Weather Patterns and Severe Storms

20.1 Air Masses

Air Masses

• Weather patterns result from a movement of a large body of air.

• Characterized by a similar temperatures and amounts of moisture at any given altitude.

• Could take several days for an air mass to move across an area = constant weather

Movement of Air Mass

• When the air mass moves out of the region it formed in ---- it takes the temp and moisture with it.

• The air mass changes as it moves as well.

Classifying Air Mass

• Source Region – the area that an air mass gets it characteristics properties of temp and moisture.

• Polar (P) and Tropical (T) – describe temperatures

• Continental (c) and Maritime (m) – where they form and moisture characteristics

Weather in N. America

• Majority of weather is influenced by the cP and mT air masses.– Form in Alaska/N. Canada, and the Gulf of Mexico

• cP (continental polar) – uniformly cold and dry in winter, can cool and dry in summer.– Lake – effect snow = when the temp of the lake and

land are extremely different causing heave and unstable air, leads to heavy snow (precipitation)

• m(T) – warm, loaded with moisture, usually unstable– Responsible for most of the precipitation in the

eastern US– In summers brings high temps and humidity

• m(P) – originate off the coast of eastern Canada– Can bring with it heavy snow/rain (from Siberia to

western coast of US)– Nor’easter = snow and cold temperatures as low

pressure center passes, going counterclockwise

• c(T) – least influence on the weather in NA– Hot dry air masses begin in the southwestern US

and Mexico during summer– Cause extremely hot, droughtlike conditions in the

Great Plains in the summer– Indian Summer – mild weather in the Great Lake

region (unseasonably warm and mild)

20.2 Fronts

Formation of Fronts• When two air masses

meet, from a front, a boundary that separates two air masses.

• Fronts are narrow, associated with precipitation, usually acts as a barrier that travels with an air mass, classified according to the temp of the advancing front.

Warm Front

• When warm air moves into an area formerly covered by cooler air.– Gradual slope as warm air rises it produces clouds

leads to precipitation– Slow moving front– Light to moderate precipitation

• Shown by red lines and semicircles on a weather map that point towards the cooler air.

Cold Front

• Forms when cold dense air moves into a region occupied by warmer air.– Becomes a steep slope, advance rapidly– Violent weather (heavy downpours, gusty wind)– Behind a cold front weather is dominated by a

cold air mass

• Shown by a blue line with triangles that point toward the warmer air mass

Stationary Front

• When the surface position of the front does not move– The flow of air is neither toward the cold air mass

or the warm air mass– Gentle to moderate precipitation can occur

• Shown by blue triangles on one side and red semicircles on the other of the front.

Occluded Front

• An active cold front overtakes a warm front– The cold air mass wedges the warm front upward– Complex weather (due to warm air going up)– Can make its own light precipitation

• Shown on a weather map by blue triangles and red semicircles on the same side of the front line.

Middle Latitude Cyclones

• Main weather producers in the country– Centers of low pressure that generally travel from

west to east, cause stormy weather– Move in a counterclockwise direction (toward the

center of the low)– Most have a cold front – lifting causes the

formation of clouds and abundant precipitation

• Low pressure systems shown on a map by a “L”

How do they form?

1.) Development of a front (two air masses with different temps move in opposite directions)2.) Front takes on a wave shape3.) Warm air moves towards Earth’s poles, cold air moves toward the equator4.) The change in airflow near the surface is associated with a change in pressure (resulting in a counterclockwise direction)5.) Occluded front forms – storm gets stronger6.) When all the warm air is gone the friction of the air and the surface slows the airflow and it “dies”

The Role of Airflow Aloft

• More often then not, air high up in the atmosphere fuels a middle latitude cyclone.

• Usually can find anticyclones and cyclones next to each other.

20.3 Severe Storms

Thunderstorms• Vertical movement of

warm unstable air can lead to a thunderstorm.

• Generates lighting and thunder, produce gusty winds, heavy rain, and sometimes hail

• Could be a single cumulonimbus cloud or a large stretch along a cold front.

• Greatest number of thunderstorms occur in the tropics – due to warm, moisture rich, and unstable air

• Development – when warm, humid air rises in an unstable environment (lifespan about 2hrs)– 1.) Strong upward movements of warm moist air– 2.) Mature Stage – amount and size of

precipitation due to the updraft is to large to be supported (most active stage)

– 3.) Dissipating Stage – when the downdrafts dominate, cooling effect of the falling precipitation causes the storm to die

Tornadoes

• Violent windstorms that take the form of a rotation of air called a vortex that extends downward.

• Tornado Season = April through June (December and January less frequent)

• Form with severe thunderstorms

• Mesocyclone – important to form a tornado, a vertical cylinder of rotating air that develops in the updraft

Tornado Intensity and Safety

• Wind speed is measured by the Fujita Scale – winds cannot be measured directly so determined by the amount of damage done.

• Tornado Watch – the possibility of a tornado (favorable conditions)

• Tornado Warning – tornado has actually been see in the area or on the weather radar

Hurricanes/Typhoons/Cyclones

• Tropical cyclones that produce winds of at least 119 km/hr

• Most powerful storm on Earth

• Can bring to shore strong winds, storm surge, and flooding

• A higher threat now that more people live and work near the coastline

Development of a Hurricane

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

• Develop most often in the late summer when water temps are warm enough to provide heat and moisture to the air

• Tropical disturbance to a category 5 hurricane– Eye – wall – is formed and usually helps to sustain

the hurricane (strongest wind gusts are here)

Hurricane Safety

• Intensity measured by the Saffir – Simpson Scale

• Storm Surge – water that precedes the hurricane and sweeps onto the shore/land.

• Hurricanes weaken as they move over colder water (adequate heat and moisture) or land (due to friction and lack of moisture)

21.3 Climate Change

Climate Change

• Climate is always changing (short-term or long-term)

• Natural Process that change climate– Volcanic Eruptions– Ocean Circulation– Solar Activity– Earth Motions

Volcanic Eruptions

• Emits large amounts of ash and dust into the atmosphere

• Aerosols in the air increase solar radiation reflected back into the space – lowering the atmosphere temperature (Volcanic winter)

Ocean Circulation

• El Nino is the change in ocean circulation

• Short term climate fluctuations

Ex.) An area that usually has a lot of rain could experience a drought during an El Nino

Solar Activity

• Changes in the output of the solar energy

• Sunspots = warm periods in Europe and North America

• Short term Effects

• No long term variations due to solar activity exist (yet)

Earth Motions

• Most are long-term changes• Tectonic plate movement – change in position

on land and ocean• Shape of orbit and tilt of the Earth on the axis– Smaller the tilt the smaller the temperature

difference between summer and winter

Human Impact on Climate Change

• Greenhouse Effect – a natural warming of both Earth’s lower atmosphere and surface– Major gasses involved – water vapor and CO2

– Makes life possible on Earth– Human impact has increased CO2 levels in the

atmosphere (burning of fossil fuels and clearing of forest)

• Global Warming – a result of the increases in CO2 and other gases ---- global temperatures have increases

• Sea level rising, flooding, heat waves, droughts, more water vapor in the air, temperature increase across the globe.