Ch. 3: Weather Patterns

Sect. 1: Air Mass & Fronts

Sect. 2: Storms

Sect. 3: Predicting the Weather

Sect. 4: Weather forecasters use

advanced technologies

Ch. 3 Weather Fronts and Storms

Objective(s) • 7.E.1.3 - Explain the relationship between the movements of air

masses; high and low pressure systems, and frontal boundaries to

storms (including thunderstorms, hurricanes, and tornadoes) and

other weather conditions that may result.

• 7.E.1.4 - Predict weather conditions and patterns based on

information obtained from:

– • Weather data collected from direct observations and

measurement (wind speed and direction, air temperature,

humidity and air pressure)

– • Weather maps, satellites and radar

– • Cloud shapes and types and associated elevation

Learners Objective(s) • “You will understand that…storms are the

result of interactions between moving air

masses, high and low pressure systems

and frontal boundaries.“

• Essential Vocabulary

• -Air mass

• -Front

• -Pressure systems

• -Weather patterns

*remaining vocabulary terms located at mrspruillscience.weebly.com

I. Sect. 3.1: Weather Changes as air masses move

• A. Air masses are large bodies of air

• Air mass – is a large volume of air in which

temperature, humidity and air pressure are nearly the

same in different locations at the same altitude

• An air mass forms when the air over a large region of

Earth sits in one place for many days

• Where Earth’s surface is cold, the air becomes cold

• Where Earth’s surface is wet, the air becomes moist

• As an air mass moves, it brings its temperature and

moisture to new locations

B. Characteristics of an Air mass

– Air masses are classified by 2 characteristics

1. Humidity

2. Temperature

• A hot desert produces dry hot air masses, while

cool ocean waters produce moist, cool air masses

• Each category name is made of two words – one

for moisture, one for temperature

• First word tells whether the air mass formed over

water or dry land. The 2nd word tells whether an air

mass formed close to the equator (temperature)

– The characteristics of an air mass depend on the

temperatures and moisture content of the region over

which the air mass formed.

o Continental: air masses formed over land

Air becomes dry as it loses its moisture to the dry land below it

o Maritime: air masses formed on oceans or seas

Air becomes moist as it gains water vapor from the water below it

o Tropical: warm, air masses formed in the tropics

Air becomes warm as it gains energy from the warm land or water

o Polar: cold, air masses formed north or south of 50º latitude

Air becomes cool as it loses energy to the cold land or water

– The colder the air the higher the air pressure subsequently the hotter the air the lower the air pressure.

• Cold air more dense

• Hot air less dense

• Types of Air masses – There are 4 major types of air masses that affect the weather of

the U.S.

o Maritime tropical - air mass that is moist and warm

o Continental polar - air mass that is dry and cold

o Maritime polar - air mass

that is moist and cold

o Continental tropical - air

mass that is dry and warm

C. Movement of an Air Mass

- 2 primary methods for air mass

movement 1. Prevailing Westerlies

– Pushes air masses from west to east.

2. Jet streams

– Pushes fast moving air masses from west to east.

• When air masses move to a new region, it carries

along its characteristic moisture and temperature

• As the air moves over Earth’s surface, the

characteristics of the surface begin to change the

air mass

http://www.youtube.com/watch?v=vPC5i6w3yDI&feature=related

• Fronts are the boundary between two air

masses.

• Storms & different types of weather phenomena occur

along fronts.

– Air masses do not easily mix with each other due to

the differences in…

1. Density (Air pressure)

2. Temperature

3. Moisture content

D. Weather changes where air masses meet

• When a new air mass moves over your area, you can

expect the weather to change

• Weather near a front can differ from the weather

inside the rest of an air mass

• Clouds can form in this rising air

• Types of Fronts • Cold front - a cold dense air mass that pushes

warmer air upward

– Occurs when a fast moving cold air mass overtakes a

slower moving warm air mass.

o Can move into regions quickly

o Often produce tall cumulonimbus clouds and

precipitation

o Brief, heavy storms are likely and after it, the air is cooler

and often very clear

2. Warm front - warm air masses that push colder air upward – Occurs when a fast moving warm air mass

overtakes a slow moving cold air mass.

o Produces cloud-covered skies – high cirrus and stratus

and low stratus clouds

o Often brings many hours of steady rain or snow and as it

passes the air is warmer

3. Stationary front - occur when air masses first meet or when a cold or warm front stops moving – Occur when a cold and warm air mass meet but

neither can move the other.

– A wide variety of weather can be found along a stationary front including clouds, prolonged precipitation, fog, and storms.

o Produces clouds that cover the sky, sometimes for days

at a time

4. Occluded front – Occurs when a warm air mass is caught between

2 cooler air masses.

– A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass.

C. High-Pressure Systems

• Letter H represents high-pressure systems or

highs

• Letter L represents low-pressure systems or lows

• Each center is the location of the highest or lowest

pressure in a region

• At a high-pressure center, air sinks slowly down.

