Download Course Learning Outcomes for Unit VII Reading

UNIT VII STUDY GUIDE
The Atmosphere in Motion
and Weather Patterns
Course Learning Outcomes for Unit VII
Upon completion of this unit, students should be able to:
8. Relate how radiation and atmospheric processes control weather and climate.
8.1 Explain the role of global circulation in producing different climates.
8.2 Describe air pressure, air masses and fronts, and their effects upon weather patterns.
8.3 Discuss how atmospheric conditions produce thunderstorms, tornadoes, and hurricanes.
Reading Assignment
Chapter 13:
The Atmosphere in Motion
Chapter 14:
Weather Patterns and Severe Weather
National Severe Storms Laboratory. (n.d.-a). Severe weather 101: Thunderstorm basics. Retrieved from
http://www.nssl.noaa.gov/education/svrwx101/thunderstorms/
National Severe Storms Laboratory. (n.d.-b). Severe weather 101: Tornado basics. Retrieved from
http://www.nssl.noaa.gov/education/svrwx101/tornadoes/
National Oceanic and Atmospheric Administration. (2010a). Global weather. Retrieved from:
http://www.srh.noaa.gov/jetstream/global/global_intro.htm
National Oceanic and Atmospheric Administration. (2010b). JetStream—online school for weather. Retrieved
from: http://www.srh.noaa.gov/srh/jetstream/synoptic/synoptic_intro.htm
Unit Lesson
Have you ever wondered why deserts form in some regions
and tropical forests in others? What creates climate? Why
are some areas more prone to precipitation? In this section,
we will explore the major factors that affect climates and
weather patterns around the world.
NOAA satellite image of Hurricane Arthur, July 3,
2014. (NOAA, 2014)
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When discussing weather patterns, it is essential to first
understand air pressure, which is the pressure exerted by
the weight of air above. There are two over-riding factors
that affect air pressure and control weather and climate on
Earth. These are solar radiation, which we discussed in
Unit VI, and the spinning of the Earth (the Coriolis effect).
These two factors will create areas of high pressure and
areas of low pressure. In general, air will always move from
an area of high pressure towards areas of low pressure.
This creates air movement, or wind. Click here for more
information from the National Oceanic and Atmospheric
Administration (NOAA) web site which further details the
Origins of wind (NOAA, 2010a).
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Local winds refer to winds that are generated by small-scale differences in air UNIT
pressure.
For example,
x STUDY
GUIDE along
the coast, land heats up more quickly than water (due to water’s higher heat capacity).
Therefore, the air
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above land will heat by convection and rise, creating a low pressure area. Over water, the air will cool,
condense, and sink, creating an area of high pressure. Wind is generated as air moves from high pressure to
low pressure. Click here for an animation that shows how these winds change direction (NOAA, n.d.). A
similar phenomenon occurs in mountain valleys. The air over the mountain slope will heat more quickly than
air at the same elevation over the valley, creating an area of low pressure. Along mountain ranges, local
winds known as Chinooks or Santa Ana, will form as a result of the rain shadow (Lutgens & Tarbuck, 2014).
As we learned in Unit VI, air on the windward side of a mountain range has more moisture than the air
descending on the leeward side. This drier air will warm as it descends and form an area of high pressure.
Therefore, the warm, dry air will flow towards the moister air on the windward side.
Larger wind patterns form high in the atmosphere, due to differences in net radiation. This creates large areas
of high and low pressure that are fairly stable and predictable. These pressure differences create strong
winds high in atmosphere that circulate around the Earth. These winds are referred to as the Jet Stream, and
largely influence patterns of weather. Click here for NOAA's (2010b) Online School for Weather for more
information and a graphical representation of the Jet Stream.
How do these jet streams form and how do they impact the world’s climates and weather patterns? Well,
remember that everything boils down to the flow of energy. Energy will always move from a state of high
energy (high solar radiation) to a state of low energy (low solar radiation). Since equatorial regions have high
net radiation, this energy will move towards to the poles (where net radiation is negative). How does this
happen? This transfer of energy (heat) happens both in the ocean and in the atmosphere. In Unit V, we
studied how the ocean gyres transferred warm water from the equator toward the polar regions (and how cold
water travelled back towards the equator). We learned how this helps to moderate temperatures around the
world. A similar pattern happens in the atmosphere. As air is warmed, it expands and becomes less dense.
