Part 1 - Surface temperature and how it varies with sun angle and surface type

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Part 1 - Surface temperature and how it varies with sun angle and surface type

Introduction

The temperature of a surface – the grass, the sidewalk, or your skin – is called the “skin” temperature. Satellites monitor Earth’s skin temperature every day. Skin temperature is an important addition to several other variables – like soil type, plant cover, etc. – to estimate how the land surface is affecting the weather. Ocean skin temperature is also an important parameter in estimating near-surface air temperature over the oceans because there are few direct observations. Since the oceans cover such a large part of Earth, these temperatures are important in keeping track of the warming of Earth’s atmosphere. Finally, surface measurements are important in evaluating skin temperature estimates from satellites.

If you have ever walked outside on bare feet, you may know something about surface temperature. The concrete next to a swimming pool is so hot on some summer days that it hurts to walk on it – unless someone in the pool splashes enough water on it to make it wet. Sidewalks aren’t much better. It feels much better to walk on grass.

Perhaps someone pointed an instrument at your forehead during the pandemic to see if you had a fever. The instrument is called a radiometer. It measures the long-wave (infrared) radiation that your skin is giving off. Infrared radiation is sometimes also called “heat” radiation, which is obvious when you feel the warmth of a hot pot on the stove without even touching it.

GLOBE has a Surface Temperature protocol for using a radiometer to measure the Earth’s skin temperature. Below, I describe some measurements I recently made near my home in Boulder, Colorado, USA and near the NSF NCAR Foothills Lab, about three kilometers to the east.

The dates and times I took data are summarized in Table 1.

Notes on Table 1:

Indoor and outdoor temperature similar except for (a) October 17, when I tried to equilibrate the radiometer according to the GLOBE protocol. I was careful to place it in guaranteed shade after it was exposed to sun for October 17a. (b) November 17, when I left the radiometer outside (in the shade) before taking observations.
Cloud abbreviations: Ci = Cirrus; Ci Cu = Cirrocumulus; Alto Cu = Altocumulus. Lee wave clouds, like ripples over rocks, are stationary. I avoided times when these clouds obscured the sun.
Local standard time (LST, in this case Mountain Standard Time or MST) started on November 5. It was important to pay attention to this, since my original notes simply recorded time, which had to be corrected to be consistent. Standard time is better to use in this context, because solar noon – when the sun is highest in the sky, is closer to noon in local standard time (LST). In Boulder, Colorado, USA, solar noon was approximately 11:45 LST.

Results

Days 1-2: Exploration

To start, three of us (me and two of my grandchildren, ages 11 and 5) sampled all sorts of surfaces, to get a feel for how skin temperature varies – including grass, bare ground, a tree trunk, the side of the house, a birdbath, sidewalks, the street, and – remembering the pandemic – our skin and clothing. We also looked for the effects of shadows on some surface types. Table 2a shows the times and local conditions. We started at 14:25 MDT (13:25 MST), about 1¾ hours after local solar noon (11:47 MST). Skies were clear, and the outdoor temperature was 24.7°C, not that different from inside (20-21°C), so we did not need to leave the radiometer outdoors to equilibrate to the environmental temperature; skies were clear. Table 2a lists the environmental conditions of our first set of measurements.

Many of the results in Table 2b were not surprising. The car mirror temperature was approximately equal to the air temperature. The sidewalk was warmer on the north (sunny) side of the street than on the south side (partially shaded by trees). The hottest measurements were from solid surfaces, with the dark areas (absorbing sunlight) on a car being the hottest (the cooler white area, reflecting more sunlight, was not as warm). We were surprised at first that the solid white rock was warmer than the blue-gray gravel, but it later made sense: The white rock surface was hit more directly by the sun, and hence heated more, while the blue-gray rocks on the ground were mostly flat with the sun’s rays hitting their surface at a shallower angle, and shadows between the rocks likely made the measurement cooler (Figure 1a).

One fun discovery – reinforced by other measurements – was how cool it was on the sidewalk north of parked cars, with the effect largest for the area that had been in shadow the longest (a difference of about 10°C from recent shadow to longest time in shadow). This shouldn’t be much of a surprise. Indeed, after it snows, the snow on the sidewalk lasts much longer in the shadow of a car parked all day than where the sidewalk is exposed to full sunlight, as illustrated by the picture I took on Tuesday, November 28, a few days after about 9 inches of snow fell on Boulder (Figure 1b).

Figure 1a: Top: blue-gray rocks – the larger rocks are of the order of 3-4cm across; Bottom: large right rock, about 50 cm across. Left is toward the west.