1. Weather Patterns and Severe Storms

2. 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

3. 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.

4. 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

6. 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)

7. 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

8. 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, drought like conditions in the Great Plains in the summer
Indian Summer – mild weather in the Great Lake region (unseasonably warm and mild)

9. FRONTS
20.2 Fronts

10. 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.

11. 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.

13. 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

15. 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.

17. 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.

19. 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”

20. 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 shape 3.) Warm air moves towards Earth’s poles, cold air moves toward the equator 4.) 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 stronger 6.) When all the warm air is gone the friction of the air and the surface slows the airflow and it “dies”

22. 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.

23. Severe Storms
20.3 Severe Storms

24. 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.

25. 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

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

27. 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

28. 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

30. 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

31. 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)

33. 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)

35. Climate Change
21.3 Climate Change

36. Climate Change Climate is always changing (short-term or long-term)
Natural Process that change climate
Volcanic Eruptions
Ocean Circulation
Solar Activity
Earth Motions

37. 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)

38. 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

39. 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)

40. 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

41. 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)

43. 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.