Ch. 20 Notes

Chapter 20

Weather Patterns and Severe Storms

Air Masses

There are several types of severe storms.

- Thunderstorms- Hurricanes- Tornadoes Lets look at atmospheric conditions that

most often affect day to day weather.

Air Masses and Weather

Think about these examples:1. Winter cold spells under clear skies are

often followed by cloudy, snowy, relatively warm days that seem warm in comparison to the recently experienced frigid weather.

2. Summer days of high temperature and high humidity often end when a series of storms pass through your area. This is generally followed by a few days of cooler weather.

Air Masses

• An air mass is an immense body of air that is characterized by similar temperatures and amounts of moisture at any given altitude.

• Because of their size, air masses bring fairly constant weather conditions when they pass over an area.

Movement of Air Masses

• As the air mass moves, the characteristics of an air mass change and so does the weather in the area over which it moves.

Classifying Air Masses

• The area, which an air mass gets its characteristic properties of temperature and moisture, is called its source region.

• Air masses are named according to their source region.

Describes temperature:Polar (P) – form at high latitudesTropical (T) – form at low latitudes

LAND VS. WATER

• In addition, air masses are classified according to the surface from which they form

 Continental (c) – form over landMaritime (m) – form over water

Types of Air Masses

There are four basic types of air masses:

1 – cP – continental Polar2 – cT – continental Tropical3 – mP – maritime Polar4 – mT – maritime Tropical

Weather in North America

• Much of the weather in North America, especially weather east of the Rocky Mountains, is influenced by continental polar (cP) and maritime tropical (mT) air masses.

• The cP air mass originates in….. CanadaThe mT air mass originates in………….The Gulf of Mexico, the Caribbean, or

the near by Atlantic Ocean

Continental Polar Air Masses

• Cold and dry air in winter and cool and dry in summer.

• They are generally associated with fair weather conditions.

 • The only major amount of precipitation

from this air mass occurs as the air passes over the Great Lakes bringing lake effect snow to the leeward shore in autumn or winter.

How does this work? • In autumn and early winter, the difference

in temperature between the lakes and the adjacent land areas can be large.

• When a cP air mass moves southward across the lakes, the air acquires large quantities of heat and moisture from the lake surface.

• By the time the air mass reaches the far shore is humid and unstable making large amounts of snow possible.

Maritime Tropical Air Masses

• These warm unstable air masses play a dominant role in the weather of the United States.

• As these mT air masses move into the U.S. they bring heavy precipitation to the eastern 2/3s of our country.

• In summer when they move into the central and eastern part of the United States the not only bring heavy rains, but high humidity as well.

Maritime Polar Air Masses

• These air masses come from the Northern Pacific, but actually form over Siberia.

• The cold, dry cP air changes into relatively mild, humid unstable mP air during it journey east.

• As this mP air mass passes over the west coast it brings low clouds and showers.

• As this air is lifted over the western mountains, it produces heavy rain or snow on the windward side of the mountains.

Atlantic Polar

• Maritime Polar air masses also form off the coast of eastern Canada.

• In winter, when New England is on the northern or northwestern side of a passing low-pressure center, the counterclockwise winds draw the mP air.

• The result is a storm characterized by snow and cold temperatures, known locally as a nor’easter.

Continental Tropical Air Masses

These cT air masses have the least effect on weather in the U.S.

These hot, dry air masses originate in southwestern U.S. and Mexico during the summer.

When cT air masses move from their source region, it can become extremely hot and drought like in the Great Plains.

In the fall, they cause mild weather in the Great Lakes region, often called an Indian summer.

Front

Formation of Fronts• When two air masses meet, they form a

front, which is the boundary that separates the two air masses.

• Fronts are usually associated with some kind of precipitation.

• They are usually narrow. • Ideally, the air masses on both side of

the front move in the same direction and at the same speed.

Frontal Collision

• In most cases, the distribution of pressure across a front causes one air mass to move faster than the other.

• When they collide, some mixing

occurs. • This can often lead to precipitation.

Types of Fronts

Fronts are often classified according to the temperature of the advancing front.

 There are eight types:1 – warm fronts2 – cold fronts3 – stationary fronts4 – occluded fronts

Warm Fronts

Forms when warm air moves into an area formerly covered by cool air.

• The slope of the front is gradual. • As the warm air rises, it cools to produce

clouds, and frequently precipitation. • The first sign of an approaching warm

front is the appearance of…… cirrus clouds.

Warm Fronts

• As the front nears, the cirrus clouds change to cirrostratus clouds, which blend into dense sheets of altostratus clouds.

• About 300 kilometers ahead of the front, thicker stratus and nimbostratus clouds appear, and rain or snow begins.

Warm Fronts

• Because of their slow rate of movement and very low slope, warm fronts usually produce light-to-moderate precipitation over a large area and for an extended period of time.

