Salt Lake Community College
Geography 1000 – Physical Geography
AJ Allred, Fall, 2014
Quiz #6
Explanations
Preface to Question #1: Stormy weather usually brings a combination of cooler air and more humid
air. Using the “humidity bucket” metaphor, in order to get vapor saturation the capacity of the air to
hold moisture must be reached or exceeded. Because warm air can hold more water vapor, relative
humidity will rise just by lowering the air temperature. So, when clouds form before a storm, more
solar energy is reflected back into space, resulting in cooler air and land surfaces. Further increase
in relative humidity can be accomplished by then adding more water vapor to air that is already less
able to hold vapor.
So, a storm situation (rain, snow and/or lightning and hail and high winds) is best created by a
combination of cooler air and more water.
1. When a rain storm moves into Salt Lake Valley, which of the following usually occurs?
a. Water vapor increases in the sky, raising relative & absolute humidity
b. Clouds thicken and form a lower altitude as humidity rises
c. Relative humidity rises because cloudy, cooler air is closer to dew point temperature
(saturation)
d. Surface heating can cause air to rise, cool by decompression and form more clouds
e. All of the above are true
Preface to Question #2: Adiabatic processes are simply: change in temperature leads to change
in temperature. Energy is neither created nor destroyed. Instead, compressed air concentrates
heat; de-compressed air disperses heat or thermal energy. So, rising air will decompress because
air pressure is much lower at altitude. Cooling occurs when air is decompressed and energy is
dispersed or diluted. Likewise, air temperature is higher when descending air compresses by
descending to lower altitude.
2. Adiabatic cooling is caused by:
a. Rising air that cools by decompression and destroys energy forever
b. Rising air that decompresses
c. Air aloft being more dry than surface air
d. Clouds forming at high altitude by releasing stored cold energy through condensation
e. Air that sinks toward the surface on a cold day
Preface to Question #3: Fog is simply a cloud on the surface. Clouds and fog are liquid water - - not
vapor. Vapor is the most energetic (heated) state of water. For fog to form, air must reach
saturation such that vapor begins to go back to liquid. Vapor saturation can occur when air cools
enough to lose its ability to maintain energetic vapor state. Fog can also occur when water vapor is
added to air that is not cooling. In any case, air going up a mountain (orographic fog) can occur
when rising air decompresses, cools and condenses vapor back into visible liquid. Fog can also
form at night when earth radiates so much heat back to space that air near the ground is too cold to
keep vapor from condensing back to liquid.
“Dew” on the grass in the early morning represents water that condensed out of vapor during a
cooling night.
3. Advection fog can form when warm air flows over a cold surface, causing cooling that leads to
condensation. Air that has cooled to vapor saturation will release latent heat as invisible
vapor becomes visible as liquid water.
True ___
False ___
Preface to Question #4: For water vapor to “leak” out of air and return to visible liquid it is helpful for
water to have a surface on which to condense or “stick”. Dirt, ice, salt, and other solid substances
enhance the ability of vapor to condense back to liquid. Super-cooled air can occur without fog
forming in air that is super clean because condensation nuclei or surfaces are not sufficiently
available.
So, “rain makers” sometimes toss aerosols into the atmosphere, hoping that humid air will condense
on those particles. This activity can help disperse fog by turning it to rain, or coax vapor to condense
into clouds that precipitate later on.
4. It is much more difficult for fog or clouds to form in very clean air because there is not enough
dirt, aerosol or other surface on which vapor can “stick” or condense back into liquid.
True ___
False ___
Preface to Question #5: Thunderstorms and other severe weather usually involves rapidly rising air
that is much lighter weight than nearby cooler, drier air. Vertical wind speeds in a thunderstorm can
exceed 60 mph. High in the storm snow and rain may begin as vapor condenses on aerosols or ice
particles. These tiny droplets grow larger as they fall and collide with other water droplets.
It is also true that at high altitudes air temperature is below freezing, so even on a hot day at the
surface, freezing conditions and ice are common in clouds high above. As ice begins to fall it may
melt back into rain and then be blown back up into a storm cloud where an additional layer of ice
may be added. This process may repeat several times resulting in a hail stone that eventually
becomes too heavy to avoid falling to earth.
In dry conditions near the surface, hailstones may melt and/or evaporate enough to not be noticed at
ground level. In cooler, more humid areas, hailstones can reach the ground, resulting in damage to
property.
