Section 17.1 NotesSection 17.1 NotesWeather changes as air Weather changes as air

masses movemasses move

Air Masses Are Bodies of AirAir Masses Are Bodies of Air

• Air Mass Air Mass – A large volume of air in which – A large volume of air in which temperature and humidity are nearly the same in temperature and humidity are nearly the same in different locations at the same altitude.different locations at the same altitude.

• An air mass forms when the air over a large An air mass forms when the air over a large region of Earth sits in one place for many days. region of Earth sits in one place for many days. The air gradually takes on the characteristics of The air gradually takes on the characteristics of the land or water below it.the land or water below it.

• Earths surface can affect the air on Earth.Earths surface can affect the air on Earth.

The Four Types of Air The Four Types of Air MassesMasses

• ContinentalContinental air masses form over land. Air air masses form over land. Air becomes dry as it loses moisture.becomes dry as it loses moisture.

• MaritimeMaritime air masses form over water. Air air masses form over water. Air becomes moist as it gains water vapor.becomes moist as it gains water vapor.

• TropicalTropical air masses form near the equator. Air air masses form near the equator. Air becomes warm as it gains energy from the becomes warm as it gains energy from the warm land or water below.warm land or water below.

• PolarPolar air masses form far from the equator. air masses form far from the equator. Air becomes cool as it loses energy from the Air becomes cool as it loses energy from the cold land or water below.cold land or water below.

The Four Types of Air The Four Types of Air MassesMasses

Movement of an Air MassMovement of an Air Mass

• Air masses can travel away from the regions Air masses can travel away from the regions where they form. They move with the global where they form. They move with the global pattern of winds.pattern of winds.

• When an air mass moves to a new region, it When an air mass moves to a new region, it carries along its characteristic moisture and carries along its characteristic moisture and temperature.temperature.

Cold FrontCold Front• FrontFront – A boundary – A boundary

between air masses.between air masses.• A cold front forms when A cold front forms when

a cold air mass pushes a a cold air mass pushes a warm air mass and warm air mass and forces the warm air forces the warm air mass to rise.mass to rise.

• Cold fronts often Cold fronts often produce cumulonimbus produce cumulonimbus clouds, which can cause clouds, which can cause heavy storms.heavy storms.

Cold Front

Warm FrontWarm Front• A warm front forms when a A warm front forms when a

warm air mass pushes a cold warm air mass pushes a cold air mass and the warm air air mass and the warm air rises slowly over the cold air.rises slowly over the cold air.

• As a warm front approaches, As a warm front approaches, you may first see high cirrus you may first see high cirrus clouds, then stratus clouds, clouds, then stratus clouds, then lower stratus clouds.then lower stratus clouds.

• Warm fronts often brings Warm fronts often brings many hours of steady rain or many hours of steady rain or snow.snow.

Warm Front

Stationary FrontStationary Front• A stationary front A stationary front

occurs when 2 air occurs when 2 air masses push against masses push against each other without each other without moving. moving.

• A stationary front A stationary front becomes a warm or becomes a warm or cold front when one air cold front when one air mass advances.mass advances.

Stationary Front

Weather FrontsWeather Fronts

High Pressure SystemsHigh Pressure Systems• Formed when air Formed when air

moves all the way moves all the way around a high-around a high-pressure center. pressure center.

• Air sinks and spreads Air sinks and spreads out towards locations out towards locations with low pressure.with low pressure.

• Generally brings clear Generally brings clear skies and calm air or skies and calm air or gentle breezes.gentle breezes.

Low Pressure SystemsLow Pressure Systems

• A large weather system A large weather system that surrounds a center that surrounds a center of low pressure.of low pressure.

• Air circles and moves Air circles and moves upward.upward.

• The rising air produces The rising air produces stormy weather.stormy weather.

Section 17.2 NotesSection 17.2 NotesLow-pressure systems can Low-pressure systems can

become stormsbecome storms

Hurricanes form over warm Hurricanes form over warm ocean waterocean water

• Tropical storm – a low-pressure system that Tropical storm – a low-pressure system that starts near the equator and has winds that blow starts near the equator and has winds that blow at 65 km per hour.at 65 km per hour.

• Hurricane – a tropical low-pressure system with Hurricane – a tropical low-pressure system with winds blowing at speeds of 120 km per hour.winds blowing at speeds of 120 km per hour.

Formation of HurricanesFormation of Hurricanes

• Hurricanes most often strike between the Hurricanes most often strike between the months of August and October.months of August and October.

• Ocean water has to be 80Ocean water has to be 80ooF or higher for a F or higher for a hurricane to form.hurricane to form.

• Tropical storms and hurricanes normally move Tropical storms and hurricanes normally move west due to the trade winds.west due to the trade winds.

• Hurricanes die out when they move over land.Hurricanes die out when they move over land.

Formation of HurricanesFormation of Hurricanes

• The center (eye) of a hurricane is calm.The center (eye) of a hurricane is calm.• The eye wall (surrounds the eye) has heavy rain The eye wall (surrounds the eye) has heavy rain

and tremendous winds.and tremendous winds.

