Lapse Rate Homework Instructions
In this homework exercise, you will make a graph of the estimated environmental lapse rate
in the Orem/Salt Lake City area.
Recall that the environmental lapse rate (lapse rate for short) describes how the temperature of the
air changes with altitude. Usually the air is colder at higher altitude, and the change is expressed
as the number of degrees C that the temperature drops per 1000 meters you go up in altitude. Sometimes it
gets warmer with altitude.
Ideally, the lapse rate is determined by sending a weather balloon up into the atmosphere, which
measures the temperature and altitude and radios the information to a weather station. We don't have a
weather balloon and our disposal, but we do have weather stations
in the mountains that can give us a decent indication of the temperature at altitude above the valley
floor. So to complete the exercise, you will get the temperature and elevation from a number of nearby
weather stations and then make a graph of the data. Detailed step-by-step instructions follow; please be
sure to follow all of the instructions carefully
1. Obtaining elevation and temperature data. Go to the
Utah Mesonet website.
Click on the link called "Salt Lake/Utah Valleys". You will see a map that has temperature plus wind speed and
direction for a number of telemetered weather stations in the area. If you move your cursor over a
weather station, a small pop-up will appear that gives current information for the station. Included in the
information will be the elevation of the station as well as the temperature and the time the temperature
was recorded at. Find and write down the temperature and elevation for at least 15 stations that
range in altitude from 4500 to 11,000 feet elevation, and note the time at which you are collecting the
temperatures. Also, be sure that you have at least one station for each 1000 feet of altitude; in other words,
you must have at least one station in the 4000-5000 ft range, one from the 5000 to 6000 range, one from 6000 to
7000, etc. You also need to note the weather conditions at the time the temperature data were measured and you
recorded them (e.g., clear skies, partly cloudy, overcast, raining/snowing etc.). Oh, and one last thing: make
sure that the time listed in each pop-up is fairly current; sometimes the stations
break and an old temperature is listed.
2. Making the graph. Now you are ready to graph your data. I've prepared a
graph for you to plot your data on,
an example that I did.
Download, save, then open and print the graph (right-click on the link, choose "save as" and note where your
browser saves the file to, then open it in Adobe Acrobat and print it). Note that the x-axis of your graph is
temperature and the y-axis will be elevation, and that they are properly labeled with values and the units (degrees
Fahrenheit and feet). The axes of graphs should always be labeled. Your task is to plot the information (elevation
and temperature) that you obtained from the weather stations in step 1. Plot the data now.
3. Determining the lapse rate. The last step of the exercise is to estimate the lapse rate from the data
that you've graphed. The lapse rate essentially is the slope of the line, but expressed in degrees C per 1000 meters
(and technically, its the slope times negative one).
a. Eyeball your data and see if they (the points) look to roughly follow a line. The should do so, at least somewhat.
Draw in the line that the points lie along using a ruler (note that the line should be a "best-fit," and we could find
it mathematically. The line must be straight, so be sure to use a ruler to draw it.
b. Now you need to figure out how much the temperature changes for every 1000 meters in altitude. To do this, first
find the temperature at the bottom end-point of your best-fit line and at the top. Convert each of those temperatures from
Fahrenheit to Celcius (This is easy! Recall that the formula to convert from Fahrenheit to celcius is C = 5/9 x (F - 32).
Just put plug your degrees Fahrenheit number in the F spot, crunch the numbers and you'll have the celcius equivalent).
Next, find the elevation difference between the top and bottom of your line (i.e., find the elevation for the top end-point
of your line and for the bottom end-point, and subtract the bottom value from the top value). At this point you could say that
the lapse rate is _____ degrees per ____ feet. However, we want the lapse rate as a number of degrees celcius per 1000 meters.
c. The second to last calculation is to convert from feet to 1000's of meters. This is easy to. There are 3280 feet per
1000 meters, so just take your elevation difference (from the top to the bottom of your best-fit line) and divide
d. The last little calculation is to take the temperature number you obtained in part (b) and divide it by the
number you got for (c) (which is the number of 1000's of meters of elevation from the top to the bottom of your line).
e. Reporting your answer. You have now estimated the lapse rate. To state it, you say that it is _______ degrees
celcius per 1000 meters, where your answer to (d) goes in the blank. If its more than about 10 degrees, something
went wrong. Write down the lapse rate you found in the space below your graph, note the date and time on which the data were
measured and recorded, note the weather conditions, make sure your name, site, date and the time at which you recorded
the temperature data are on the top of the graph, include your data (the temperatures and altitudes that you used)
photocopy your work and submit it to your facilitator.