As Dr. C wrote during his Surface Temperature Field Campaign, the weather in mid-December was cold in many parts of the United States. At our house here in Boulder, Colorado (Figure 1), this morning’s minimum temperature was -21 degrees Celsius. Just 20 kilometers east of here, the minimum temperatures was 27 degrees Celsius below zero, and about 50 km to the southeast of here, the minimum temperature reached -28 degrees Celsius. The weather reports were saying that those of us who live closer to the mountains weren’t having temperatures as cold as those to the east of us.

Figure 1. Map showing location of Boulder and CASES-99. The colors represent contours. The Rocky Mountains are yellow, orange, and red on this map. The colors denote elevation, with yellows, oranges and reds indicating higher terrain.

How does the air temperature relate to the surface temperatures that the students measured during Dr. C.’s field campaign? To answer this question, I looked at how the surface temperature related to the air temperature at our house.

The air temperature at our house was measured at 1-1.5 meters in our carport, and also on a thermometer I carried with me on our early-morning walk around the top of our mesa. That temperature, as noted above, was -21 degrees Celsius. To get the surface temperature, I put the thermometer I was carrying on the surface after I finished my walk. I am assuming that this temperature is close to the temperature that would be measured by a radiometer like the one used in GLOBE. I took the reading ten minutes later.

Just for fun, I also measured the temperature at the bottom of our snow (now 10 cm deep) and at the top of the last snow (about in the middle of the snow layer). At these two places, I put the snow back on top of the thermometer, waited ten minutes, and then uncovered the thermometer and read the temperature. The new snow was soft and fluffy, while the old snow was crusty; so it was easy to find the top of the old snow.

All of the measurements were taken close to sunrise, when the minimum temperature is normally reached, and the area where I took the measurements was in the shade.

Figure 2 shows the temperatures that I measured.

Figure 2. Temperature measurements at the snow surface, between the old and new snow, at the base of the snow layer, and at 1-1.5 meters above the surface at 7:30 in the morning, local time.

That is, the temperature was coolest right at the top of the snow. The temperature was warmer at the top of the old snow, and warmest at the base of the snow. As noted in earlier blogs, the snow keeps the ground warm.

The temperature at the top of the snow was also cooler than the air temperature. The surface temperature is often cooler than the air temperature in the morning, especially on cold, clear, snowy mornings like this one. However, on hot, clear, days in the summertime, the ground is warmer than the air.

Here are two sets of measurements taken in the Midwestern United States in October of 1999. Could you guess which measurements were taken at night, and which measurements were taken during the day even if the times weren’t on the labels? The first plot is from data taken after sunset, while the second plot was from data taken at noon.

Figure 3. Data from the 1999 Cooperative Atmosphere Exchange Study (CASES-99) program in the central United States, courtesy of J. Sun, NCAR.

For years, I have been measuring the rain in our back yard using a standard rain gauge similar to the ones used by the U.S. National Weather Service (Figure 1). Like the gauge used by GLOBE students, rain goes through a funnel into a tube whose horizontal cross-sectional area is one-tenth that of the outer gauge, so that the measured rain is ten times the actual amount of rainfall. This year, I took a GLOBE-approved plastic gauge home. We put this one on a fence along the east side of our yard (Figure 2).

Figure 1. Rain gauge used for observations in my backyard. Normally, there is a funnel and small tube inside, but it doesn’t fit very well, so we pour the rain into the small tube after each rain event. This gauge is similar to those used by the U.S. National Weather Service. This gauge is about 25 cm in diameter.

Figure 2. Plastic raingauge matching GLOBE specs. This gauge is about 12 cm in diameter. Note the tall tree in the background.

Neither gauge is in an ideal location. In both cases, there are nearby trees (Fig. 2, map) which might impact the measuring of the rain. This is a problem a lot of schools have: there is just no ideal place to put a rain gauge. We were particularly worried about the plastic gauge, which was closer to trees than the metal gauge.