Effects of HurricanesEffects of Hurricanes

• Hurricanes can lift cars, uproot trees, and tear Hurricanes can lift cars, uproot trees, and tear the roofs off buildings due to strong winds.the roofs off buildings due to strong winds.

• Heavy rain can also cause severe flooding.Heavy rain can also cause severe flooding.• Storm surge – A rapid rise in water level in a Storm surge – A rapid rise in water level in a

coastal area that occurs when a hurricane coastal area that occurs when a hurricane pushes a high mass of ocean water, often pushes a high mass of ocean water, often leading to flooding and widespread destruction.leading to flooding and widespread destruction.

Winter storms produce snow Winter storms produce snow and iceand ice

• Winter storms are formed when 2 air masses Winter storms are formed when 2 air masses collide (a cold, dry, dense air mass and a moist collide (a cold, dry, dense air mass and a moist air mass).air mass).

• Blizzards – Blinding snowstorms with winds at Blizzards – Blinding snowstorms with winds at least 56 km per hour and low temperatures.least 56 km per hour and low temperatures.

• Lake-Effect Snowstorms – Cold air gains Lake-Effect Snowstorms – Cold air gains moisture and warmth as it passes over a lake moisture and warmth as it passes over a lake and then moves over cold land where it cools and then moves over cold land where it cools again and snows.again and snows.

Winter storms produce snow Winter storms produce snow and iceand ice

• Ice Storms – When rain falls onto freezing-cold Ice Storms – When rain falls onto freezing-cold ground and covers everything with heavy, ground and covers everything with heavy, smooth ice.smooth ice.

Section 17.3 NotesSection 17.3 NotesVertical air motion can cause Vertical air motion can cause

severe stormssevere storms

Thunderstorms form from rising Thunderstorms form from rising moist airmoist air

• Thunderstorm – A storm with lightning and Thunderstorm – A storm with lightning and thunder.thunder.

• Can form at a cold front or within an air mass.Can form at a cold front or within an air mass.

Formation of Thunderstorms: Formation of Thunderstorms: Step 1Step 1

• Rising humid air forms a cumulus cloud. The Rising humid air forms a cumulus cloud. The water vapor releases energy when it condenses water vapor releases energy when it condenses into cloud droplets. This energy increases the into cloud droplets. This energy increases the air motion. The cloud continues building up air motion. The cloud continues building up into the tall cumulonimbus cloud of a into the tall cumulonimbus cloud of a thunderstorm.thunderstorm.

Formation of Thunderstorms: Formation of Thunderstorms: Step 2Step 2

• Ice particles form in the low temperatures near Ice particles form in the low temperatures near the top of the cloud. As the ice particles grow the top of the cloud. As the ice particles grow large, they begin to fall and pull cold air down large, they begin to fall and pull cold air down with them. This strong downdraft brings heavy with them. This strong downdraft brings heavy rain or hail – the most severe stage of a rain or hail – the most severe stage of a thunderstorm.thunderstorm.

Formation of Thunderstorms: Formation of Thunderstorms: Step 3Step 3

• The downdraft can spread out and block more The downdraft can spread out and block more warm air from moving upward into the cloud. warm air from moving upward into the cloud. The storm slows down and ends.The storm slows down and ends.

Effects of ThunderstormsEffects of Thunderstorms

• Flash floods – heavy rainfall in the same area Flash floods – heavy rainfall in the same area that can cause rivers to overflow their banks.that can cause rivers to overflow their banks.

• Winds – winds as high as 170 mi/hr that can Winds – winds as high as 170 mi/hr that can knock down entire forests.knock down entire forests.

• Hail – can wipe out entire fields, damage roofs, Hail – can wipe out entire fields, damage roofs, and kill livestock.and kill livestock.

• Lightning – damage power lines and equipment, Lightning – damage power lines and equipment, cause forest fires, and kill people.cause forest fires, and kill people.

Tornadoes form in severe Tornadoes form in severe thunderstormsthunderstorms

• Tornado – A violently rotating column of air Tornado – A violently rotating column of air stretching from a cloud to the ground.stretching from a cloud to the ground.

• More tornadoes occur in North America than More tornadoes occur in North America than anywhere else in the world.anywhere else in the world.

• Occur when warm air masses meet cold, dense Occur when warm air masses meet cold, dense air and form thunderstorms normally in the air and form thunderstorms normally in the central plains of the United States.central plains of the United States.

Effects of TornadoesEffects of Tornadoes

• Powerful winds can pick up or destroy Powerful winds can pick up or destroy everything in the path of a tornado.everything in the path of a tornado.

• Most tornadoes are small and last only a few Most tornadoes are small and last only a few minutes.minutes.

• Larger tornadoes are less common but have Larger tornadoes are less common but have stronger winds and last longer.stronger winds and last longer.

• The path of a tornado is unpredictable.The path of a tornado is unpredictable.