Name _______________________________
Date _________
Period ________
Atmospheric Moisture
1. Atmospheric moisture - ____Water____- exists in three states of
matter: ____solid_____, ____liquid_____, and ____gas_____.
a. Gaseous water in the air is called __water vapor____.
b. The oceans cover 70 % of the Earth’s surface, and is the __main_
source of moisture in the atmosphere. Other sources are rivers,
lakes, __glaciers___, and __volcanoes___.
2. Water vapor enters the atmosphere by:
a. _Evaporation_____ -liquid to gas
b. _Sublimation_______
-solid to gas
c. _Transpiration_______ -liquid to gas from plants
d. Collectively, evaporation and transpiration are called
evapotranspiration
Energy of Evaporation and Transpiration of Water
1. Large amounts of _energy__ are required to change liquid water into
water vapor during the process of evaporation (an endothermic reaction).
According to your reference tables, it takes ___540___ calories/gram
for evaporation (vaporization).
2. Why do you get cold when you step out of the shower?
a. The evaporating water is __stealing___ the heat energy from your
body and the other water molecules. How does this endothermic
process work?
The average water molecule has less_than 540 cal/gram of energy in the liquid
state, so it cannot escape as a gaseous water vapor. The warm, high-energy skin
molecules collide_ with the water molecules. After many collisions, some of the
water molecules gain _more__ than 540 cal/gram of energy, and can now escape
into the air (instead of giving the energy back to the slower moving skin molecules).
Process of Evaporation
1. As stated earlier, the average water molecule has less than 540 cal/gram
of energy (called _kinetic_ energy), but some have more. These water
molecules evaporate into the air, and become __water vapor__.
2. Some water vapor molecules in the air hit the surface of the water and
___condense___ back as liquid water due to the attractive force of the
liquid water molecules (condensation rate).
3. If __more___ water molecules evaporate than condense, then eventually
all of the water will evaporate into water vapor. The rate of evaporation
is __greater_ than the rate of condensation. This is what happens as a
mud puddle dries up.
4. If the rate of condensation ___equals__ the rate of evaporation, then
the mud puddle does not dry up. This is called __dynamic___
_equilibrium_______. This occurs in a closed 2-liter of coke.
Factors Affecting Evaporation Rates of Water
1. The overall (net) evaporation rate (evaporation rate vs. condensation
rate) of water at a location is determined by:
a. The amount of __energy____ available. Warmer air temperatures
___increase______ the evaporation rate.
b. The __surface____ area of the water. Water in a cookie dish
evaporates faster than water in a 2-liter bottle.
c. How much water vapor is ___already__ in the air. More water vapor
in the air decreases the overall evaporation rate, because of the
increase in ___condensation_____.
d. Faster winds __increase___ the overall evaporation rate.
Humidity, Temperature, and Dew Point
1. Like moisture, humidity is a general term that refers to the water vapor
___content___ in the atmosphere.
2. Absolute (specific) humidity is the __amount____ (mass) of water vapor
in a certain amount of air, and is measured in grams of water vapor per
cubic meter of air.
3. Moisture capacity is the __maximum___ amount of water vapor that the
air can hold. As temperature increases, moisture capacity increase, and
as temperature decreases, moisture capacity _decreases____.
4. Relative humidity is a __comparison___ between the amount of water
vapor in the air (absolute humidity) to the maximum amount it can hold
(capacity). *note: the answer will be in percent (%).
5. The formula for relative humidity is:
Relative Humidity = Absolute Humidity X 100
Moisture Capacity
-So. if the absolute humidity in the air during the day is
8 grams/meter3, but the air can hold 16 grams/meter3, then what is
the relative humidity? 50%
-At sunset the temperature lowers, which lowers the moisture
capacity to 10 grams/meter3. What is the new relative humidity?
80%
-As the night gets even cooler, the moisture capacity lowers to 8
grams/meter3. Now what is the relative humidity?
100%
6. The closer the absolute humidity is to the maximum capacity of the air to
hold moisture, the ___higher_____ the relative humidity will be.
7. Saturated means filled to _capacity_____. This is when the absolute
humidity is at its maximum capacity. Relative humidity is _100_%.
8. Dew Point is the temperature at which the air is completely
_saturated______ with water vapor. Absolute humidity equals moisture
capacity, so relative humidity is 100%.
a. If the temperature cools even more, then the absolute humidity
exceeds its capacity, and ___dew___ begins to form on the grass,
__fog___ begins to form near the ground, or ___clouds__ begin to
form in the air.
b. For example, if the absolute humidity is 8 grams/meter3 and the
maximum amount of moisture the air can hold (moisture capacity) is
only 7 grams/meter3, then there is a 1 gram/meter3 _excess___.
c. This 1 gram/meter3 excess cannot be gaseous water vapor, so it turns
to ___liquid___ water (dew, fog, or clouds) or __ice__ (frost or
snow).
d. The dew point temperature depends upon absolute humidity. The
more water vapor in the air, the dew point temperature _increases_.
The less water vapor in the air, the dew point ___decreases____.
Measuring Relative Humidity and Determining Dew Point
1. Relative humidity and dew point can be measured indirectly with an
instrument called a _sling__ _psychrometer____.
2. This instrument contains two thermometers.
a. The ordinary thermometer measures the air temperature, and is
called a ___dry bulb___ thermometer.
b. The other thermometer has a wick around its bulb, and is called a
___wet bulb_____ thermometer.
3. When the wick is moistened and the thermometers are whirled in the air,
the temperature of the wet bulb __cools__ because of the cooling
effect of the evaporation of the water.
a. In dry air, there is _more__ overall evaporation, so the wet bulb
temperature decreases (or ___depresses__ ) significantly.
b. In humid air, there is __less__ overall evaporation, so the wet bulb
temperature decreases (or depresses) little.
4. Using the charts on page _12__ of your reference tables will allow you to
determine the relative humidity and dew point temperature.
a. Wet bulb depression means the difference between wet bulb and dry
bulb temperatures.
5. Use the examples below and determine the relative humidity and
dewpoint.
Dry Bulb
20
20
20
20
10
-6
Wet Bulb
Dewpoint (C) Relative Humidity(%)
14
16
18
20
8
-8
___10___
___14___
___17___
___20___
___6____
__-14____
___51____
___66____
___82____
___100___
___76____
___48____