ATMS 100: Introduction to Meteorology
Hands-On Meteorology
Moisture in Your Everyday Life
The questions on the next several pages are related to real life experiences that you have
had or may encounter at some time in your life. Answer as many questions as you can as
completely as possible in the time provided. You should work with at least one other
person. Your group will be given a specific question number to start with. At the end of
the allotted time each group will discuss their answers.
1. You live in Kansas and it is January. Your clothes dryer just broke and you have
to hang your clothes out to dry on the clothesline. The weather conditions indicate
a temperature of 40 F and a dewpoint temperature of 20 F. Will the clothes dry?
Explain.
Yes, the clothes will dry. Clothes dry when water evaporates from them.
As long as the air is not saturated, water can be evaporated into it. Once
the air is saturated it contains as much water as it possibly can (for that
temperature) and you cannot add any more vapor into the air. The
clothes will dry because the dewpoint and air temperature are far apart,
indicating that the air is far from being saturated.
2. Explain why does it take longer for vegetables to cook at higher elevations. (Hint:
consider the elevation and the temperature at which water will boil)
At the surface, the boiling point of water is 100 C (212 F). As water
boils, bubbles of vapor rise to the top of the liquid and escape. The
saturation vapor pressure exerted by the bubbles must equal the pressure
of the atmosphere, otherwise the bubbles would collapse. So, boiling will
only occur when the saturation vapor pressure of the escaping bubbles is
equal to the total atmospheric pressure.
As you move to higher elevations, air pressure decreases. This means that
you will be able to get water to boil at lower temperatures (92 C). Recall
that SVP is related to temperature. Lower temperatures produce lower
SVP. Once water boils, the temperature remains constant. If you continue
to heat the water, the energy is used to convert the liquid water to water
vapor.
Because the vegetables are boiling at a lower temperature, they will have
to boil longer to cook thoroughly.
3. Would you expect water in a glass to evaporate more quickly on a windy, warm,
dry summer day or on a calm, cold, dry winter day? Explain.
The water would evaporate more quickly on the summer day. The windy
conditions will enhance evaporation (as opposed to the calm winter day
conditions.) Both days are “dry”, however they are very different
Moisture in your everyday life
ATMS 100
temperatures. The air has a larger capacity for water vapor at higher
temperatures and so more water will evaporate into the air on a warmer
day than a colder day.
4. If you take a hot shower in the bathroom, the mirror will fog up. Explain why this
happens. Explain why aiming a stream of air from a hair dryer at it will make the
mirror clear again.
With the hot shower on, you are increasing the dewpoint in the bathroom
as water evaporates from the stream of water from the shower into the
air. When the amount of water vapor in the air is the maximum amount
possible, then the air is saturated (the dewpoint temperature = air
temperature). At this point, adding any more vapor into the air will cause
condensation to occur. Water droplets begin to condense on the mirror
(they are easy for us to see but they are condensing on all the surfaces in
the bathroom). If you blow air over the condensed water, evaporation
will start to occur. If the air is warm, the saturation vapor pressure of the
air near the mirror will increase, thereby decreasing the relative
humidity and causing evaporation to occur.
5. Two people with long thick hair live in different parts of the country; one in
Arizona and one in North Carolina. In the summer if they both wash their hair and
leave it to air-dry. One will have dry air within an hour while the other persons
hair not dry all day. Whose hair will remain damp and why?
The person living in North Carolina will have damp hair. Hair dries as
water evaporates from it. When the air is close to saturation it is more
difficult to evaporate water into the air. When the air is further from
being saturated water will evaporate easily into it. In North Carolina in
the summer, the relative humidity is typically high because there is ample
moisture in the air from the Gulf and the Atlantic Ocean. In Arizona, the
air is dry as there is not plentiful moisture source available.
6. Explain what is meant by the phrase “It’s not the heat, it’s the humidity.”
This phrase is often used in conjunction with hot summer temperatures.
