Download Geography 1000

Salt Lake Community College
Meteorology 1010
J. Allred, Adjunct
Support for Quiz #7
Students: Quiz #7 includes topics from Chapter 7 and Chapter 8. To help you keep track of where these topics
are found in the textbook I have included page numbers for some topics.
Notice that the notes to some questions can be helpful for other questions as well.
Notes to Question #1: Page 222 describes an “air mass” being as large as a continent or part of an ocean
basin, possibly 1,000 miles across. If the region is dry and cold, then the air mass above it is also dry
and cold.
Large continents and large oceans produce persistent climate conditions that cause overlying air masses
to take on similar features. High-latitudes exhibit colder water, therefore colder air. Tropical regions
show high humidity caused by solar energy that evaporates and holds vast amounts of vapor.
Most of the time these masses are stable or even ‘stagnant’.
Oceans are almost always more humid than adjacent land areas. Jet streams tend to push these air
masses along, causing brief weather changes. Jet streams are sometimes called the “storm track”
because high-speed wind at high altitude tends to draw lower elevation air along with it.
When air masses briefly move to other areas, rapidly changing weather can sometimes be severe. The
interior of major continental land masses depends on moisture being delivered by wind from humid
oceans. For instance, Utah would be extremely dry were it not for jet streams bringing maritime (ocean)
moisture from the coast and/or from the Gulf of Mexico when air masses move. Florida would
experience drought were it not for maritime tropical air masses that deliver warmth and humidity.
Florida also experiences occasional frost when Artic air invades from Canada.
Notice that in Figure 8-3(a) and (b) on page 223 that the United States really has no well-developed
continental air mass of its own, but frequently hosts invasions of air masses from elsewhere. The result
is widely varying weather and some of the world’s worst storms.
1. Air masses have ‘personalities’ or persistent traits that derive from the characteristics of surface
conditions of the ‘source region’. Air masses have to be stable or even ‘stagnant’ long enough to pick
up those ‘source region’ traits. For instance, air masses may form over any of these:
-
dry land
water, cold or warm
high-latitude cold air
low latitude wet & warm air
etc.
True ___
False ___
Notes to Question #2: Based on class discussion and the textbook on page 204 and 206, westerly winds
tend to move from west  east. High-altitude jet streams also move west to east. Utah is far from the
ocean but still receives most of its stormy weather from the west coast. As shown on page 204, Utah
sometimes receives moistures from the Gulf of California and/or Gulf of Mexico, especially during
monsoon summer conditions when clockwise high pressure can draw moisture up from the south.
High mountains impede wind flow, but also can ‘capture’ some rain and snow that would otherwise
move on to the east. Lack of clouds in Utah encourages daytime surface heating that can cause air to lift
enough to start the typical ‘storm’ process:
-
air moves and lifts
air cools by expansion as it reaches lower air pressure at higher altitude
moisture in the air may condense into clouds of visible liquid water that also release latent heat
air that warms itself by releasing latent heat may build into a substantial thunderstorm
precipitation falls
later on, air becomes cooler and drier, sinking back toward the surface
Only rarely does wind blow from Colorado back to Utah because the jet streams and westerlies prevail
from the west. Monsoon winds arrive mostly from the south.
On rare occasion, a cold high-pressure mass of air in Wyoming will push into Utah, but that wind is dry
and does not bring rain or snow.
2. Utah is far from the ocean, and experiences persistent high-pressure due to being close to the
descending side of Hadley cells where air is sinking and warming by compression. Which of the
following additional statements is not true?
a.
b.
c.
d.
e.
Utah would be even more dry were it not for abundant sunshine that causes intense surface
heating and rising air that can lift moisture high enough to cause brief thunderstorms
Utah would be even more dry were it not for high mountains that induce air to rise and cool by
decompression for occasional precipitation
During summer monsoons, clockwise winds in high-pressure cells may “scoop-up” moisture
from the Gulf of Mexico or Baja, California and bring it northward for precipitation in Utah
On occasion, strong winds from the central Pacific Ocean bring heavy precipitation to the dry
mountains of the western United States, including Utah. Such conditions may be called the
“Pineapple Express” because of moist air arriving briefly from the region near Hawaii.
