Name ______Date: / /
Teacher ______per. ______
LAB # _____
TITLE: Humidity, Dewpoint and Cloud Formation
OBJECTIVE: Why does it sometimes feel sticky outside and other days dry as a bone? That is the humidity level. A sling psychrometer can tell us the humidity level and dewpoint. It works by measuring the amount of water that evaporates into the surrounding air. Two thermometers are placed in flowing air, one thermometer bulb being covered by a wet wick. Evaporation from the wetted bulb causes its temperature to be depressed relative to that of the dry bulb. In dry air, evaporation will be rapid and the depression will be greater, giving a low relative humidity reading. In humid air there will be little evaporation and the depression will be small, giving a high relative humidity reading. If the air is completely saturated no evaporation can take place, therefore both the wet and dry bulb readings will be the same. This equates to a relative humidity of 100%. Cumulus clouds are our “puffy” fair weather clouds. They are often flat on the bottom and rounded on top. The distance from Earth’s surface to the bottom of these clouds is often the same for a large group of them. Clouds can only form if a specific temperature, called the dewpoint, is reached. Since the air temperature decreases with height above Earth’s surface, clouds may form if the air temperature is cold enough to be at the dewpoint at some altitude. In this lab, we will study the relationship between the relative humidity, dewpoint temperature, and the height above Earths surface at which clouds form.
HYPOTHESIS: If moisture in the air increases, then…because…
VOCABULARY: dewpoint, relative humidity, psychrometer, wet-bulb, dry-bulb, wet-bulb depression, and cloud base
MATERIALS: Sling psychrometer (procedure D)
PROCEDURE A:
Use your Reference Table to complete the chart in the data section for procedure A and B You will complete the last column Cloud Base Altitude later in procedure C.
PROCEDURE: B:
Refer to the “Cloud Base Altitude Chart” which shows decreasing temperature with changing altitude and answer questions 1 through 5 below.
1. What change occurs in the dewpoint as altitude increases?
2. How does the air temperature (dry-bulb temp.) change with increasing altitude?
3. Which changes more rapidly with increasing altitude, air temperature or dewpoint temperature?
4. At what altitude do the dewpoint and air temperature become the same if the surface air temperature is 0˚C and the dewpoint temperature at the surface is -20˚ Celsius?
PROCEDURE C: Use the information you calculated in procedure A and B to determine the cloud base altitudes.
PROCEDURE D:
1. Go to the various locations and use a sling psychrometer to measure the wet and dry bulb temperatures.
2. Complete your data table by determining the relative humidity.
3. Convert your data form °F to °C and complete the table.
4. Determine the dewpoint temperatures and relative humidity.
QUESTIONS: (Answer in Complete Sentences)
1. Why does the height of the cumulus cloud base change from day to day
2. Visit the website below in order to answer question 3. http://profhorn.meteor.wisc.edu/wxwise/AckermanKnox/chap4/cloudbase.html
What happens to the cloud base altitude as the temperature and dew point approach one another?
3. What would happen to the height of the cloud base if the dewpoint temperature were lower?
4. How would it be possible to have a day without any clouds?
5. What relationship would you expect to find between the air temperature and dewpoint temperature at ground level if the area is covered by fog?
6. What happens to the air temperature of a descending mass of air?
7. What happens to the dewpoint temperature of a descending mass of air?
8. Explain why a descending (dropping) air mass would tend to become drier.
CONCLUSION: Describe, step by step, how you can determine the base altitude at which clouds form?
ABOVE & BEYOND: Include several pictures of labeled clouds showing the low line of cloud formation around the horizon.
DATA: Procedure A
Dry Bulb Temperature(°C) / Wet Bulb Temperature
(°C) / Wet & Dry Bulb
Depression
[difference]
(°C) / Dewpoint
(°C) / Cloud Base Altitude
(km)
20 / 17
16 / 5
24 / 20
8 / 4
26 / 3
1 / 16
2 / -4
Location / Wet bulb
(°C) / Dry bulb
(°C) / Difference
(°C) / Dewpoint
Table
(°C) / Relative Humidity
Table (%)