Geography 1000 Mid-Term Essay Option
Sample Responses for Mid-Term Exam Essay Options – Each of the complete sentences below would be
worth at least one point.
Optional essay questions for up to 50 total points. Respond to any, all or none in as much detail
(or not) as you like. Consider that each significant point you make is worth up to about one point
each. Simple drawings, lists or brief phrases may be the best way to get your ideas across.
1. Why does a can of compressed air get cold when you release pressure by spraying.
When a can of compressed air is filled at the factory, the compression process causes the can to
become hot to the touch, because energy in the air is concentrated as the contents are compressed
into the can.
After the can is filled, it sits on a shelf and slowly cools back down to surrounding (ambient) air
temperature, losing heat by conduction (direct contact) and by radiation. So, there is now less
energy inside the can than before it was filled, because the total energy held by the concentrated
gas or liquid in the can is at the same temperature as ambient surrounding conditions.
So, later on when the contents of a can of compressed air (or liquid) are sprayed out, the
compressed contents are decompressed, which disperses, or de-concentrates that substance,
resulting in lower measured temperature (measured as heat).
The adiabatic process in the Earth’s atmosphere operates the same way. When air rises it
decompresses because lower air pressure at higher altitude allows a parcel of air to expand, filling
a large volume or space. This process is easy to experience in a swimming pool: down deep the
weight of water overhead presses down, causing water pressure you can feel in your ears. As you
rise, you can feel that pressure diminish as the weight of water overhead is reduced.
Likewise, rising air in the atmosphere cools by decompression, not because energy was
destroyed, but because it is dispersed or de-concentrated as decompression occurs. Heat in the
air, as measured by temperature, appears to be lower as a parcel of air rises in the atmosphere.
Likewise, as a parcel of air descends in the atmosphere, its energy content is compressed or
concentrated into a smaller space, resulting in higher measured temperature (heat). Compression
occurs because a descending parcel of air is covered by the weight of more air above. Energy in
the parcel has not been created, only concentrated, showing a rise in measured temperature (heat).
Earth climates where descending air masses dominate tend to be dry, because descending air is
warming, and warming air has greater capacity to absorb water vapor. Water that is evaporated
into the atmosphere is unavailable for precipitation until something changes.
See pages 87-88 in the textbook.
2. Describe how Utah experiences any or all of the four types of precipitation or
‘storminess.’ Use words, drawings and/or labels.
Convective precipitation can occur any time the air becomes vapor saturated. Cooling a parcel of
air is the most effective way to cause air to become vapor-saturated. The easiest way to cooldown a parcel of air is by lifting it to a higher altitude, leading to de-compression, as explained in
topic #1 above.
Typically, warm, humid air is buoyant enough to rise on its own, leading to cooling by
decompression, then condensation of vapor into clouds of liquid water droplets that can
eventually fall as precipitation. Convective precipitation is most common in tropical regions
where sunny conditions and abundant water lead to massive evaporation, the first stage in
preparing for precipitation later.
Utah is a dry region, but there is sometimes enough water vapor and heat in the atmosphere to
promote buoyancy that leads to precipitation.
Most often, Utah precipitation is caused by air rising as wind blows over mountains, called
“orographic” effect. Less buoyancy from water vapor is needed for air to rise and decomparess,
because wind has no choice but to rise over the mountain obstacle. That is why mountain peaks
in the western United States are often snow-capped when valleys are very dry. Most of the year,
afternoon clouds appear over mountains as wind pushes over mountains, cools adiabatically and
condenses vapor back into liquid water. If that happens enough, precipitation may occur.
Convergent air masses tend to rise because colliding air masses of similar contents have nowhere
else to go but up. Convergence can occur over sun-warmed islands as heated air rises. The most
common occurrence of convergence is near the equator, where the “ITCZ” receives incoming air
masses from places north and south that take the place of masses of sun-heated rising air that is
also buoyant due to high humidity. Humid air is chemically lighter weight than dry air. Warm
air is more buoyant than cool air because heated air molecules exhibit more energetic motion that
causes them to push apart. Heated air masses expand rather than pressurize because the
atmosphere is not a sealed container.
Cooler, drier air from regions to the north and south converge in the tropics where they gather the
same heat and humidity that caused previous air to rise, continuing the cycle. This process forms
the equatorial segment of “Hadley Cells”. At the far ends of Headley Cells, away from the
equator, air masses become cooler and drier, causing them to descend and move back toward the
equator for more rising. Hadley Cells that help govern world climates: Rain forest dominates in
the tropics, while deserts dominate at the north and south ends of Hadley cells where air masses
are generally descending, and warming by compression. Warm, dry air can absorb available
moisture, so terrain underneath descending air tends to be dry with little prospect of precipitation.
Utah can experience “frontal” or wedge storms that can include gusty winds, precipitation and
even hailstones and lightning. Tornadoes also occur in Utah, even in mountainous areas. Frontal
weather patterns can be more vigorous than simple convective storms because the presence of
cooler/drier air can promote humid, warmer air to rise even more rapidly. Even cold air can rise
if surrounding air is even colder. The existence of Coriolis force helps promote cyclonic,
accelerating wind, adding further to the vigor of frontal storminess.
