. Low-level warm, moist air; upper-level warm, moist air

b. Low-level cool, dry air; upper-level warm, moist air

c. Low-level warm, moist air; upper-level cool, dry air

d. Low-level cool, dry air; upper-level cool, dry air



Rank the three thunderstorm components (air temperature, moisture, a lifting mechanism) in order of their significance in causing thunderstorms. Justify your ranking.



Tornadoes = narrow, funnel-shaped spirals of rapidly

converging and rotating air that form in association with

thunderstorms.

Near-circular low-pressure systems that rotate

counterclockwise in the Northern Hemisphere

Pressure gradient is much more intense for tornadoes

Tornadoes generate the strongest natural winds on

Earth!

People have been “storm chasing” for about 50 years. Tourists pay companies to get them into tornado prone areas to get footage of

tornadoes. Groups of tourists will pile out of minivans and cheer as a tornado rips apart a

farmhouse. Is this sensible?



The worst tornado outbreak in U.S. history – April 3, 1974.

Tornadoes were spotted from Canadian border all the way down to Gulf coast, and from Illinois to Virginia.

A total of 148 tornadoes touched down in 13 states.

330 people killed, over 5,000 injured.

A massive single tornado threw two tractor-trailers onto the roof of a bowling alley in Xenia, OH as the storm sped through at 50 mph.

33 people killed, 1300 buildings destroyed in Xenia that day.



Tornadoes are ranked on the Fujita intensity scale, which places them in a category based on the destruction they

cause.

Rule of thumb: Weak tornadoes can tear shingles off the roof of a house, strong tornadoes can tear the roof off a house, and

violent tornadoes can tear up the whole house.



Union City, OK tornado May 1974.

a. Early stage

b. Mature stage



Characteristics of tornadoes:

• Funnels are less than 2,000 feet wide

• Average funnel velocities are 31 mph, with highs of 125 mph

• Path of destruction typically 3-16 miles long

− Some may stay on ground for over an hour and travel over 62 miles

Tornadoes follow the path of their parent thunderstorms (most travel east or northeast, as they are often associated with midlatitude cyclone thunderstorms.



a. Early stage. Friction slows winds near surface. Higher wind velocity moving upward from the surface. Contrasting vertical wind speed generates counterclockwise winds about a central horizontal axis.



b. Updraft stage. Updrafts below a thunderstorm draw spiraling horizontal winds upward to form a mesocyclone within the larger storm cloud. These are rotating thunderstorms that can be seen on radar (up to 6 miles across).



c. Tornado stage. Rotation in the mesocyclone forms small intense spiraling winds within a newly formed tornado. These winds then extend downward from a cloud base toward the ground surface.



Why do you think U.S. tornado fatalities have declined in the past 50 or so years?

Better forecasting and warnings.



Tornado Alley. The U.S. is home to the majority of the world’s tornadoes, averaging about 1200 per year. These states have an annual average of more than 5 strong to violent tornadoes, 1950-

1995.

Tornadoes occur when thunderstorm activity is at an optimum across much of the nation.

More common in Gulf coast and SE states in early spring, migrates to Great Plains in late spring.



Explain why the number of tornadoes counted each year has increased, while the number of days with at least one tornado sighting has remained essentially unchanged for several decades.


Go to the next section: Severe Weather: Hurricanes


Severe Weather: Hurricanes


Hurricane Katrina was the worst natural disaster to strike the United States in the last century.

Are we doomed to repeat history? Can a hurricane strike the same place twice?

More than 1,300 people died

An estimated $100 billion in damage in communities in Louisiana, Mississippi, and Alabama

Much of the damage had long been predicted – A dozen major hurricanes had made landfall along coastal Louisiana in the last century

Government reports, scientific articles, and newspaper stories had all predicted what would happen if a major hurricane made landfall near New Orleans



Who should address the issue?

Scientists – best equipped to determine the probability and destruction of a hurricane, but no funds or resources to do much about it.

Government – have power and funds for levees or evacuation plans, but don’t always understand the seriousness of the threat.

“We learn from history that we learn nothing from history.” George Bernard Shaw

New Orleans before and after Katrina.

