air masses and severe weather

Air Masses and Severe Weather

Integrated Science

Air Mass

Huge volumes of air that can cover entire continents or oceansSeveral types that each have specific characteristics see text p. 689

Atmospheric LiftingClouds are indicators of weatherFor clouds to form, air must be liftedThree types of lifting cause clouds.

Lifting Type #1Convectional LiftingEarth’s surface is heated by the sun, which heats the air mass above it. The heated air will then rise.

Convectional Lifting

Convectional Lifting

If cooling occurs near the air mass’ saturation temperature, often cumulus clouds may form

Convectional LiftingOften happens in the mid-west during summer

Atmospheric Lifting Type #2

Orographic LiftingAn air mass is pushed upward over an obstacle, such as a mountain range.

Atmospheric Lifting Type #2Orographic

As the air mass is pushed over the obstacle, the air mass will lose moisture in the form of clouds, rain, or snow

Atmospheric Lifting Type #2

Atmospheric Lifting Type #2

Atmospheric Lifting Type #2

The clouds dump their load of rain on the windward slopeThe down slope (leeward) air will be dry = rain shadow

Rain shadow- orographic lifting

Rain shadow Western Kansas is so dry only certain short grasses can grow; not enough rain for trees

Rain Shadow; Big Hatchet Mt, New Mexico. Notice how dry the land is

Orographic Lifting – Chinook Winds

At the high elevations of the top of a mountain, the air pressure is low.As the air descends the mountain is under more pressure.The increased air pressure heats the air causing warm winds called Chinook WindsTemperatures can rise 20-300 as air moves down the mountain

Chinook Winds

Atmospheric Lifting Type #3

Frontal LiftingThis occurs at the mesh point between two different temperature air masses.One air mass (warm) is pushed upward by the other (cold).This can be associated with thunderstorms.

Frontal LiftingThree types of frontal liftingA) warm frontB) cold frontC) Stationary front

Frontal LiftingCold Front: When a cold air mass moves into a warm air mass.Cold, denser air forces the warm air up where it cools and condenses, forming clouds

Cold Front:Warm air is abruptly pushed upward, cooling, condensing moisture into cumulus or cumulonimbus clouds

Cold FrontNotice how

steep the angle is between the two air massesTypically brings sudden, heavy

rains and storms

Cumulonimbus: Lake Superior

Warm FrontWARM FRONT: when a warm air mass moves into a colder,denser air mass.Warm air rides up and over the colder air

Warm front

Warm FrontNotice the angle of slope between the two air masses.

Warm FrontFrom the ground, one would observe the following cloud sequenceA) cirrusB) cirrostratusC) altostratusD) stratusE) nimbostratus

Warm FrontThe weather during a WARM FRONT starts with cirrus clouds about 24-48 hours before the rain beginsCirrus clouds are “at the front of the front”

Warm FrontAs more warm air is pushed upward, more moisture condenses forming cirrostratus clouds

Cirrostratus on Lake Superior

Warm FrontAs warm more warm air is pushed up, heavier clouds form mid-way up over the cold airAltostratus and stratus

Altostratus clouds: Lake Superior

Warm FrontThe final cloud type in a warm front is the nimbostratus“nimbo” = “rain”

Warm FrontWarm front: rain or snow is steady over several hours or days

Quiz Yourself!!! T or F

QUIZ: T or F

For clouds to form, air must be liftedThere are FOUR different types of liftingOrographic lifting refers to air moving up and over a mountain

QUIZ: T or F

Convectional lifting is the circulation of warm air rising, cold air sinkingA rain shadow is caused by WET air descending down the leeward side of a mountainA WARM FRONT means warm air is riding up over cold airA WARM FRONT produces sudden, severe rain and storms

Severe Weather

Thunderstorms Cold FrontsBegins with humid air rising, cooling, and condensing into a single cumulus cloud.Cloud builds as they are “fueled” by warm, moist air from below.

Thunderstorms

The droplets of water grow larger until they are so big that they fall as rain.

Cumulonimbus clouds: L Superior

Thunderstorms (con’t)Lightning may be associated with T.stormsHits Earth 100 times per secondOver 200 people in the US die each year from lightning.

LightningLightning is the cause of thunderreturns nitrogen to the soilmay cause fires May have helped start pre-biotic formation of amino acids(?)

Lightning FormationDuring thunderstorms strong updrafts cause molecules to bump together and their charges to separateNegative charges concentrate at the base of the clouds

Lightning FormationThe ground has lost electrons, giving it a positive (+) chargeOpposites attract: The (-) from the cloud base is “pulled” to the (+)First downward movement of (-) is called a leader

Lightning FormationWhen the (-) and (+) touch, they create a conductive path to the ground.The other (-) rush down this path = lightning

Lightning on L.Superior

Lightning on L.Michigan

Lightning strike

Quiz Yourself!! TRUE or FALSE

TRUE or FALSE?????

Cumulonimbus clouds can become thunderstormsStratus clouds produce sudden, severe thunderstorms and lightningLightning forms because of unequal charges

Tornadoes

A funnel shaped cloud that extends downward from a cumulonimbus cloud.Called a tornado only once it hits the ground.Winds inside travel at up to 800km per hour in counterclockwise direction.

Hurricanes

Caused by a warm, vertical wind interacting with an existing storm and causing the storm to turn inward and spiral.Wind speeds of up to 300km per hour.Hurricanes grow as more moist air rises into it.Severe winds rotate around a calm, low pressure zone in the middle, which is the eye of the storm.