3. Describe how evaporative cooling works (“swamp coolers”).
Evaporative cooling can convert hot outdoor air at 100°F and 25% humidity to as cool as 75°F
and about 50% humidity. Air is drawn by electric fan across a fabric soaked in water.
Evaporation occurs, as sensible energy (heat, measured as high temperature) is converted to
latent, or hidden heat. Vapor is a more energetic state of water, and requires absorption of energy
in order to change from liquid to vapor. That energy is not lost or destroyed, it is simply
converted to latent, or hidden heat.
Adding water to the air as vapor is measured as higher humidity. It is important to note that in
dry climates like Utah adding water vapor is usually acceptable, or even pleasant. In fact, with
prevailing dry air, it is easy and economical to evaporate water.
In contrast, humid regions where humidity is already high cannot absorb much additional water
vapor, so evaporative coolers are less effective. In fact, even in Utah, on a humid day, swamp
coolers seem to do little good. That is because air that is already humid is ineffective in
absorbing more vapor. Likewise, people often feel uncomfortable in weather that is both hot and
humid because natural body perspiration is not evaporated in air that is already humid. Without
evaporation, body perspiration just runs off without cooling. In fact, dehydration can occur.
Swamp coolers may also be ineffective in very hot climates, such as the height of summer in
Arizona, because the air contains more heat than can be converted to latency by evaporation. For
instance, at 115°F, a swamp cooler can provide a temperature drop of perhaps 25 degrees, which
only reduces air temperature to about 85°F, not enough to provide comfort. Trying to evaporate
more energy using more water may just raise humidity to an uncomfortable, making air feel like a
steamy swamp.
These limitations help explain why “expansion” cooling, also called “central air” is popular.
4. Describe how expansion cooling works, also known as refrigeration or “central air”.
Mechanical cooling, sometimes called “compression/expansion” cooling is similar to the
adiabatic process that occurs naturally in the Earth’s atmosphere as parcels of air rise and fall.
In conceptual terms, air is compressed in order to concentrate its energy. Temperature rises and
heat energy is concentrated into a smaller volume. Concentrated heat or energy is then more
easily “blown off” or transferred to a heat-absorbing fluid. The “anti-freeze” in an automobile
radiator is a good example of an effective “heat exchange” fluid.
After heat is drawn out, the air is allowed to cool by expansion. This process can be repeated or
enhanced to achieve any desired temperature. This kind of mechanical air conditioning requires a
great deal of energy (using expensive electricity) in order to “squeeze” the energy or heat out of
the air. So, the use of “central air conditioning” generally results in substantially higher
electricity cost during summer months. In contrast, evaporative cooling is relatively inexpensive,
because it uses small amounts of low-cost water and a relatively small amount of electricity to
operate a fan that promotes simple evaporation by blowing air across a wet surface.
5. Describe how evaporative coolers are the opposite process of severe thunderstorms,
tornadoes and hurricanes.
Evaporation converts liquid water to a more energetic vapor state. The energy required for
moving water to a more energetic state becomes ‘latent’ or hidden in the air. Likewise,
evaporated water also becomes hidden as invisible vapor in the air. Evaporation is the process of
storing energy in humidity. Humidified air is buoyant and can transfer large amounts of latent
heat to other places by wind motion. When evaporation is occurring, air tends to become cooler
and more humid.
Condensation is the opposite of evaporation. Condensation releasing water vapor back into liquid
water that is visible as clouds or rain, snow, dew, etc. Condensation also releases the latent heat
that was holding water as vapor, resulting in higher air temperature. So, unlike evaporation that
makes water and heat hard to see and feel, condensation converts water vapor to liquid water, and
allows that energy to be felt again as heat (“sensible” heat).
Vigorous storms can occur when rising air de-compresses by expansion and cools to the point
where condensation occurs, releasing the hidden or latent heat that was associated with water
vapor.
Warming air by condensation also results in greater buoyancy, inducing humid air to keep rising,
possibly condensing further and re-warming again and again as long as sufficient vapor remains.
Under such conditions, rising air induces low atmospheric pressure that provokes surface winds
that may add yet more latent heat to the process.
We can call this process convective, because humid air is moving, transferring heat as vapor
along the way. A “frontal” weather condition tends to be even more vigorous than simple
convection, because humid, warm air rises more rapidly when air nearby is cooler and/or drier
(less buoyant).
6. Describe how expansion cooling is similar to the ‘adiabatic’ process.
Rising air cools by de-compression because air pressure is lower with altitude. Descending air
warms by compression because gravity causes air above to press down on air at lower elevation.
Descending air is thus compressed by the weight of air above, causing heat energy to be
compressed or concentrated, resulting in a rise in measured temperature.
7. Explain why Arizona tends to exhibit hot weather much of the year, while Ecuador (at
the Equator) does not.