Weather conditions and storms in Colorado tend to move toward Utah. What happens in
Colorado happens soon afterward in Utah.
Notes to Question #3: Consider the maps shown on pages 197, 199 and 206 and elsewhere in the book.
Remember from class discussion that the “westerlies” are important winds that prevail from west to east,
bringing most of the weather changes that affect the U.S. west coast, Utah and so on. See also page 204
on “westerlies”.
Westerly winds typically bring moisture and latent heat from the ocean to places like Oregon. By the
time that air reaches the east coast, it tends to have adopted the weather conditions from the interior of
the United States: hot in summer from intense surface heating and very cold in winter, from being
isolated from ocean warmth far away. So, by the time the westerlies reach the U.S. east coast they are
very different from the mild, wet winds that reached Portland, Oregon from the nearby ocean.
The U.S. west coast receives substantial ocean warmth even during winter, so conditions are mild along
the coast. Incoming ocean air from the west often brings fog and light rain to places like San Francisco
and Portland, Oregon. Drizzle rain and persistent cloudiness prevails in Seattle.
3. At 40 degrees north latitude a city near the west coast of a continent will likely have a milder climate
than a city on the east coast of that continent. The U.S. east coast gets much of its weather from
conditions in Chicago and southern Canada that tend to be severe: hot, humid summers, cold, snowy
winters. Meanwhile, Portland, Oregon is relatively mild due to cool, but not cold air and humidity
from the Pacific Ocean.
True ___
False ___
Notes to Question #4: See pages 210-212 in the textbook. Trade winds are persistent world-wide winds
that push from east  west around the world. Trade winds arise from high-pressure cells at about 30ºN
and 30ºS of the Equator that are rotating. See pages 197, 199, 200 and 209.
Trade winds are persistent, but they do sometimes slow down, stop or even reverse briefly, such as
during an El Niño season.
4. During an El Niño condition, trade winds fail to push warming water and warming air westward
across the Pacific Ocean. As a result, the normal pattern of storms in the western Pacific turns to dry
or drought instead. Meanwhile, the warmth and moisture that did not get pushed westward ends-up
producing rainy weather along the east side of the Pacific (USA and South America).
True ___
False ___
For Question #5 see notes above to Question #4: Keep in mind that trade winds are persistent, but not
perfectly so. For centuries they have been reliable enough for sea traders to move goods across the
ocean with great confidence. These winds are located above and below the Equator, not at the Equator.
With no real Coriolis force at the Equator, and with no contrasting cold/dry air available, storms at the
Equator tend to be simple convective events that are brief and not violent. Warm, moist air rises, cools
by condensation and drops rain.
5. Trade winds occur only on occasion, but when they do occur, severe weather can result, such as tornadoes and/or
hurricanes at the Equator.
True ___
False ___
Notes to Question #6: Storms in the United States are typically “Mid-Latitude Cyclones” with winds
that rotate counter-clockwise around a low-pressure center. Mid-latitude cyclones are discussed many
places in the textbook, however, looking ahead a little to Chapter 9 can be a big help. See pages 240
through 250 in the textbook.
Mid-latitude cyclones typically involve collision of differing air masses, often pulled together by jet
streams and Coriolis force. Air masses move in curves, with cool air following in behind warm air.
Wind moves these air masses, so in most cyclones wind brings warm air and then wind brings cooler air.
Mid-latitude cyclones typically involve warmer air that pushes through bringing moisture. Remember,
warm air holds more water than cold air. Afterward, a rush of colder air arrives, helping the warm air to
rise more quickly. Clouds form, the temperature drops, winds become gusty and precipitation usually
falls.