As the air nears the ground, it spreads out toward

areas of lower pressure

• High pressure system – is formed when air moves

all the way around a high-pressure system

o Are often large and change slowly

o When one stays in one location for long time, air mass

may form – warm or cold, moist or dry

o Often brings clear skies and calm air or gentle breezes

F. Low-Pressure Systems

• Low-pressure system – is a large weather system

that surrounds a center of low pressure

o It begins as air moves around and inward toward the

lowest pressure and then up to higher altitudes

o Rising air produces stormy weather

o In the northern hemisphere, the air in a low-pressure

system circles in a counterclockwise direction

http://www.youtube.com/watch?v=3HAth0D0Pt4&feature=related

Pressure System Weather

II. Section 3.2 – Low-pressure systems can

become storms

A. Hurricanes form over warm ocean water

• Near the equator, warm ocean water provides the

energy that can turn a low-pressure center into a

violent storm

• Tropical storm – is a low-pressure system that

starts near the equator and has winds that blow at

40mph or more

• Hurricane – is a tropical low-pressure system with

winds blowing at speeds of 74 mph or more

o Are called typhoons or cyclones when they form over the

Indian Ocean or the western Pacific Ocean

B. Formation of Hurricanes

• Energy from warm water is necessary for a low-

pressure center to build into a tropical storm and

then into a hurricane

• Tropical storms generally move westward with the

trade winds

• As long as the storm stays over warm water it can

grow bigger and more powerful

• Once the hurricane moves over land or cooler

water, it loses its source of energy

• Eye – at the center of a hurricane that is a small

area of clear weather (about 20-50 kilometers/10-

30 miles in diameter)

http://www.youtube.com/watch?v=HJydFJORWf4&feature=related

Anatomy of A Hurricane

• C. Effects of Hurricanes • In the space below, list several effects that a

hurricane can have – Lift cars

– Uproot trees

– Tear the roofs off buildings

– May produce tornadoes

– Can cause river banks to overflow and flood nearby areas

• Storm Surge http://www.youtube.com/watch?v=gd8WiiXNxho&feature=relmfu

– Huge mass of ocean water.

– Sea levels rise several meters, backing up rivers and

flooding the shore

– Can be destructive and deadly Above the storm http://www.youtube.com/watch?v=eT5K6FR_eVs&feature=related

http://www.youtube.com/watch?v=4f45jA5UxB0

How hurricanes are formed!!!

• Stages of Hurricane Development 1. Stage 1: Tropical disturbance; 10-23 mph

2. Stage 2: Tropical depression; 23-39 mph

3. Stage 3: Tropical storm; 40-73 mph

4. Stage 4: Hurricane; 74 mph

• The Saffir-Simpson scale – Scale used to determine the severity of a hurricane.

• Category 1: wind speed 74-95 mph; storm surge 4-5 feet.

• Category 2: wind speed 96-110 mph; storm surge 6-8 feet.

• Category 3: wind speed 111-130 mph; storm surge 9-12 feet.

• Category 4: wind speed 131-155 mph; storm surge 13-18 feet.

• Category 5: wind speed 155+ mph; storm surge 18+ feet – Hurricane Katrina was a category 3 hurricane when it made

landfall near New Orleans, La on August 29, 2008.

» Costliest natural disaster

» 6th strongest to form, 3rd strongest to make landfall

» 1 of the 5 deadliest

– Hurricanes

• Tropical cyclone (low pressure) that typically

measures 300-500 miles across with winds from

70-200 mph.

• Comes from the West Indian word Huracan or “big

wind.”

• Called Typhoons when formed in the Pacific

Ocean;

– Chinese word, Táifēng or “great wind.”

• Hurricanes are named by the World Meteorological

Organization.

• Guided or directed by the Trade winds.

• Can only form over water that is at least 80ºF.

• Typically forms during the months of late July to

early October.

D. Winter Storms produce snow and ice

• Most severe winter storms in the US are part of low-

pressure systems

• The systems that cause winter storms are formed

when two air masses collide

• Blizzards

– Are blinding snowstorms with winds of at least 35 mph

and low temperatures (usually below 20 degrees F)

– Occur in many parts of the northern and central US

– Wind and snow can knock down trees and power lines

– Water pipes can freeze

• Lake-effect Snowstorms

– Heavy snow fall in the areas just east and south of

the Great Lakes

– Cold air from the NW gains moisture and warmth as it

passes over the Great Lakes

– Over cold land, the air cools again and releases the

moisture as snow

http://www.youtube.com/watch?v=pQeWSl3laXI

• Ice Storms

o Cold rain freezes as it touches the ground and other

surfaces with a heavy, smooth ice

o Ice-covered roads become slippery and dangerous

o Drivers may find it hard to steer and to stop their cars

o Branches or even whole trees may break from the weight

of ice

o Falling branches can block roads, tear down power and

telephone lines, etc

o Damage from ice storms can sometimes shut down

entire cities