1. Chapter 12
The Causes of Weather

2. What is Meteorology?
It is the study of the physics, chemistry, and dynamics of atmospheric phenomena.
Atmospheric phenomena are known as meteors.
Cloud droplets and precipitation (rain, sleet, snow, & hail) are known as hydrometeors; smoke, haze, dust, & other particles suspended in the air are known as lithometeors; thunder and lightning are known as electrometeors.

3. Weather vs. Climate
Short term variations in atmospheric conditions that interact and affect the environment and life on Earth is called weather. Can take place over minutes, hours, days, weeks, months, or years.
Climate is the long term average of variations in weather for a particular area.
Weather determines the clothes we wear from day to day and climate determines what crops we plant in a given region.

4. Heating Earth
Why is Miami, FL warmer in January than Detroit, Michigan?
Earth’s tilt on it axis causes Miami to receive a greater number of hours of daylight and solar radiation than Detroit.
And the Earth is round, which means different areas are at different angles.
Air & water are constantly moving around the Earth, which redistributes Earth’s thermal energy around the world. This causes our weather.

6. Air Masses
When air near Earth’s surface is heated, it rises. This causes a formation of an air mass.
It is a large volume of air that has the same characteristics, such as humidity & temperature, as its source region.
A source region is an area over which the air mass forms. Most form over tropical or polar regions.

7. Types of Air Masses Five types of air masses:
Maritime Tropical Air Mass
Maritime Polar Air Mass
Arctic Air Mass
Continental Polar Air Mass
Continental Tropical Air Mass

8. Tropical Air Masses
Maritime tropical air masses form over tropical bodies of water. In the summer, they bring hot, humid, moist air to the eastern US.
Continental Tropical air masses bring hot and dry air to the southwestern US.

9. Polar Air Masses
Maritime polar air masses form over the cold waters of the North Atlantic and North Pacific.
They can bring mild to very cold and humid air to the west and northeast coasts of the US. Usually bring heavy rain to the Pacific northwest.
Continental polar air masses form over the interior Canadian region and Alaska. They bring frigid air in the winter, and cool, dry air in the summer.

10. Arctic Air Mass
Siberia and the Arctic Basin are the source regions for arctic air masses.
During winter, these areas receive almost no solar radiation, which makes them extremely cold with frigid temperatures.
In the summer, it is very cold and dry.

11. Air masses do not stay in one place
Air masses do not stay in one place. They move and transfer thermal energy from one area to another. When they move over land or water, they can develop characteristics of that water and land.

12. End of Section 1

13. Section 2 Weather Systems

14. Global Wind Systems
Earth’s rotation on its axis causes what is known as the Coriolis Effect.
This results in fluids and objects moving in a curved pattern rather than in a straight line.
Air curves to the right in the Northern hemisphere and to the left in the southern hemisphere.
They take cold air to the warmer regions and warm air to the colder regions.
These global wind systems help equalize thermal energy on Earth.

17. Three Wind Systems Polar Easterlies Prevailing Westerlies Trade Winds
These occur in each hemisphere

18. Polar Easterlies
Between 60 N latitude and the North Pole & 60 S and the South Pole
Dense polar air that sinks
Deflected in an easterly direction away from poles
Cold, weak, and sporadic

19. Prevailing Westerlies
Between latitudes 30 N and 60 N & 30 S and 60 S.
These are midlatitudes & originate from the west
Surface winds move in a westerly direction toward the poles
Steady winds that move much of the weather across the US and Canada

20. Trade Winds Between latitudes 30 N and 30 S
Air sinks and moves toward equator in an easterly direction then warms and rises moving back towards 30 N & 30 S
These winds create Horse Latitudes – a belt of weak surface winds created by an area of high pressure.
Earth’s major deserts are located at the Horse Latitudes (Sahara Desert)

21. Intertropical Convergence Zone
ITCZ
Near the equator, trades winds meet and join causing uplift (thunderstorms)
Air is forced upward and creates an area of low pressure – convergence
This zone can vary with seasons following the position of the sun
Creates doldrums, equatorial calms, which are light ocean currents & winds; sailors dread this area because there was no wind

23. Jet Streams
At boundaries between wind zones, atmospheric conditions and events occur which influence Earth's weather.
On the sides of the boundaries, temperature & pressure vary greatly which in turn causes wind.
Jet Streams – strong westerly winds - are a result of this.
Jet streams vary with speed and position depending on the season.
Occurs mainly in the areas where there is the greatest temperature difference.

24. Types of Jet Streams
Major – polar jet streams – separate the polar easterlies from the prevailing westerlies in the northern and southern hemispheres.
Minor – subtropical jet streams – occur where the trade winds and the prevailing westerlies meet.
Storms form along jet streams
Take cold surface air toward the tropics and warm surface air toward the poles.

25. Fronts
Air masses with different characteristics can collide and result in dramatic weather changes – front – a narrow region btwn two air masses of different densities.
The density of an air mass results from its temperature, pressure, and humidity.
Fronts can form across thousands of km
Cold, Warm, Stationary, & Occluded

27. Cold Front
When cold, dense air displaces warm air, it forces the warm air, which is less dense, up along a steep slope.
Intense precipitation and thunderstorms

28. Warm Front Advancing warm air displaces cold air
Develops a gradual boundary slope
Widespread precipitation

29. Stationary Front
When two air masses meet but neither advances, the boundary btwn them stalls.
Occurs btwn air masses where there is a small temperature/pressure difference.
Light winds & widespread precipitation

30. Occluded Front
When a cold air mass moves so rapidly, it takes over a warm front and forces warm air upward.
Two cold air masses collide
Strong winds & heavy precipitation

31. Pressure Systems
Sinking air is associated with high pressure and rising air is associated with low pressure.
Air always flows from high to low pressure.
Low-Pressure Systems & High-Pressure Systems
Low – air rises; air from outside the system replaces the rising air and spirals inward toward the center & then upward.
In the Northern Hemisphere, air moves counter-clockwise and clockwise in the Southern Hemisphere.
Cloudy weather & precipitation

32. Pressure Systems
High Pressure Systems – sinking air moves away from the system’s center when it reaches Earth’s surface.
Air circulates in a clockwise direction in the northern hemisphere and a counter clockwise in the southern hemisphere.
Fair weather
In subtropical oceans with pleasant weather

33. End of Section 2