Blue areas = flooded areas



Key observations of Katrina:

Large size

Dense mass of clouds surrounding central clear “eye”

Swirling spiral pattern of clouds

Centered over the ocean (Gulf of Mexico)



Hurricanes originate in areas of the world’s oceans where the temperature is greater than about 80°F. Hurricanes are most common in the summer

seasons of each hemisphere.



What are hurricanes:

High winds

Heavy rainfall

Storm surges (elevated water levels) along coastlines

Not all hurricanes make land fall

Biloxi, Mississippi before and after Katrina.



North Atlantic hurricane season June 1 – November 30

Hurricanes only develop over warm (>80°F) water down to ~200 ft

Needs sufficient evaporation and condensation to foster large volume of moisture to foster growth of huge cloud masses

Warm surface waters move north from equator in the summer in N. Hemisphere creating optimal hurricane conditions

Development of Atlantic hurricanes that make landfall:

2-3 weeks prior to landfall – Cloud mass develops over warm ocean water. Most storms generated in east Atlantic take about 3 weeks to reach the coast of N. America

Katrina – developed in west Atlantic near the Bahamas.



Hurricanes begin to grow when warm, humid air is forced aloft.

The rising air cools and condenses to form cumulus clouds that will develop into cumulonimbus cells.

Earth’s rotation imparts a counterclockwise rotation to the storm in the Northern Hemisphere. This rotation is zero at the equator, therefore the majority of hurricanes originate between 10° and 20° N or S of the equator.

To maintain wind speed, inflow of air into developing low-pressure system must be matched by outflow of air in the upper troposphere.



1-2 weeks prior to landfall – Tropical depression develops (winds 23-39 mph). After about 5 days this develops into a tropical storm (winds 39-74 mph). Finally, a hurricane develops (winds at least 119 mph).

Air pressure is lowest in the eye, where warm air is rising.

The lower the pressure inside the hurricane, the faster the winds.

Hurricanes will continue to grow in size and intensity as long as underlying water temperature remains above 80°F.

Precipitation concentrates within 124 miles on either side of eye, releasing up to 20 billion tons of water per day.



3-7 days before landfall – Atlantic hurricanes are driven west by prevailing winds at 6-16 mph.

Florida and Texas experience more landfalls than any other state.

Hurricane may turn parallel to the east coast or pass south of Florida to strike the Gulf Coast or Caribbean islands.

2-3 days before landfall – The likely landfall site is identified.

Evacuations should be in full gear by now.

1-2 days before landfall – Size and slow motion of hurricanes means their impact is drawn out over several days.

Effects can reach coast before worst affects from the eye.

Measurements are made off coast by buoys to gather info about eye

Waves over 100 feet high during Katrina

Predicted landfall site of Katrina. It hit within 19 miles of the predicted site.



Winds in the NE quadrant blow onshore, piling up water in a storm surge.

Surges cause ~90% of the damage in coastal areas.

Can potentially reach inland areas up to 6-12 miles from shore.

Katrina – water reached heights of 30 feet above sea level in NE quadrant of storm.

Winds in NW quadrant blow offshore. In the case of Katrina, winds in NW quadrant pushed water from Lake Pontchartrain (located NW of the city) over levees into the city.



Hurricanes can take days to make their way onshore.

Hurricanes lose intensity over land because of frictional drag and loss of a moisture source.

Damaging winds near the core have speeds similar to F1-F3 tornadoes.

Can rain up to 24 inches of rain in just a few days over inland regions.

Water causes major erosion and damage from flooding.

Erosion from hurricane Ivan eroded most of N segment of Chandeleur islands, LA. In box – lighthouse.



Katrina – category 3 hurricane

In 1973 in Bangladesh – Cyclone pushed onshore from Bay of Bengal

22-foot storm surge produced widespread flooding on the low lying plain

300,000 deaths



Draw a diagram that illustrates how the four components of the earth system (atmosphere, biosphere, hydrosphere, and geosphere) interact during a hurricane.



Use the Venn diagram provided to compare and contrast the features of midlatitude cyclones, tornadoes, and hurricanes. Identify at least 12 features.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

Tornadoes Midlatitude cyclones

Hurricanes

The End



Chapter 15: Weather Systems Copyright © The McGraw-Hill Companies, Inc. Permission required for...

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