A ‘cold front’ storm can be vigorous, as cold air pushes underneath slower-moving warm, wet air that
will rise more quickly as a result. Warm, humid air always rises more vigorously when it collides with
air that is much more stable. Dry, cold air tends to be stable.
In the Salt Lake area, the weather report sometimes mentions that before a storm arrives, air may warmup and then cool-off dramatically as the main cold-front storm arrives.
In some parts of the world, winter air can be so cold that there is not enough water in the air to snow at
all. So, snow storms often involve relatively warmer air that brings enough moisture to provide
precipitation. In such cases, people may say “the warm before the storm”.
6. In the United States, there is often:
Wind before the warm
Warm before the storm
This pattern is part of normal cyclonic mid-latitude cyclones. Typically, warm air flows counter-clockwise around a
low-pressure center, while colder air follows it. At any given location, warm winds usually arrive before colder air
that signals the main part of the storm.
True ___
False ___
Notes to Question #7 See maps on pages 6, 110, 113, 197. 199, 200, and especially page 213.
7. Utah is a dry-weather state because we are located:
a. far from a major source of water
b. near the edge of a world-class ‘Hadley cell’ that provides high-pressure, sinking air that warms
as it compresses
c. in a region where abundant sunshine can evaporate water
d. in a region where prevailing winds can carry that evaporated water away from us
e. All of the above are true of Utah
Notes to Question #8: Based on class discussion and the textbook, stormy weather is almost always
about rising air that tends to turn counter-clockwise in the northern hemisphere. So, when a rotating
mass of air moves into our area from the west, the first winds to arrive will appear to come from the
southwest. As the storm passes on elsewhere, the last winds to flow through the valley will be on the
back side of the storm, tending to flow from north and west or north and east.
The graphic on pages 177 and 178 shows low-pressure systems over the United States. These systems
tend to move west to east, so the first winds to arrive will be the front of the storm, with winds moving
in from the southwest or the south, even though the flow of the overall storm is from the west. The back
side of the storm will produce winds from the other side, moving mostly from the north and west.
8. Take a moment to visualize a typical storm front moving into Salt Lake valley. Which sequence is
most applicable?
a. Wind from the southwest, followed by wind from the northwest as the storm passes by
b. Thick stratus clouds, followed by cirrus clouds that bring brief, heavy precipitation
c. Cumulo-nimbus clouds, followed by cirrus clouds that bring slow, steady precipitation
d. Winds from the east, followed by thin alto-stratus clouds from the east
e. Winds from the north, followed by alto clouds that thicken into heavy cirrus clouds
Most stormy weather that arrives in Utah is coming from either prevailing westerly winds or summer
monsoon winds from the south.
In most cases, the front edge of a low-pressure cyclonic pattern will show winds hooking from the
southwest because cyclonic movement is counter-clockwise. The overall flow may be from the west or
south, but cyclonic turning will show crossing winds as the front of the storm arrives.
Likewise, after the low-pressure cell passes by, the cyclonic turning will be arriving from a more
northwesterly direction.
Notes to Question #9: For the jet streams, see pages 205-207 in the textbook. As the sun moves to higher
and lower latitudes during the year, the two jet streams tend to follow. The ITCZ also moves more or less
with the Sun, so that during June the ITCZ is mostly in the northern hemisphere where the Sun is directly
overhead at noon. For the ITCZ see pages 199, 200 and 202.
9. Northern hemisphere jet streams undulate west  east like high-altitude stream of fluid around the
globe. They tend to follow the sun’s summer movement northward and winter movement southward.
In contrast, the ITCZ near the Equator does not exhibit this migration north and south with the sun’s
position in the sky but stays positioned year-round over the Equator.
True ____
False ____
Notes to Question #10: Based on class discussion and many places in the textbook, when colder, drier
air collides with warmer, wetter air, the natural buoyancy in the lighter, wetter air will rise even more
vigorously. The speed of rising air can accelerate even more as condensation forms and latent heat
returns to ‘sensible’ higher-temperature air that adds buoyancy.