• A gradual increase of temperature occurs with the passage of the front.

• A wind shift, from east to the southwest is associated with a warm front.

Cold Fronts

• A cold front forms when cold, dense air moves into a region occupied by warm air.

• Generally, cold fronts are steeper and advance more rapidly than warm fronts do.

• Because of their steepness and rate of movement, cold fronts produce more violent weather.

Cold Fronts

• The forceful lifting of air along a cold front can lead to heavy down pours and gusty winds.

• After the cold front passes, temperatures drop and wind shifts.

• The weather behind a cold front is dominated by a cold air mass, so the weather is clears.

Stationary Front

• Occasionally, the flow of air on either side of the front is almost parallel to the line of the front.

• In such cases, the surface position of the front does not move, and a stationary front forms.

• Sometimes, gentle to moderate precipitation occurs along a stationary front.

Occluded Front

• When an active cold front overtakes a warm front, an occluded front forms.

• Weather associated with occluded fronts is complex.

• Precipitation ranges greatly depending on the fronts involved in the occlusion.

Middle-Latitude Cyclones

• These are the main weather producers in the United States. Middle-Latitude cyclones are large centers of low pressure that generally travel from west to east and cause stormy weather.

• Air moves counterclockwise and in toward the low.

• Most middle-latitude cyclones have a cold front and a warm from, extending from a central area.

• Forceful lifting causes the formation of clouds that drop heavy precipitation.

Cyclone DevelopmentHow do the cyclone develop and form?• The front forms as two air masses with

different temperatures move in opposite directions.

• Over time, the front takes on a wave shape.

• As the wave develops, warm air moves towards the poles invading an area formerly occupied by colder air.

• Meanwhile, cold air moves toward the equator.

• This changes the airflow near the surface.

Cyclone Development

• Since cold fronts move faster than warm fronts, the cold front closes in and lifts the warm air producing an occlusion.

• As the occlusion begins, the storm often gets stronger.

• Pressure at the storm’s center drops and wind speed increases.

• In winter, these conditions produce heavy snowfalls and/or blizzards.

Cyclone Development

• As more warm is forced to rise, the amount of pressure change weakens.

• In a day or two, the entire warm area is displaced.

• Only cold air surrounds the cyclone at low levels.

• The horizontal temperature difference that existed between the two fronts is gone.

• The cyclone has exhausted its energy. • Friction slows the air near the surface.

The Role of Airflow

• Airflow aloft plays an important role in maintaining cyclonic and anticyclonic circulation.

• In fact, these rotating surface wind systems are actually generated by upper level flow.

The Role of Airflow

• Cyclones can often last for a week or longer. • Surface convergence must be offset by outflow

somewhere higher in the atmosphere. • As long as the spreading out air higher up is

equal to or greater than the surface inflow, the low-pressure system can be sustained.

• There is a close relationship between cyclones and anticyclones. Generally, they occur together.

Severe Storms

There three types of severe storms and their classes.

• Thunderstorms• Tornadoes• Hurricanes

Thunderstorms

Thunderstorms form from the vertical movement of unstable warm air.

Thunderstorms:• Generate lightning and thunder• Strong gusty winds• Heavy rain• Occasionally hail

Thunderstorms

• Thunderstorms may be produced from a single cumulonimbus cloud or they may be associated with clusters of cumulonimbus clouds stretched along a cold front.

Occurrence of Thunderstorms • At any given time there are 2000

thunderstorms in progress on Earth. • About 45,000 thunderstorms take place

each day. • That means more than 1.6 million

thunderstorms occur annually. • The United States experiences about

100,000 thunderstorms each year most of which occur in the Florida, Gulf Coast region.

• The west coast has little thunderstorm activity due to the warm, moist, unstable maritime tropical air seldom penetrates this region.

Development of Thunderstorms

Thunderstorms form when warm, humid air rises in an unstable environment.

Three stages of development:

1 – Strong updrafts supply moist air and each surge of moist air causes the cloud to rise vertically.

2 – The mature stage usually occurs with in hour of the initial updraft.

The size and amount of precipitation is too great for the updrafts to support.

Heavy precipitation is released.

This is the most active stage producing gusty winds, lightning and thunder, and sometimes hail.

3 – The dissipating stage, the cooling affect of the falling precipitation and the flowing in of colder air from high above cause the storm to die down.

The life of a single cumulonimbus cell is about an hour or two, but fresh supplies of warm, humid air can generate new cells to displace those that are scattering.

Types of Thunderstorms

•Air mass thunderstorms - widely scattered, form over areas where the surface is strongly heated

Types of Thunderstorms

•Frontal thunderstorms - usually form in warm, moist air on or ahead of cold fronts.

•If they occur in lines, this line is called a squall line.

•These storms may last hours.