Why do we have two gauges? The metal gauge was hard to use: its funnel didn’t fit easily into the gauge, so we had to pour the rain from the large gauge into the small tube after every rainfall event. We got the plastic gauge to replace the metal one. We put the gauge on the fence because it was well-secured. But the first six months we used the new gauge, the rainfall seemed too low compared to totals in other parts of Boulder. So, I put the metal gauge back outside and started comparing rainfall data.

Figure 3. Map of our backyard. Left to right (west to east), the yard is about 22 meters across. The brown rectangular shape is our house; the circles represent trees and bushes. The numbers denote the height of the trees and bushes. The 10-m tree is an evergreen; the remaining trees and bushes are deciduous. The southeast corner of the house is about 3 m high.

How did the gauges compare?

Starting this summer, I started taking data from both gauges. Unfortunately, it didn’t rain much. And sometimes, we were away from home: so this is not a complete record. But I don’t need a complete record to compare the rain gauges.

Table: Rain measurements from the two rain gauges

The results (in the table, also plotted in Figure 4) look pretty good. With the exception of the one “wild” point on 6 October 2008, the measurements are close to one another. We think that the plastic gauge was filled when the garden or lawn next door was watered. This would not be surprising: we have found rain in the plastic gauge when there was no rain at all.

I learned after writing this blog that Nolan Doeskin of CoCoRaHS (www.cocorahs.org) has compared these two types of gauges for 12 years, finding that the plastic gauge measures slightly more rain (1 cm out of 38 cm per year, or about 2.6%).

Figure 4. Comparison of rainfall from the two rain gauges in our back yard. Points fall on the diagonal line for perfect agreement.

I learned two things from this exercise.

First, I probably should have used the two gauges before I stopped using the metal one. That way, my rainfall record wouldn’t be interrupted if the new gauge was totally wrong. (I was worried that the trees were keeping some rain from falling into the gauge. This would have led to the plastic gauge having less rainfall than the metal gauge. And, since the blockage by the trees would depend on wind direction and time of year, I wouldn’t have been able to simply add a correction to the readings.) Fortunately, the new and old gauges agreed.

In the same way, if you want to replace an old thermometer with a new one, it’s good to take measurements with both for awhile, preferably in the same shelter. Suppose the new thermometer gives higher temperatures than the old one. If you want to know the temperature trend, you can correct the temperatures for one of the two so that the readings are consistent.

The second thing I learned is that it is o.k. to reject data if there is a good reason (such as people watering their lawns). It’s also important to note things going wrong – like my spilling a little bit of water on 15 August. If you keep track of things going slightly wrong (or neighbors watering the lawn), you can often figure out why numbers don’t fit the pattern.

I will continue to compare records for awhile, to see whether the readings are close to one another on windy days. If they continue to be similar, I will be able to try a method to keep birds away from the rain gauge that was developed by a GLOBE teacher – Sister Shirley Boucher in Alabama. Keep posted!

Did you know that you could count cricket chirps to estimate temperature? I heard this a number of years ago, but never thought much about it until I heard it mentioned on television this summer. Was this true, or just an urban myth? I decided to go outside and see for myself. Starting in August, I started listening to crickets. I estimated the “cricket temperature” from the first formula I found on the Web:

Cricket temperature in degrees Fahrenheit = number of chirps in 15 seconds + 37.

I measured the actual temperature by taking the average from two thermometers. One is mounted on the house at about eye level (1.5 m) beneath the overhang where we park our car. The second lies on the table on our deck, at about 1.5 m above the ground. A louvered sun-roof on the deck keeps the thermometer from cooling too much. In both places, there is enough wind for ventilation. But I had to ignore the house-mounted thermometer if our car was warm (i.e., recently driven). Though I did not have a GLOBE instrument shelter, the height matches that for the GLOBE air temperature protocol.

It took me a week or two to figure out how to count cricket chirps. 15 seconds was too short a time — I kept ending up with numbers like 30-and-a-half chirps. Or I would lose track or start too early or too late. So I tried 30 seconds. That way if I was between 60 and 61 chirps, the resulting error would be divided by two.