The cycle may repeat several times as wet air rises, cools, condenses and precipitates. Eventually,
moisture is absorbed by the ground or is blow elsewhere by winds moving west to east.
The other major storm types exhibit some common characteristics as well. A convective storm will
quickly run out of energy as it lifts and cools. An orographic storm will often “settle down” after
passing the top of a mountain. Frontal storms are introduced on page 114. For good examples in the
textbook, you can look ahead to Chapter 9, starting on page 240.
When a cyclone subsides, cool air has fully pushed underneath warmer, lighter air that lifted previously.
There is no more uplift, so winds calm down. Light or steady precipitation may continue for a while,
but heavy downpours have ended. The storm has run out of vigor. Warm air aloft has no reason to
move because it is already aloft. Cold air below has no reason to rise because it has no buoyancy. This
“occluded” condition is studied in more detail in Chapter 9.
When a cyclone occludes, there is no more uplift because warm air is fully aloft while cool air covers
the surface. This is similar to a momentary weather inversion, because normal air is unstable as surface
heating lifts air.
In fact, soon after a storm occludes, the normal process of instability builds again as daily surface
heating produces rising air while cooler air aloft tends to subside.
10. A mid-latitude cyclone weather “frontal” storm tends to become more vigorous or even dangerous
when cold air has fully wedged underneath warm air aloft. This layering of warm air over cold air
signals the main part of the storm when winds, rain, lightning and hail become most intense.
True ___
False ___
Notes to Question #11: See notes for Questions 6, 8 and 10 above.
11. In a classic mid-latitude cyclonic cold front, cold air chases warm air around the perimeter of a lowpressure center. Colder air tends to move faster than warmer air, so it eventually catches up and
wedges underneath the slower-moving warm air, causing the usual storm steps: rising,
decompressing, cooling, condensing, precipitation. Cold fronts tend to be vigorous, with rapid lifting
and strong winds that may provoke lightning and hail stones.
True ____
False ____
Notes to Question #12: See pages 137 to 140. For radiation fog  When land surfaces cool quickly at
night, the air directly above the ground can cool quickly also. Outgoing radiation during the night does
not leave enough heat at the surface to keep humid air from cooling down to condensation.
Condensation level, also called ‘vapor saturation’ or ‘dew point’ occurs anytime the air “bucket” fills to
overflowing. Cooling air is a smaller ‘bucket’ that can overflow, leaving wet dew on the grass and fog
in the air close to the ground.
Advection fog can occur when humid air cools to condensation or ‘dew point’ after passing over very
cold water or land.
For steam fog  A jet airplane at high altitude, or a person going outside on a cold day can see humid
air quickly cool enough to form a cloud of condensation. In winter, car exhaust is visible as fog or
steam. Very humid air will release vapor back to liquid when it cools below vapor saturation
temperature.
For orographic fog  Humid air rising over a mountain can also decompress enough to see air
temperature fall below condensation level, or vapor saturation. Clouds often form near the top of
mountains as the air becomes too cold to hold water as vapor. Vapor condenses to liquid water that is
visible as a white cloud.
12. Which of the following statements is false?
a. Radiation fog occurs when severe ground cooling at night causes surface air to cool and
condense water vapor back to liquid water (clouds)
b. Advection fog results from water air passing over cold surfaces, causing the air to cool and
release vapor back to visible liquid (clouds)
c. Steam fog results when a person exhales on a winter day. Warm, moist air from a person's lungs
quickly cools to condensation.
d. Up-slope fog, or "orographic" fog occurs when heavy, moist air flows downhill, causing vapor to
condense into liquid when it contacts warm air at the bottom of the hill
e. A high-altitude jet airplane produces humid exhaust that quickly cools to condensation,
producing "contrail" clouds that may take a long time to evaporate and disappear
Notes to Question #13: Page 72 in the textbook shows the latitude of Salt Lake City. As explained
many times in class discussion, westerly winds prevail from the west at roughly 40ºN latitude. Trade
winds blow from the east at about 20 ºN. The south and central United States is a collision zone
between these two air masses. Easterly trade winds bring warmth and moisture that collides with cooler,
drier air coming from the west at higher latitude (about 40ºN).
