Salt Lake Community College – Meteorology 1010
Quiz #2
J. Allred, Adjunct
Answers and explanations
1. Which of the following atmospheric conditions probably contains the most latent heat:
a.
b.
c.
d.
e.
45° F and 65% humidity
60° F and 70% humidity
70° F and 55% humidity
90° F and 40% humidity
95° F and 50% humidity
According to the textbook, a 10% increase in temperature can allow a 100% increase in absolute humidity. In other
words, air with high temperature can hold much more water. So, the obvious choice above is to go with the
“biggest bucket” or highest temperature -- as long as the other choices are cooler and not much more humid.
For instance, choice ‘d’ has only about half the capacity to hold water as does ‘e’ - - and it is even less humid at
40%. Choice ‘c’ is much, much smaller than ‘e’ and has only the same relative humidity.
The fact that some of the much cooler temperatures are holding a bit more “relative humidity” does not mean a lot
as long as there are such large differences in temperature.
2. A cubic foot of air at 10,000 feet can hold more energy than a cubic foot of air at sea level because it is closer to
the sun. In most cases, it actually does hold more energy.
True ___
False _X__
A cubic foot of air at 10,000 is probably a lot less dense on any given day. As such, thinner air has less ability to
hold energy, because there is less mass available. A complete vacuum (zero air pressure) would not be able to
hold any energy at all. The higher you go in the atmosphere the thinner the air, and usually the cooler the
temperature.
Being closer to the sun helps in a small way, because the upper atmosphere does get hit with solar energy first, and
some of it is captured up there.
3. A warm, sunny day is more likely than a rainy day to exhibit dangerous levels of Ozone air pollution.
True _X__
False ___
Photochemical “smog” is created by solar energy interacting with certain types of air pollution, such as oxides of
nitrogen and sulfur. When air pollution is already a problem, a hot, sunny day will make things worse by
chemical reactions between pollution and solar energy. In contrast, a cloudy day will reduce the amount of
solar energy available for producing ground level ozone.
Meanwhile, atmospheric ozone, in a band of gases located at about nine miles above the surface of the earth, is not
created by air pollution, but instead helps reduce the amount of harmful ultraviolet solar energy that would
otherwise make life unbearable at the surface of the earth.
4. Lifting a parcel of air to a higher altitude will tend to help it reach vapor saturation.
True _X__
False ___
Lifting a parcel of air (adiabatic) means that the parcel will de-pressurize and thereby cool, as energy is dispersed
or de-concentrated. Cooler air has less capacity to hold moisture (smaller bucket). At some point, the bucket is
too small to hold whatever moisture is available (the absolute humidity), so condensation occurs as ‘saturation’
or dew point is reached.
5. An ice cube in the freezer will slowly evaporate for two reasons:
- it is surrounded by very dry air that has at least some capacity for holding more vapor pressure
- cold air in the freeze still has enough heat to cause sublimation of ice directly to vapor.
True _X__
False ___
Even in a freezer there is substantial energy, enough to help slowly evaporate the ice. Because the freezer is cold,
evaporation skips the liquid water stage and goes directly to vapor. Typically, the water vapor then just refreezes on the walls of the freezer or goes out as fog when the freezer door is opened. The process is slow, of
course.
6. A weather “inversion” will exhibit which of the following conditions:
a.
b.
c.
d.
e.
Unstable conditions that typically include colder air aloft and warmer air below
Stable conditions, with colder air aloft and warmer air below
Cloudy, stagnant and polluted air, often with rain and low or falling air pressure
Stability, relatively high air pressure, no precipitation and colder at the surface than up in the atmosphere.
Unstable conditions, typically with rising air and precipitation.
Weather inversions involve stable air that is NOT rising. It may even be descending. Stable conditions tend to be
stagnant, dirty, hazy, making it even harder for sunlight to warm up the surface so that stale air can rise and
clear out.
Inversions also typically include an unusual condition: air aloft is often warmer than air below, which helps keep
things stable – warm above aloft has no reason to descend into heavier air. Cooler, more dense air at the bottom
has no way to rise. Thus, stable conditions exist. Local air pollution builds up.
Low air pressure is NOT associated with inversions because low pressure is associated with rising air, instability,
turbulence, air movement, freshening and sometimes precipitation. Storms are about rising air, not stable,
stagnant air.
Weather inversions can occur any time of year, but are most common during winter, when there is less solar energy
for heating the earth and atmosphere. Also, during winter nights radiation from the surface can allow even
more earth heat to escape into space, making the cycle worse.
7. A vapor-saturated parcel of air will tend to rise if it is surrounded by a region of dry air.
True _X__
False ___
Water vapor is actually lighter in weight than regular atmosphere, so it will tend to rise. When water vapor cools
enough it will condense into a cloud or fog, thus releasing latent, or hidden heat. That newly available heat
adds warmth to the surrounding air, causing it to rise further. As long as there is enough water available, there
will also be latent, or hidden heat available. So, rising air may just keep rising until it finally “dries out” or
cools off enough to the same temperature as other air. Without either heat or moisture, air will tend to be stable.
8. Wet air that is blowing up over a mountain will tend to precipitate (rain or snow) because:
a. Rising air pressure going up the mountain helps squeeze humidity out of the air
b. Air pressure is lower at higher elevations, causing temperatures to drop and leading toward vapor
saturation
c. Wind helps evaporate air out of clouds, causing rain and snow
d. Snow caps near mountain tops encourage clouds to drop moisture by reflection off white surfaces
e. All of the above are true.