Then I discovered that the crickets didn’t always chirp together (CHIRP CHIRP CHIRP) but sometimes got out of synch (chir-rurp chir-rurp chir-rurp). In this case, I would count the chirps when they were in unison, and try to maintain the beat until they got back in unison again. To make things more accurate, I’d count chirps for five 30-second periods, average the number, and then divide the average by two. If there were two sets of crickets that weren’t always chirping at the same time (say an “alto” group and a “soprano” group), I’d count the alto chirps for one 30-second period and then count the soprano chirps for the next 30-second period.

I ended up with a lot of data for temperatures above 70 degrees. But getting numbers at the cooler temperatures was harder.

Since temperatures are the coolest around sunrise, I had to start getting up around 2:00-3:00 a.m. and 5:00 a.m. to get data for the cooler temperatures.
How well did the formula work? You can see from the first graph, in Figure 1. If everything (the formula, my counting, the thermometers) worked perfectly, all the red dots would fall on the black line. That is — along the black line the “cricket temperature” is equal to the measured air temperature.

Figure 1. Air temperature measured from cricket chirps. For this graph, the number of chirps during 15 seconds is added to 37 to get the air temperature. The highlighted temperature readings are for the red “best-fit” line.

In the graph, the data are close to the black line, but not always on it. The red line is the straight line that best fits the data. Notice that the red line drops below the black line for high temperatures. Thus from the red line, for a “cricket temperature” of 80 degrees Fahrenheit, the measured air temperature is only 78 degrees Fahrenheit. Similarly, from the red line, a cricket temperature of 50 degrees Fahrenheit corresponds to a measured temperature of 52 degrees Fahrenheit — not exactly right.

So I plotted cricket chirps against air temperature to get a better method: Count cricket chirps for 13 seconds, and then add 40 degrees. Using this method, the points (and the red line) are much closer to the black line.

Figure 2. Air temperature measured using cricket chirps. For this graph, the number of chirps during 13 seconds is added to 40.4 to get the air temperature. As in Figure 1, the black line is where the points would fall if the method was perfect, and the red line is the line that best fits the data.

Clearly the approach in Figure 2 works slightly better. The red “best-fit” line through the data lies almost on top of the “perfect fit” black line. I later found this method was also on the Web.

If you find it hard to count chirps for 13 seconds, count chirps for a longer period (say 30 seconds) and then multiply by 13/30 to get the chirps in 13 seconds. Or if you are really patient, count chirps for a full minute and multiply by 13/60.

Notice that both the “cricket temperature” and the measured temperature stay above 50 degrees Fahrenheit. Both my husband (who helped collect data when I was gone) and I heard no cricket chirps at all when the measured temperature was 49 degrees Fahrenheit or lower. This suggests that 50 degrees Fahrenheit is at about the lower limit for when crickets chirp.

I had suspected that the lowest temperature for chirping crickets would be 50 degrees Fahrenheit or less. Why? Because meteorologists who study winds using radar have noticed that insects stop flying (and producing radar echoes) at 50 degrees Fahrenheit (or 10 degrees Celsius). I reasoned that it would take about the same energy — or less — to chirp than to fly, since chirps are produced by crickets rubbing their wings together, which should consume less energy than flying.

For comparison, a posted article from Dartmouth College lists 55 degrees as the minimum temperature for cricket chirps. Both my husband and I noticed fewer crickets (one or two) were chirping when the temperatures were in the lower 50s, so some crickets probably did stop chirping at that temperature, but not all.

Finally, let’s plot the data to show the relationship between cricket chirps and the temperature in Celsius degrees. Figure 3 shows this relationship. Again, the “best-fit” red line and the data are close to the “perfect fit” black line.

Figure 3. Relationship between cricket chirps and the temperature in degrees Celsius. As in Figures 1 and 2, the black line is where the points would fall if the method was perfect, and the red line is the line that best fits the data.

You can find several approaches on the Web, but it is not certain where they came from, and the raw data aren’t available, so you don’t know how many measurements were taken to determine the approach. Here I provide the data set so that you can play with it — or add your own observations. Have fun! It will be interesting to see whether chirps from crickets in the other parts of the U.S. and world relate to temperature in the same way.

Cricket Chirp Data – Boulder Colorado, USA. All dates 2007