5. Hail stones form when air rises so quickly in a cumulonimbus cloud that ice pellets and rain
drops get blown back up where ice is added. This process can occur many times until
hailstones are too large for wind to keep them aloft. Near the surface, air could be so dry that
even a massive hailstone evaporates to nothing before it hits.
True ___
False ___
Preface to Question #6: Air pressure drops very quickly with altitude. At only about 3.5 miles above
sea level, air pressure drops by about half. At 10 miles altitude, air pressure is down to 10%of
average surface pressure. These severe changes in air pressure between high and low altitude are
normal and usually mean nothing in terms of weather.
Horizontal changes in air pressure have very important effects on weather. A difference of as little
as 10mb in air pressure between two locations at the surface can cause breezy winds. A change of
50mb between locations or at the same location typically causes a vigorous storm or other
substantial change in weather. A 100mb change at the Earth’s surface is likely to be lethal to people
and severely damaging to property. Wind speeds in such a storm could exceed 100 mph.
6. Vertical differences in atmospheric pressure are extremely important. Even a change of
10-20mb in air pressure between the surface and an altitude of 10 miles will cause a severe
storm to develop. By comparison, a horizontal pressure change of 40-60mb from one day
to the next means little or no change in weather.
True ___
False ___
Preface to Question #7: Water vapor is chemically lighter weight than dry air: hydrogen in water has a
molar or atomic weight of just 1.0. Most of the atmosphere is composed of nitrogen with a mass or
molar weight of 14. So, humid air will rise rapidly when placed adjacent to dry air.
Likewise, warm air rises quickly over cool air because heated air molecules vibrate vigorously, pushing
apart to create less density than in less agitated cooler that is more dense.
7. Adding atmospheric water vapor through evaporation makes air heavier and less likely to
cause stormy weather. High humidity makes everything feel heavy, thick and slow to move.
Adding heat to humid air makes things even more stable and/or even stale, and may lead to
air inversions that collect air pollution.
True ___
False ___
Preface to Question #8: A hot day will produce low air pressure because thermal energy will cause air
molecules to vibrate and push apart, creating less density. If the Earth had a “lid” on it, then such
heating would cause high pressure, but there is no container to keep warming air from expanding and
rising, resulting in low pressure as higher density air can push in from the sides. High pressure flows to
low pressure.
At night, cooling air becomes more dense as air molecules move with less kinetic force or motion.
Stormy weather is almost always about rising air that:
-
Rising by being lighter-weight than surrounding air
Cools by decompressing, as there is less air pressure at higher altitudes
Condenses from vapor to liquid as cooling air loses heat energy needed to keep vapor from
returning to liquid
With enough condensation cooling, precipitation will fall.
8. A hot day will always cause high air pressure. At night, air cools and becomes less dense, so it
sinks to the surface, increasing the likelihood of stormy weather developing.
True ___
False ___
Preface to Question #9 In most of the United States, storms and weather changes tend to arrive from
the west, as westerly winds prevail along with the jet stream that blows from west to east.
Storms from the west tend to bring cooler air and water vapor. These two conditions combine to form
clouds that drop precipitation. Gusty winds are common as water vapor rises in the air. With enough
vigor, rising air can provoke lightning and hail.
With enough water vapor and cooler conditions, the condensation level (vapor saturation or dew point)
can drop close to the surface. Surface fog may form.
By comparison, on a normal dry, sunny day in Utah, cloud levels are extremely high above the ground
because only at such high altitudes is it cold enough for air to lose the ability to keep water as vapor
instead of liquid (or ice).
9. A recent storm in the Salt Lake are included these features:
a. Prevailing high pressure moved eastward while a low-pressure condition moved in from the
West
b. Low-pressure air brought water vapor that led to clouds and cooling as sunshine was
blocked
c. As the storm developed, air became lighter due to humidity, but slightly heavier due to
being cooled as solar-blocking clouds developed
d. Clouds were thicker and lower because of more water in the air and less ability of the air to
hold that water as vapor
e. All of the above are true
Preface to Question #10 When actual air temperature drops close to dew point temperature then
clouds may begin to form because the air is does not contain enough thermal energy to keep vapor from
condensing to liquid that is visible as clouds or precipitation.
It is true that clouds help keep daytime temperatures cooler by reflecting solar energy back into space.
At night, clouds keep the Earth warmer by absorbing outgoing thermal radiation and sending it back
toward the surface. So, clouds are like a blanket that keeps energy out,during the day and keeps energy
from escaping at night.