Which occurs 1st?

thunder or lightning

Lightning

Thunder occurs when•an area where lightning occurred that was super heated.

•The super heated air leaves quickly and the area is filled with cooler air producing the noise

Producing Lightning•The negative charges that drop from clouds are called stepped leaders. When they touch a + charged object, ( tree, another cloud, or etc.)they become grounded and the lightning bolt is released.

Producing Lightning

•In reality then, lightning travels from the ground up & from the clouds down, but at the speed of light, who can tell.

Distance calculation

•Sound travels at 1,090 ft./sec.

•Count the time between the lightning and the thunder

1 one thousand, 2 one thousand and so on

Distance calculation

•If the time is 5 seconds, then the distance was approx. 1 mile.

•2.5 seconds = 1/2 mile•1 second = 1,090 ft.

Types of lightning• Cloud to ground bolts or strikes• Cloud to cloud• Ball lightning

Heat lightning - is really a thunderstorm that is at a distance. You can not hear the thunder.

Is Ball Lightning Dangerous?

One of the first scientists to experiment with thunderstorm electricity (even before Ben Franklin) was killed by BL. In 1752, George Wilhelm Reichmann attempted to reproduce one of Franklin's thought-experiments. Lightning struck his metal mast, and witnesses said that a ball of fire flew out and struck him on the forehead, killing him instantly.

Byproducts of lightning are……….

ozoneNitrogen in the soil

Tornadoes

Tornadoes are violent windstorms that take the form of a rotating column of air called a vortex.

The vortex extends downward from a cumulonimbus cloud.

Some tornadoes consist of a ingle vortex, but within stronger tornadoes, smaller vortexes rotate with a main funnel.

Tornadoes

These smaller vortexes have a diameter of about 10 meters and rotate very rapidly.

This accounts for times when one building is destroyed and another a short distance away is untouched.

Occurrence and Development of

Tornadoes• In the U.S. about 770 tornadoes

are reported each year. • The frequency of tornadoes is

greatest between April and June, but actually can occur at any time of the year.

Tornadoes

• Most tornadoes form in association with thunderstorms.

• An important process in the formation of many tornadoes is the development of a mesocyclone.

• A mesocyclone is a vertical cylinder of rotating air that develops in the updraft of a thunderstorm.

Tornadoes

• The formation of this large vortex causes the atmosphere to roll.

• Strong thunderstorm updrafts cause this rolling air to tilt.

• Once the air is completely vertical, the mesocyclone is well established.

• Actually, few mesocyclones produce tornadoes.

Tornado Intensity

• Pressures with some tornadoes have been estimated to be as much s 10% lower than the pressures immediately outside the storm.

• The low pressure near the surface causes air to move into the tornado from all directions.

• As the air streams inward, it spirals upward until it merges with the airflow of the cumulonimbus cloud that formed the storm.

• The tremendous pressure change can produce winds in excess of 480 kilometers per hour.

Fujita scale is a measure of tornado intensity.

Tornado over water?

• Is a waterspout

Hurricanes

• Hurricanes are whirling tropical cyclones that produce winds of at least 119 kilometers per hour (75 mph).

• Hurricanes are considered the most powerful storms on Earth.  

• Hurricanes bring tremendous amounts of rainfall.

• Strong winds produce a storm surge flooding coastal areas.

• They are becoming more of a threat as more people move into and live in coastal areas.

Occurrence of Hurricanes • Most occur between 5 and 20 degrees

latitude both north and south of the equator.

• Hurricanes tend to form from tropical disturbances over ocean water.

Development of Hurricanes

• A hurricane is a heat engine that is fueled by the energy given off when huge quantities of water vapor condense.

• Hurricanes develop most often in the late summer when water temperatures are warm enough to provide the necessary heat and moisture to the air.

• The tropical disturbances resulting over this warm ocean water are disorganized.

• Their low pressures have little or no rotation.

Hurricanes

• An organized storm occurs when an inward rush of warm, moist air moves toward the core of the storm.

• The air then turns upward and rises in a ring of cumulonimbus clouds.

• This doughnut-shaped wall that surrounds the center of the storm is called the eye wall.

• Here the greatest wind speeds and heaviest rains occur.

• The outward flow of air at the top of the storm makes room of more inward flow.

• At the very center of the storm is an area of relatively quiet weather called the eye.

• The air within the eye gradually descends and heats by compression making it the warmest part of the storm.

Hurricanes

Hurricane Intensity

• Hurricane intensity is described using the Saffir-Simpson scale.

• The greatest damage is produced by storm surges.

• A Storm surge is a 65-80 kilometer wide dome of water that sweeps across the coast where a hurricane’s eye moves onto land.

Losing Energy

• As hurricanes move onto land friction with the surface slows wind speed and it can no longer pick up an adequate amount of moisture.

• Winds subside and the storm dies out.