Because it is less dense, it rises. When it reaches a certain point, it will cease to rise. As more air rises
beneath it, it forces that air to travel horizontally (towards either the North or South Poles). Eventually, this air
cools to the point that it will once again sink towards the Earth’s surface. This air is then pushed by the air
behind it to return to the starting point, forming a cycle of air movement.
Of course, this is a very simplified explanation of the Earth’s air circulation. If the Earth were not rotating, we
would see warm air rise at the equator, travel to the poles, then cool, and sink to return to the equator near
the Earth’s surface. However, the Earth is constantly spinning, creating what is called the Coriolis Effect. We
briefly discussed this in Unit V, as this effect will cause water to move in a clockwise direction in the Northern
Hemisphere and a counter-clockwise direction in the Southern Hemisphere.
In the atmosphere, the Coriolis Effect creates smaller cells of air circulation (see Figure 13.17) Click here for
more information about global circulation (NOAA, 2010c). How do these cells create world climatic
conditions? First, let’s discuss the area near the equator. Keep in mind that this area receives the most net
radiation. As the air warms and rises, it creates an area of low pressure. This is referred to as the intertropical
convergence zone (ITCZ) (NOAA, 2010d). As this warm, moist air rises and cools, clouds and precipitation
form, which makes the tropical region very wet. This air begins to sink again around 20-30 degrees latitude
(North and South), creating a high pressure system called the Subtropical High. This air is very dry (having
spent all of its moisture in the ITCZ), which explains why so many of the World’s deserts are found in these
regions. This sinking air will then be pushed either North or South, where it either returns to the ITCZ or
reaches about 60 degrees latitude. In both cases, it gains heat and moisture as it passes over land and sea,
and once again rises to form an area of low pressure and precipitation.
These cells of circulation represent the general movement of air, and explain why you see areas of high and
low air pressure. Keep in mind that the tilt of the Earth causes seasonal changes in solar radiation, which will
cause the ITCZ to move as much as 20 degrees, either North or South. This accounts for the wet and dry
seasons that occur in the equatorial region. The differential heating of land and water will also affect air
pressure over continents and oceans. Click here for a summary of the world’s climates and where they are
found (NOAA, 2010e).
Around the globe, there will form areas of air that have fairly uniform temperature and moisture conditions.
These are known as air masses (NOAA, 2010f). The movement of these air masses can have a significant
impact on a region’s weather. Air masses have a source region where they form. These source regions are
generally area where the climate is fairly stable—like tropical regions or polar regions. As they move from
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their source area, air masses can bring a change in weather to other regions. UNIT
Thesex air
masses
are largely
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responsible for the wet humid conditions in the Southeastern United States and
the winter snows on the
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Northern United States. The boundaries of these air masses are known as fronts and mark the changes in
weather patterns. Where a tropical air mass moves into an area, it is referred to as a warm front. A polar or
arctic air mass will bring a cold front. A warm front is generally slower moving and is less dense than cold
front, which can move in quickly and force the warm front upwards. The collision of fronts often brings clouds
and precipitation, as the warm moist air is forced upwards, where the moisture will condense to form clouds.
When a warm front and cold front collide along the jet stream, it can form an area of low pressure, which can
affect very large areas. These are known as cyclones. Refer to this NOAA diagram to see how they form
(NOAA, 2010g). These cyclones are often responsible for the formation of thunderstorms and, occasionally, a
tornado.