When air temperature is high, but there is little moisture in the air (low
humidity), our bodies are able to cool easily by evaporating perspiration.
When temperatures are high and humidity is high, it is more difficult to
cool our bodies and so it feels oppressively hot.
7. During hot summer days in California, many people seek refuge in the shade
where it is much cooler than in the sun. Explain why this method of keeping cool
does not work in Illinois in mid-summer.
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In California, humidity is typically very low because there is no
significant moisture source (ocean is too cold to effectively evaporate
large quantities of water vapor). When it is hot in the sun, you can move
into the shade and immediately feel a significant temperature difference.
During mid-summer in Illinois the humidity is high and so it feels a lot
hotter than it really is (see #6). When you move into the shade, you lose
the direct sunlight but the humidity does not change under the tree and
so it still feels relatively hot.
8. From a scientific perspective, what time of day would be the best time to water
your lawn?
For the most effective watering you want to have the least amount of
evaporation. This would occur when the relative humidity is high. When
RH is high, it is difficult to evaporate much more water vapor into the
air. The RH is typically highest in the early morning hours and so this
would be the most effective time to water the lawn.
9. Why can you see your breath on a cold morning? Does the air temperature have to
be below freezing for this to occur?
When the air temperature is cold, the amount of water vapor needed to
saturate the air is also low (saturation vapor pressure is low). When we
exhale, we add moisture into the atmosphere. Adding moisture raises the
dewpoint temperature. If the air is already close to saturation, the
moisture we add to the atmosphere may bring the air to saturation and
eventually condensation may occur; hence we see our breath as a fog. The
air temperature does not have to be below freezing for this to occur, just
close to saturation.
10. Explain why icebergs are often surrounded by fog.
The icebergs are frozen and very cold and so they have a chilling effect on
the surrounding air. (Air cools by conduction of heat from air to iceberg;
small amount of ice may melt and evaporate into the atmosphere.) If the
air is cooled to the dewpoint temperature, a fog will form (condensation
takes place).
11. During a summer visit to New Orleans, you stay in an air-conditioned motel. One
afternoon, you put on your sunglasses, step outside, and within no time your
glasses are “fogged up.” Explain what has apparently caused this.
Inside the motel, the air temperature is cool, and it is relatively dry (air
conditioning tends to dry out the air). Your sunglasses therefore are also
cool. Cooler temperatures do not require as much water vapor to saturate
the air (lower saturation vapor pressure). When you step outside, you
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encounter warmer, more humid air. There is plenty of moisture available
outside (high humidity or high dew point). Since your glasses are still
cold, the temperature of your glasses and the dew point temperature are
probably close together, near or at saturation. Water vapor condenses on
the glasses creating a fog.
12. On a winter night, the air temperature cooled to the dew point and fog formed.
Before the formation of fog, the dewpoint remained almost constant. After the fog
formed, the dew point began to decrease. Explain why.
Fog is composed of liquid water drops. When fog forms, water changes
from a vapor to a liquid. Dewpoint temperature is a measure of the
amount of water vapor in the atmosphere. If the amount of water vapor
in the atmosphere decreases, then the dewpoint will decrease also.
13. Is humid air or dry air more dense?
Humid air is less dense than dry air. If the air is completely dry then
there is no water vapor in it. Approximately 99% of the atmosphere is
Nitrogen (N2) and Oxygen (O2), which have molecular weights of 28 and
32, respectively. This means that the molecular weight of dry air would be
approximately 29.
If the air is humid then there is water vapor in it. This means that some of
the dry air must be replaced with water vapor. Water vapor has a
molecular weight of 18 (2 Hydrogen = 2; 1 Oxygen = 16). Therefore, you
are replacing some of the 29 molecular weight air with 18 molecular
weight air, thereby making the air “weigh” less. Density is a mass (or
weight) per unit volume and therefore humid air is lighter and less dense
than dry air.
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