Rising air will de-compress, and thereby cool down by expansion. Available energy is essentially diluted or
dispersed in the expanding parcel of air. Eventually, enough cooling may occur to reach vapor saturation (dew
point or “bucket full”). Precipitation often follows.
So, air pressure is lower at higher elevations. Therefore, air tends to be cooler at higher elevations. Cooler air
cannot hold as much moisture as warmer air - - precipitation may follow.
9. When clouds form, heat is released. When water freezes, heat is released.
True _X__
False ___
When vapor saturation occurs, dew point temperature is reached and the air “bucket” is full. The appearance of
water vapor as visible water means that hidden heat is being released. So, when fog forms temperatures often
rise a little because heat was ‘hidden’ in the air along with the vapor that was invisible before saturation
occurred.
In order for water to freeze, heat in the water has to be released or handed-over to something else. So, when water
freezes, the temperature of nearby objects, such as air, or grass or dirt, tends to rise a little. If water can’t give
away its heat, then it can’t change state from water to ice.
10. Rain is more likely to occur after moist air flows over the top of mountains and cools and condenses on its way
down the other side.
True ___
False _X__
Precipitation is associated with rising air, such as air going UP over a mountain. Rising air cools down and
becomes saturated, allowing precipitation to occur.
On the back side of the mountain, air is more likely to head back downhill. Going down means rising air pressure
and a tendency for the air to warm up by being compressed. Energy in the air gets concentrated by being
pressurized as it goes down, so air temperature tends to be higher. Warming air has increasing capacity to hold
water, so there is little chance of rain. As air descends, rising temperature means decreasing relative humidity,
because warmer air has more ability to hold water, not less.
That’s why the lee side or back side of mountains tends to be dry - - the rain “shadow.” The precipitation occurred
on the front side - - the “going-up” side. The “going-down” side should be warmer and drier.
Descending air is often considered ‘thirsty’ which helps explain why high pressure systems are often found over
deserts.
11. A hot, sunny sky in Arizona may well contain more absolute humidity than a foggy, winter sky in Alaska.
True _X__
False ___
As discussed in Question #1 above, hot air has great capacity to hold water in the form of invisible vapor. So, a hot
day in Arizona with 20% humidity probably represents a much larger amount of absolute humidity (actually
mass of water) than does a cold day.
Cold air has so little capacity to hold moisture that fog can be produced when only a small amount of atmospheric
moisture is present. Cold air simply can’t hold much vapor. Fog can appear even when the air is dry enough to
make your skin crack from dryness.
A big reason why rain is uncommon in Arizona is not just because the region is dry, but also because the air is so
warm that it can hold vast amounts of water in vaporized form. During the warm season, Arizona air is seldom
able to cool down enough to condense out any clouds, let alone produce rain, even though there may be more
water vapor available than on a cold day in Alaska.
12. To keep her orange grove from freezing, a Florida farmer will be encouraged to spray cold water on her trees.
True _X__
False ___
Liquid water contains a great deal of heat. Spraying water on green plants means “spraying heat” on them. Even
more, when water freezes, it gives off additional heat to nearby objects.
Water “sacrifices” its heat to nearby objects, such as plant leaves.
13. On a 90°F day with extremely dry air, at what altitude above that place would you expect to find
atmospheric ice crystals?
a.
b.
c.
d.
e.
11,500 feet or higher
Between 17,500 feet and 19,500 feet
22,500 feet or higher
17,500 feet or higher
None of the above is a good estimate.
Based on class discussion, we agree that totally dry air will tend to be cooler by 5.0°F for each 1,000 feet of
increasing altitude. So, at 1,000 feet higher elevation that dry air should be cooler by 5.0°.
In order to find frost in that air column, you would have to go up high enough for the air temperature to drop from
90°F to just 32°F. That is a drop of 58 degrees. In order to lose 58 degrees at five degrees per thousand feet,
you would to go up more than 11,000 feet:
90° – 32° = 58°
58/5 = 11.6 units of altitude at 5° loss each
The best answer option is  11.6 x 1,000 = about 11,500 feet
14. As temperatures rise during the day, absolute humidity will tend to go down.
True ___
False _X__
Remember, absolute humidity is the actual weight or volume of water in the air. Relative humidity is a comparison
between how much water is in the air, and how much could be in the air based on the size of the “bucket.”
Warmer air can hold more moisture, so as the day warms up, the capacity of the air to “hide” water vapor goes up.
So, without adding or removing any water, a warming day will show gradually decreasing humidity. The bucket
has gotten bigger, so relative humidity is lower.
For example, using simple class discussion, if the morning begins with five units of water available (as absolute
humidity) a temperature of 50° in the morning could show fog - - the bucket is full at 5/5 = 100%.
At the hottest part of the day, an air temperature of 100° would mean a “ten gallon bucket” with only “five gallons” in
it, so relative humidity drops to just 50%.
In reality of course, a day-time temperature change from 50° to 100° would mean an even greater drop in relative
humidity - - but for this class we can just use simple “bucket” models.
15. The actual weight or volume of water in a parcel of air is best considered a measure of:
a. Absolute humidity
b. Relative humidity
c. The likelihood for rainfall to occur
d. Dew point
e. All of the above are true.
Absolute humidity represents the actual weight of water in the air - - its actual atomic mass in grams or ounces.
That water does not usually mean much unless you compare it to how much the air COULD hold - - now you have
relative humidity.