Most regions along the Earth’s equator are dominated by ocean. Water is one of the most
effective substances for absorbing heat. A great deal of energy is required for water temperature
to change, so places near the ocean tend to exhibit milder swings in temperature than do nearby
land areas.
In addition, solar electromagnetic rays or photons are more vertical and concentrated near the
Equator and are blocked by less atmospheric gas than elsewhere on Earth. The combination of
abundant solar energy and abundant water results in vast evaporation of water into vapor and
latent heat. The result is cooling of the air and promotion of frequent cloud cover as humid air
eventually condenses back to water vapor.
So, equatorial regions are protected from excessive air temperatures by the cooling effect of
evaporation and by the “sun screen” effect of frequent cloud cover.
Arizona has neither of these advantages, so during summer months dry earth surfaces become
very hot as they absorb solar energy. These surfaces have less capacity to absorb energy without
a commensurate change in heat as temperature. High-termperature objects re-radiate excess
energy back into surrounding air, resulting in yet higher temperatures.
Winds that are generated by evaporative cooling and buoyancy are self-promoting: rising air
provokes more wind that causes more evaporation. Solar energy and water availability also
promote vigorous plant growth, which is a means of storing solar energy in the form of
hydrocarbon fuels. Energy stored in woody and grassy fuels can be released by combustion later
on.
In addition, winds generated by evaporation and rising air in the tropics can also transfer vast
amounts of latent heat to places elsewhere. These same winds provoke ocean currents that also
transfer vast amounts of energy to places far away, helping keep the tropics cooler and helping
keep higher latitudes warmer.
8. Explain Mr. Allred’s “Magic Rules for Climate”.
A climograph shows a monthly profile of air temperature across an average or typical year. The
extremes of monthly air temperature are an important measure of climate mildness or severity,
and are largely governed by distance to or from:
1) the equator
2) sea level
3) a large body of water
Being downwind from a large body of water also provokes mildness and relatively greater
precipitation because water is adept at absorbing heat during hot weather and releasing heat
during cooler weather. Likewise, wind blowing across water carries heat (in humidity) to dry
land, resulting in less severe cold weather, and cloud cover and/or precipitation any time vapor
saturation occurs.
During the day, clouds reduce solar energy reaching the ground; during the night, water vapor in
clouds absorbs and re-radiates long-wave radiation that helps re-warm ground surfaces rather than
escaping to outer space.
9. Explain the process of turning sunshine into gasoline.
Living plants use solar energy to convert minerals and water into more plant mass. The effect is
storage of solar energy in biomass or hydrocarbons. Biomass is a form of fuel that can be burned
(combusted) by rapid oxidation. Across natural history, large amounts of bio-mass or
hydrocarbons are not combusted, but instead accumulate and decompose or metamorphose into
classes of material that may be called “peat” or “lignite”. Over millions of years, these organic
materials further convert into deposits of fossil fuels (coal, oil, and natural gas), trapped in
sedimentary. rock. Distillation of fossil fuels can separate natural gasoline and diesel oil from
other products.
Modern chemistry can produce “bio-fuels” without waiting millions of years for natural decay
processes to occur. Used oils, grease and food waste can be converted to fuel that can match or
exceed the quality of gasoline and diesel fuel that tend to contain too much sulfur.
10. Describe the chemicals and processes involving atmospheric and ground-level ozone.
Ozone is a molecule composed of three atoms of oxygen, for a total molecular weight of 48,
based on oxygen having an atomic weight of 16.
Ozone is produced by a variety of processes, such as ocean waves, lightning, electric motors,
industrial chemistry, sunshine, and ultraviolet solar energy interacting with atmospheric oxygen.
The result is two locations in the atmosphere where ozone is important to life on earth:
1. Atmospheric ozone “layer” in the stratosphere. This natural source of ozone is both created
and destroyed by collisions between incoming solar ultraviolet energy and oxygen molecules
(O2). The absorption of ultraviolet energy by ozone results in atmospheric heating in the
“ozone layer” and relatively less solar energy reaching the surface of the earth. So, the
atmospheric ozone layer not only protects everything from damage by ultraviolet rays, but
reduces the amount of solar energy at the earth’s surface where it would tend to convert to
heat.
2. Ground-level ozone near the surface of the earth. Human-produced ozone results from many
processes, but mostly commonly by air pollution that interacts with sunshine to produce yet
more air pollution. The process most commonly begins with combustion of petroleum and
coal that produces vast amounts of air pollution, such as dusty aerosols and oxides of
nitrogen, carbon and sulfur. These pollutants are bad enough by themselves, but they do even
more damage by interacting with ordinary sunshine to produce additional ozone.
This ozone is near the ground where it is harmful, rather than in the stratospheric ozone layer
where it might assist in blocking harmful ultraviolet rays.
Desirable atmospheric ozone is destroyed by release of man-made chlorofluorocarbon
compounds (CFCs). Each CFC molecule can destroy thousands of atmospheric ozone molecules,
resulting in undesirable amounts of ultraviolet solar energy reaching the surface of the earth.
Ordinarily, about 95% of Uv is absorbed by the ozone layer.