Wood-chip temperatures
On 10 October, by myself this time, I had another surprise: how hot that wood chips, often used in landscaping are, with temperatures reaching 60-65°C at 13:00 MST, when the air temperature was 26.1°C. Seeing this, I decided to take several more measurements of wood-chip skin temperature. I chose to focus on the wood chips at the west end of the top of the mesa we live on, since on cloudless days they were exposed to full sun from noontime onwards.
Table 3a shows the environmental conditions for each observing session, while Table 3b shows the corresponding wood-chip skin temperatures. The warmer skin temperatures tend to occur when the sun is nearer to its highest point in the sky (nearer to 11:45 LST). It is not surprising that the coolest skin temperature is at the latest time of day (lowest sun angle). For this dataset, it is harder to pin down the effect of air temperature. Clouds may have also played a role.
The skin temperature also varied with the angle at which I pointed the radiometer. The warmest temperatures were mostly when my back was to the sun, but there was a randomness to the temperatures as well, I think due to how much shadow the radiometer was “seeing.”
Note on Table 3b: “Sigma” or standard deviation is a measure of the variation of the observed skin temperatures. As if often true, the standard deviation increases as the spread (range of measurements, “sun – shade” in the table) increases.
Compare Table 2b to Table 3b. The warmest wood chip temperatures in the sun in Table 3bare comparable to the highest temperatures in Table 2b – those on the white rock and the black part of the car. Why was the skin temperature so high for the wood chips? They were old and dry. There had been no recent rain. My hypothesis was that the skin temperature was warm because the heat at the surface wasn’t conducted downward, since dry wood chips aren’t very good conductors of heat. Indeed, they are good insulators! In contrast, metal, like a saucepan, is a good conductor of heat, which you know if you ever touched one on a hot stove.
Because of this, I suspected that the skin temperature of the wood chips would cool rapidly in the shade. This suspicion was already supported by the fact that the temperatures of the wood chips in shade was significantly cooler than the temperature of the wood chips in the sun in Table 3b. To measure this, I took temperature readings with my back to the sun, and then moved so that the same spot was in shadow. From Table 3b, it looks like the skin temperature drops almost instantly. This is consistent with the heat being concentrated near the surface.
Wood-chip cooling
To look for how fast and how much the wood-chip skin temperature would cool, I went back to those wood chips on the west end of our mesa. This was on November 17, when skies were clear, although there was some haze. As in the earlier case, I measured the temperature of the wood chips pointing in four roughly orthogonal directions (at close to right-angle increments in direction, mostly toward NW, NE, SE, SW) and with the sun to my back, pointed at roughly the same spot. Then pointing to the same spot but now in my shadow, I took several successive measurements. It took about two minutes to take 10 measurements. Again, there was no precipitation for several days, so the wood chips are dry. The results appear in Figure 2.
Figure 2a. Left: skin temperature of wood chips at two locations. Right: the wood chips. Measurements taken around 13:00 Local standard time (LST) November 17; air temperature 14.6°C; skies clear. Solar noon is around 11:45 LST. Total time elapsed: about 2 min.
Note the significant and rapid fall in temperature of between 15-20°C during the roughly two minutes the chips were in my shadow.
Sidewalk and grass-leaf mix cooling
Intrigued, I took sequential measurements for two other surfaces, grass with leaves (on November 16) and on two sidewalks (on November 16 and 17). I expected the sidewalk to have far less change over a short time because its skin temperature didn’t seem to be much cooler when I took a measurement in my shadow. Also, the sidewalk would be expected to conduct heat downward, something not possible with the chips, which are good insulators. I expected the grass plus leaves to be intermediate, since there was some exposed soil, and the grass still was partially green. I wasn’t disappointed, as shown in Figure 3.
Figure 2b. Top left: cooling of sidewalk and grass plus leaves (pictured at right, top) as they cooled in my shadow (bottom right, for grass), under sunny conditions. Top left: at NSF NCAR’s Foothills Lab around 13:00 MST November 16, air temperature 19.2°C, skies clear except western horizon. Lower Left: west end of the mesa where I live, around 14:10 MST November 17, air temperature 14.7°C, skies clear, hazy. Duration of ten measurements: about 2 minutes.
As expected, the sidewalk hardly cooled in the short time it was in shadow – between 1 and 3 degrees Celsius, in the short time interval. In contrast, the grass-leaf mix cooled by about 8 degrees. Because of the short measurement time at the Foothills Lab, I repeated the sidewalk observations at the west end of the mesa where I live. Clearly, there was hardly a drop in temperature in the time series.
Why, then, was the sidewalk so cool north of the parked car? This is because the sunlight was blocked for hours, rather than minutes. My guess is that this is because its heat is being conducted slowly downward into the sidewalk and soil below. A much smaller amount is conducted through the air or carried upward by air currents, as long as the sidewalk is warmer than the air.
What then about the wood chips? Since they are poor conductors and good insulators, the skin temperature would fall until its temperature is determined mainly by a combination of its radiating upward (what we measure) and its exposure to the air.
One interesting surprise was how much cooler the sidewalk was compared to the wood chips. When taking the mesa sidewalk measurements in Figure 2, I thought at first the radiometer wasn’t working, a surprise since I had left it outside in the shade to equilibrate. However, when I pointed the radiometer at the same spot where I had a reading of about 51 Degrees Celsius previously, I obtained a similar reading 2 hours later.
What could I have done better? If you try this, it would be good to have at least three people participating. One person to take the observations with the radiometer, one person to record the measurements and time, and one person to take pictures of the surface in question. These observations should be combined quickly. Make sure you note whether your times are standard or daylight time! Your notes will also be neater than mine, which were rather messy.
If you are taking a temperature measurement, use it – but also compare it to nearby official measurements. If you use the GLOBE temperature protocol, you have a pretty good measurement. I recorded the temperatures inside (where the radiometer is stored) and outside the house (where I took the measurements), as well as the “official” temperatures (at Foothills Lab about 3 km east of where I live) and the official temperature, which is observed farther to the east at the Boulder Municipal Airport. The temperature in our carport (whose south and east walls are outside but is open to the west and north) seemed too cool – probably reflecting a daytime of shade and trapped air, and the thermometer on the south side of the house is exposed to the sun part of the day. So, I used the Foothills Lab and official temperatures, which were similar, and good enough for my objective here, which was to explore the relationships of surface types and exposure to the sun on skin temperature.
If you need to leave the radiometer outside so that it can adjust to the new environment, make sure it is shaded, since the sun can heat the radiometer to temperatures above the surrounding air if the radiometer is in the sun. Preferably in a place that is well ventilated – and, as the GLOBE protocol points out – in a safe place.
Ideally, if you are taking observations for an hour, the temperature should be recorded at the beginning and ending times, as well as sky conditions.
Things are least complicated with clear skies. Second best is clear skies near the sun.
Pictures of the exposed surface with the laser light provides an idea of just where you are measuring, which enables a more precise impression of whether your measurement is influenced by small shadows (as in the case of the blue-gray rocks in Figure 1a).
After you have explored many of these questions, you can understand how hard it is for scientists to estimate surface skin temperatures from space!