Most of the questions above contain preface statements that also help with Question #10.
10. In Utah, a rainy, foggy day might include all of the following features except:
a. Air that rises due to high pressure and dryness
b. Humid air that is rising
c. Dew point temperature and actual air temperature being about the same
d. Daily high and low temperatures being close together
e. Relatively warmer air at night if clouds stay all night rather than clearing to show stars
Preface to Question #11: Coriolis force is nearly absent at the Equator and is stronger with latitude –a
positive correlation between increasing latitude and increasing Coriolis force. Lack of Coriolis helps
prevent the world’s most severe weather: hurricanes and tornadoes by lack of wind deflection in
curves. In the northern hemisphere Coriolis force is to the right and the opposite direction in the
southern hemisphere.
11. Which of the following is not true of Coriolis force?
a. Coriolis is weak or non-existent near the Equator
b. Tornadoes and hurricanes do not occur at the Equator
c. Coriolis force is positively related to latitude
d. In the southern hemisphere, high-pressure air tends to rotate counter-clockwise
e. In the northern hemisphere, high-pressure air tends to rotate counter-clockwise
Preface to Question #12: The four major storm types all involve the same basic processes:
Step 1: Air moves
Step 2: Air rises
Step 3: Air decompresses with altitude
Step 4: Decompressing air cools by dilution or dispersion as thermal energy is spread over greater
volume
Step 5: Cooled air is less above to hold water as vapor. Vapor may condense back to liquid, which forms
clouds
Step 6: With enough condensation back to liquid, water may fall from the sky as rain or snow or ice
pellets
Step 7: If enough water vapor is available, the release of vapor back to liquid may cause instability as
rising air re-heats itself over and over each time vapor condenses back to liquid, releasing the latent
heat that was absorbed when water was originally evaporated.
Step 8: Air that keeps warming itself by releasing latent heat as vapor goes back to liquid may induce
strong, gusty winds and produce lightning and hailstones as vigorously rising air keeps the process going.
All of the four major storm types below include the steps listed above.
- air may be induced to rise over mountains (orographic)
- air may rise when it is super-saturated with vapor and very warm (convective)
- air will have to rise if it collides with another air mass coming from the other direction
(convergent)
- air will rise much more rapidly when it is very warm and very humid and collides with air that is
much more dense by being dry and/or cold (frontal or wedge)
12. Which of the following is not one of the four major types of ‘storm’ conditions?
a. Air rising over mountains may cool by decompression until clouds form
b. Dry air flowing over dry land will cause severe heating and condensation by compression
c. Humid, warm air will rise quickly when it encounters cooler, drier air
d. Humid air masses that converge have no alternative but to rise
e. Humid, warm air will tend to rise as soon as the sun appears in the morning
Preface to Question #13 below. Cumulo-nimbus clouds involve vertical development – rapidly rising air
cools and condenses vapor back into liquid water, which releases heat for more rising. Unstable air in
these storm clouds can keep rising as long as warm, humid air is available to keep releasing heat when
condensation occurs.
Storm clouds of vertical development can be quite violent as surface winds can exceed 100 mph to feed
rising air that can reach more than 60 mph. Tornadoes may form when extreme low air pressure occurs
as rapidly rising air leaves room for higher-pressure air to move in from other locations.
13. Which of the following is not true of cloud types?
a. Clouds of vertical development tend to promote stable air instead of storms because vapor
content is uniform from high to low altitude
b. Cumulo-nimbus clouds are associated with the most violent storm types because air is rising
quickly and releasing heat.
c. Low, thick stratus clouds indicate that surface air temperature and surface dew point
temperature close to the same
d. Clouds of vertical development can reach close to the tropopause during the most extreme
storms
e. On a very hot day, clouds in the sky above are often made of ice
Preface to Question #14: In most of the United States prevailing winds are from the west, so most
weather changes and storms arrive from the west. High pressure systems develop when air is either
stable or sinking. Storms develop when air is rising in low-pressure systems. These high and low
pressure air masses migrate eastward. High pressure pushing outward at the surface tends to feed air
into low-pressure systems that involve air that is rising from the surface. At the top of these systems,
rising air slows and moves laterally, forming upper-air high pressure that pushes toward air that is
beginning to sink back into high pressure at the surface. The cycle continues.
The cycle of high to low pressure can occur locally, regionally or on a world-wide basis in Hadley cells
that rising air in tropical storms and sinking air over deserts. In the tropics, air rises due to heat and
humidity. Over deserts, air sinks due to lack of buoyant water.
