# Name: __________________________________ Period: ___________ Date: _________ Lab 5-1: Dew Point/Relative Humidity/Cloud Formation 80 Minutes

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Name: __________________________________ Period: ___________ Date: _________

Lab 5
-
1
:

Dew Point
/Relative Humidity
/Cloud Formation

80 Minutes

Introduction:

Millions of tons of water enter the atmosphere through evaporation of surface water and from

plant surfaces. (Evaporation + Transpiration = Evapotranspiration) As warm, moist air rises, it cools to dew
point temperature and condenses to form clouds if condensation nuclei are present. Condensation is the change
in phase from a gas to a liquid. Pre
cipitation may occur, returning water back to the earth's surface and plants.

Dew point temperature is very important in predicting rain and snow. Define
dew point temperature.

__________________________________________________________________________

__
________________________________________________________________________

When the air temperature and the dew point temperature are close enough together,

this indicates that there is an increased chance of precipitation. For example, if the air temperatu
re is 72°F and
the dew point temperature is 70°F, that means that the air only has to cool 2°F in order for the air to be saturated
so that dew or rain forms. Define
relative humidity.

_______________________________________________________________________
___

__________________________________________________________________________

If the dew point and air temperature are close together, the relative humidity is high. If the relative humidity is
100%, the air cannot hold more water vapor and it precipita
tes.

Procedure:

A
. Use of the sling psychrometer. Soak the psychrometer in water until the cotton sock is wet. Swing it around
carefully for 30
-
90 seconds until the temperature no longer falls. Record both the wet and dry bulb temperatures
in degrees Cels

Dry bulb temperature ________

C

Wet bulb temperature ________

C

Difference in wet and dry bulb temperatures
__________

(Using
ESRT
p.12) Dew point temperature __________

C

Find the relative humidity using the
same data that you recorded above for determining dew point temperature.
(Again using ESRT p.12 but the bottom chart this time). LABEL ALL ANSWERS!

Dry bulb temperature __________

C

Wet bulb temperature __________

C

Difference in wet and dry bulb tempera
tures__________

C

Relative Humidity __________%

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B
. Cloud formation.

List the steps necessary for a cloud to form. (Use your class notes and/or read

this lab)

1._________________________________________________________________________

2.________________
_________________________________________________________

3._________________________________________________________________________

4. ________________________________________________________________________

C
.

Use the information given in Report She
et 1 below to determine the dew point temperatures and cloud base
altitudes.

Report Sheet 1

DRY
-
BULB
TEMPERATURE

24°C

4°C

25.00°C

WET
-
BULB
TEMPERATURE

12°C

-

2°C

18.75°C

WET
-
BULB DEPRESSION

DEWPOINT
TEMPERATURE

CLOUD BASE ALTITUDE

D
.
Cloud

Base Altitude Chart. If you know both the air temperature and dew point temperature, you can find
the height at which clouds form. This is known as cloud base height or altitude. Refer to the "Cloud Base
Altitude Chart" below. Answer the following questio
ns
.

3

Questions:

1.
What are the 3 variables shown in the chart?

2. As altitude increases, what happens to the air temperature? (Look at the solid lines)

3
.

As altitude increases what happens to the dew point temperature? (Look

at the dashed lines)

4. Which changes more rapidly, air temperature or dew point temperature?

5. What would be the altitude of the bottom of a cloud if the surface air temperature is 10°C and the
surface dew point temperature is 2°C? (Label your answ
er)

(
Sandra Russell
-

Starpoint High School

/

2003)

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