Severe weather is a term to describe thunderstorms, tornadoes, and hurricanes. Thunderstorms are the most
common. Thunderstorms form when warm, humid air rises in an unstable environment. Generally, in order for
a thunderstorm to form, there must be some sort of trigger to force the air up. Most thunderstorms form in the
southeastern United States. This is largely due to the subtropical climate of the area, providing plenty of heat,
moisture, and instability. However, you will also notice that a small area just east of the Rocky Mountains also
has a high number of thunderstorms. Given that this is an arid climate (on the leeward side of the Rocky
Mountains), why would this be an area of high thunderstorm activity? During the summer, a maritime air mass
moves up to the mid-latitudes of the eastern half of the United States. There is also a continental polar air
mass that moves down along the Rocky Mountains. Where these two air masses collide is where you see this
unusually high rate of thunderstorms.
Tornadoes and hurricanes are some of the most destructive weather events on Earth. Tornadoes are
vortexes of air that form around extremely low pressure centers. Because of the difference in pressure
between the center and outside the cell, winds can be extremely strong, up to 480 km per hour! Tornadoes
form from severe thunderstorms and usually occur in areas where two air masses collide. This is why the
central United States is more prone to tornadoes—where the maritime tropical air meets the continental polar
air mass. Tornadoes can form very quickly, travel fast, and are very unpredictable. Hurricanes also form
where there are extremely low pressure centers—over warm ocean waters. Unlike tornadoes, hurricanes take
time to form and travel quite slowly in a very predictable path. Because hurricanes need warm water and lots
of moisture to form, one would predict that most hurricanes form around the equator. While they do form in
this region, hurricanes cannot form right at the equator because there is no Coriolis Effect. It is the cycling of
air that forms the hurricane, and this can only happen where the spin of the Earth causes air to move either
clockwise or counter-clockwise (above 5 degrees latitude). Hurricanes are fueled by warm ocean waters. This
NOAA video demonstrates how hurricanes form and are sustained (NOAA, 2013).
The Earth’s weather is extremely dynamic. Even with complex computer models, satellite imagery, and the
latest weather monitoring devises, meteorologists still face a certain amount of uncertainty in forecasting
weather. In Units VI and VII, you get a brief overview of the many interacting factors that are responsible for
the climates and weather patterns we see. How has this information helped you understand your local
weather and climate?
References
Lutgens, F. K., & Tarbuck, E. J. (2014). Foundations of Earth Science (7th ed.). Upper Saddle River, NJ:
Pearson.
National Oceanic and Atmospheric Administration. (n.d.). NST interactive: Land and sea breezes combined.
Retrieved from http://oceanservice.noaa.gov/education/pd/oceans_ weather_climate/
media/sea_and_land_breeze.swf
National Oceanic and Atmospheric Administration. (2010a). Origin of wind. Retrieved from
http://www.srh.noaa.gov/srh/ jetstream/synoptic/wind.htm
National Oceanic and Atmospheric Administration. (2010b). The jet stream. Retrieved from
http://www.srh.noaa.gov/ jetstream/global/jet.htm
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National Oceanic and Atmospheric Administration. (2010c). Global circulations.
Retrieved
from
UNIT
x STUDY
GUIDE
http://www.srh.noaa.gov/ jetstream/global/circ.htm
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National Oceanic and Atmospheric Administration. (2010d). Intertropical convergence zone. Retrieved from
http://www.srh.noaa.gov/jetstream/tropics/itcz.htm
National Oceanic and Atmospheric Administration. (2010e). Climate. Retrieved from http://www.srh.noaa.gov/
jetstream/global/climate.htm
National Oceanic and Atmospheric Administration. (2010f). Air masses. Retrieved from
http://www.srh.noaa.gov/ srh/jetstream/synoptic/airmass.htm
National Oceanic and Atmospheric Administration. (2010g). Norwegian cyclone model. Retrieved from
http://www.srh.noaa.gov/srh/jetstream/synoptic/cyclone.htm
National Oceanic and Atmospheric Administration. (2013). NOAA ocean today: Fuel for the storm [Video file].
Retrieved from https://www.youtube.com/watch?v=9-_obMEF_2o
Suggested Reading
The links below will direct you to both a PowerPoint and PDF view of the Chapter 13 Presentation. This will
summarize and reinforce the information from this chapter in your textbook.
Click here to access the Chapter 13 PowerPoint Presentation. (Click here to access a PDF version of the
presentation.)
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