Coriolis force exists because the Earth is a sphere that rotates. Objects in flight tend to fall behind the
direction of rotation and thus appear to be curving. Coriolis is not caused by wind, but Coriolis has more
effect on high-speed wind than on gentle breezes.
A dry, high-pressure system can hold large amounts of moisture without forms clouds or precipitation.
A clockwise high-pressure flow sometimes moves water vapor into an area where it can be lifted by a
low-pressure air mass. As such, stormy weather results not in the high-pressure air, but from water
delivered by high pressure air carrying moisture to a low-pressure area where it can be lifted to produce
clouds and precipitation. In a sense, a dry high-pressure system can “feed” water into a low-pressure
storm without showing clouds of its own.
Dry, high-pressure cells can sometimes block low-pressure storm cells from their usual migration across
the United States or other places. Persistent high pressure leads to dry, sunny weather. In Utah, highpressure, dry weather is our normal climate. In other places a “blocking high” pressure system can
cause drought and prevent humid air from helping suppress wildfires.
14. Which of the following is not true of high and low-pressure air masses in the western United
States?
a. Persistent high pressure block a low-pressure storm from arriving, leading to drought and
worsening wildfires
b. Weather changes often come from the west. Low-pressure and high-pressure systems feed
into each other as they migrate eastward
c. Cooler, drier air masses from the west frequently collide with warmer, wetter air that
persists in the eastern USA
d. Coriolis force may be generated if high air pressure results from a hot day
e. A sunny, clockwise high-pressure system can feed moisture into a low-pressure counterclockwise system, resulting in a storm
Preface to Question #15: A “dry line” refers to a sharp boundary between air that is ready to rise quickly
when it collides with a distinct air mass nearby that is much more dry and cool. By metaphor, a “hot air
balloon” will appear to be much more buoyant if adjacent air is much cooler. When there is very strong
contrast between air masses, the more humid air will rise quickly by its buoyant, lighter-weight
condition. The greater the contrast in buoyancy between air masses, the more vigorously the lighter air
will rise.
In a “dry line” storm, the buoyancy difference can be severe, resulting in dangerously high winds and
rising air fast enough to form a low-pressure funnel known as a cyclone or tornado. Dry line storms are
among the most dangerous weather events anywhere in the world.
In a “frontal wedge” storm, a fast-moving mass of either cool, dry air or a mass of warm, wet air collides
with the other air mass. Vigorous lifting results when humid air rises rapidly, and then further warms
itself by releasing latent heat each time vapor is condensed back into liquid water. The re-heating
process can cause very high-speed rising air that may provoke lightning, hail, and damaging winds along
the surface as higher pressure air rushes in to take the place of rapidly rising air.
Even though a dry-line storm relies on heat and latent heat, the presence of cold air nearby can produce
some of the most extreme storms, including tornadoes. In fact, snow can be part of a tornadic storm,
because cold, dry air helps warm, wet air rise more quickly, and because the tops of severe storms are
reach far into the troposphere where air temperatures are far below zero F.
Finally, air that rose inside a vigorous cumulo-nimbus cloud tends to descend afterward. Downdrafts
can produce violent high winds known as “micro-bursts” after rising warm, wet air converts to cold dry
air at the top of the storm and then sinks back to Earth.
Stormy weather almost always involves humidity. Humidity is always reported as “relative” humidity
because we need to know how close the air is to vapor saturation. Cloud formation is not just about
water, but also about the capacity of the air to hold that water. Relative humidity is a comparison
between water vapor and water vapor capacity in the air. No matter how much or how little vapor is
available, clouds will not form without air reaching “dew point temperature” and condensation nuclei
on which vapor can condense. Like baseball scores, there is no meaning to a number without a
comparison to some other number, so relative humidity is always a percentage, a ratio.
15. Which of the following conditions is most likely to result in a severe “dry line” storm that can
provoke a killer tornado in the central United States?
a. An air mass at 50°F and 50% humidity colliding with air that is 30°F warmer and 20% more
humid
b. Air that is 80°F with 70% humidity colliding with air that is 85°F at 65% humidity
c. An entire region where air temperature is 100°F and relative humidity is uniformly 70%
d. Air that has exceeded 100% humidity and entered the realm where absolute humidity is
above 50%
e. Air that is no longer relatively humid, but has reached absolute humidity of at least 80%