Salt Lake is too far west to be part of “Tornado Alley” and too far east to receive abundant moisture
from the ocean. Instead, Utah is adjacent to the extreme north end of Hadley cell air that is dry and
warming as it sinks to the Earth’s surface at about 30ºN latitude, forming deserts or dry country.
Aside from brief summer monsoon winds that sometimes bring moisture from Mexico, most of Utah’s
weather is remnants of moisture from the coast of Oregon and California. Most of that water was
‘caught’ by mountains before reaching Utah. In fact, Utah would be even more desert-like if it did not
have mountains of its own to catch westerly moisture was not already lost before reaching Utah.
A wide variety of maps in the textbook show wind patterns in the regions surrounding Utah and Salt
Lake City.
13. Located at 20 degrees north latitude, Salt Lake City receives most of its weather from the trade
winds.
True ___
False ___
Notes to Question #14: Moisture in the air might normally be insufficient to produce rain or snow. High
clouds may form if air aloft is cool enough to reach dew point temperature, allowing vapor to come back
as visible liquid.
If daily solar heating at the surface of the Earth is intense enough, then rising air currents, often called
“thermals” may lift moisture enough to provoke precipitation. The process has been summarized in class
many times:
-
Air lifts
Air cools by decompression at higher altitude where air pressure is lower
Clouds may form if cooling is enough to reach dew point temperature
Latent heat is released when vapor returns to liquid
Latent heat can cause air to lift further, leading to genuine instability – the more heat is
released the more air rises to condensation level and releases more heat.
The process can continue as long as the air contains additional water vapor that can be
released along with latent heat.
Meanwhile, rapidly rising air in unstable conditions can produce strong winds, heavy
precipitation, lightning and hail.
When cooler, drier air is nearby, unstable conditions can become severe, producing air that
lifts and rotates so rapidly that a tornadic funnel cloud forms.
In Utah, air is often not wet enough to produce rainfall unless surface heating is intense and lifts that
moisture to an altitude high enough to provoke cooling by decompression.
After dropping rain or snow, the storm usually fades away as moisture is lost and air cools and subsides.
During summer monsoon conditions in Utah, substantial moisture may be delivered by winds from the
south. Then, daytime surface heating can provoke that moist air to rise, sometimes leading to vigorous
thunderstorms that cause flash floods and other damage.
See pages 201-203 and especially page 204 in the textbook.
14. During summer, intense daytime heating of continental interiors can produce temporary low pressure
zones. If moisture is available, then daily heating may lift moist air enough to produce brief
thunderstorms. Falling rain may evaporate and then re-heat the next day to produce another set of
thunderstorms. Daily heating and precipitation can persist until the moist air migrates slowly out of
the area, usually from west to east.
True ___
False ___
Notes to Question #15: Monsoon conditions are about wind changes. Over India, on-shore breezes in
summertime help ensure that abundant moisture arrives from the ocean for farming. In Utah, during
summer, the ITCZ is at its furthest north, so even though ITCZ precipitation does not fall on Utah directly,
winds from the south can “pull” some moisture far enough north to reach Utah.
15. During Utah’s annual summer ‘monsoon’:
a. Prevailing high pressure brings sunshine and dry, descending or stable air
b. Clockwise winds may bring moisture up from the south
c. Daily surface ‘thermals’ of rising hot air may "pop up", causing water vapor to rise, cool,
condense and form clouds
d. Small-scale low-pressure thunderstorms may occur even though high pressure still dominates
e. All of the above are typical of Utah’s monsoon season