Dimming the Sun
Certain weather variables influence the amount of water that evaporates each day from a standing body of water. In this activity, your team will collect and analyze weather data to find out what variables affect water evaporation amounts.
Your team will be collecting data once a day for four uninterrupted days. Your
teacher will provide you with the initial pan water level. Record that depth on
your "Data Sheet" handout.
Each day, gather and record the following measurements for both pans on your
"Data Sheet" handout.
Air Temperature (in °C): Read the thermometer that
is next to the sheltered pan or find the temperature for your appointed time
Relative Humidity (in percent): Read the hygrometer or record
from another weather source.
Wind Speed (in km/hr): Read the anemometer or record from another
Solar Radiation: Note the amount of cloud cover. For the
unsheltered pan: 1=all cloudy, 2=mostly cloudy, 3=equal mix of sun and clouds,
4=mostly sunny, 5=all sunny; for the sheltered pan, rate the amount of shade:
1=fully shaded, 2=mostly shaded, 3=partly shaded.
Water Depth (in mm): Use the ruler to measure the water
depth. Measure from the same place each time. If you are a member of Team 2 or
Team 4, take your reading, and then refill the pan with room temperature water
to the exact level of the original water mark. If it has rained during
the day or night, remove water from the pan until the water line is at the
exact level of the original marked starting point.
Precipitation (in mm): Record any water that has collected in the
rain gauge. If you are a member of Team 2 or Team 4, empty the gauge after you
take your reading.
When all your data has been gathered for the day, perform the following
[Water Depth - Precipitation = Adjusted Water Depth]
[Initial Pan Water Level - Adjusted Water Depth = Evaporation Amount]
Use the "Data Sheet" handout to record your data for four consecutive
days. Also enter each day's data into the all-class "Measurements Chart."
After all the data has been recorded, you will work as a class to analyze
it. When you are done, answer the questions on this page.
Write your answers on a separate piece of paper.
How did the pan evaporation amounts change in relation to air temperature?
Relative humidity? Wind speed? Solar radiation?
Which variables seem to most affect evaporation amounts? Explain your
difference, if any, was there in the amount of evaporation that occurred in the
unsheltered location compared to the sheltered location? What might account for
If the amount of sunlight reaching Earth's surface were to decrease, would
you expect the evaporation rate on Earth's surface to increase, decrease, or
stay the same? Why?
What are the limitations of the experiment?
What could the class do to have more confidence in the conclusions drawn
from this experiment?