Name: _____________________________________
Date: ________________________ Block: ________
DETERMINING CLOUD BASE INVESTIGATION
Background Information:
As air rises there is less atmosphere above it, so there is less pressure on it and the air molecules
can spread out. As they spread out, they take their heat with them. Imagine a cubic meter rising with
the air. As the air expands there will be fewer molecules in the cube, but more importantly for this lab,
there will be less HEAT in the cube which will cause a drop in the temperature. The expansion of the
air causes the temperature to drop, though no heat is actually taken from the air. Temperature
changes that occur due to expansion or contraction and without gain or loss of heat are called
adiabatic temperature changes. Normally as air rises in the lower atmosphere the temperature drops
at a rate of 10ºC for each kilometer it rises. This is known as the Dry Adiabatic Lapse Rate.
When the air spreads out (as it rises) it opens up space which more water molecules can fill, allowing
more water molecules to evaporate into the air than would have at a lower altitude. This lowers the
dew point by a rate of about 2ºC for each kilometer of rise. This is known as the dew point lapse rate.
You’ve already learned that when air is cooled below the dew point, condensation exceeds
evaporation and the amount of liquid water increases as dew or cloud/fog droplets form and grow. As
air rises, the temperature drops 10ºC for each kilometer, and the dew point drops 2ºC per km. If this
goes on long enough, the falling temperature will eventually catch up to the falling dew point. When
this happens, condensation will exceed evaporation and tiny droplets of liquid water will form in the
air. We call a mass of tiny water droplets in the air a______________ (or________ if at ground level).
In this lab you will use a graph of temperature and dew point lapse rates to help you determine the
altitude at which clouds will form.
Materials:
 Sling Psychrometer
 Dew Point & Humidity Charts
 Graph for determining Cloud
Base Altitude
How to use the graph to find the Cloud Base Altitude:
1. Locate the Dry-Bulb and Dew Point Temperatures on the x-axis.
2. Follow the slanted solid line up for the Dry-Bulb temperature and the dashed line for the Dew
Point temperature.
3. Find the point where the two lines meet and determine the altitude by using the Altitude scale
on the y-axis.
**Above this point temperature would cool below the dew point and a cloud would form.
Determining Cloud Base Investigation
Use the Dew Point and Relative Humidity Charts and the Graph for Determining Cloud Base Altitude
to complete the table below.
Trial
Dry- Bulb
Wet-Bulb
Temperature Temperature
(°C)
(°C)
1
20
16
2
20
17
3
20
12
4
14
10
5
14
14
6
28
18
7
26
15
Difference between
Dry-Bulb and Wet
Bulb Temperatures
Dew Point
(°C)
Relative.
Humidity
(%)
Cloud Base
Altitude
( km )
Classroom
Outside
Analysis Questions:
1. Does the classroom air have more or less water vapor than outside? _______________________
How do you know? ______________________________________________________________________
2. According to the background information, what happens to the air temperature as altitude increases?
____________________ At what rate? (oC/km) ____________________
3. According to the background information, what happens to the dew point as altitude increases?
____________________ At what rate? (oC/km) ____________________
4. As the air temperature gets closer to the dew point temperature, does the Relative Humidity of the air
increase or decrease? ________________________________
5. Does the height at which a cloud will form decrease or increase as the difference between the dew point
and air temperature increases? _________________________
6. Describe the relationship between relative humidity and cloud height:
_____________________________________________________________________________________
7. If there is fog in the air, is the air temperature higher, lower or equal to the dew point? ________________
How do you know? ______________________________________________________________________