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Abstract:
It has long been noted that artificial turf athletic fields can get very hot. However, no one has yet written on the thermal mechanisms involved in this phenomenon. As a result, data is taken and presented randomly resulting in widely varying reports of just how hot these fields can get. Because of ignorance of the conditions which influence temperature elevation it is difficult to plan to deal with the problem. The author presents a model for understanding the factors involved. The key observation is that these fields are excellent thermal radiation absorbers while being thermally conductive insulators.
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The Data
Reports of elevated temperature readings from artificial turf athletic fields are easy to find on the web. The details, however, vary widely. Some refer to elevations over ambient of only ten or fifteen degrees (all temperature references in this paper will be in degrees Fahrenheit, for ease of understanding by American non-technical readers). Other reports speak of temperatures over 200 degrees. In August and September of 2007 I visited various artificial fields in the Boston area and measured temperatures under varying conditions using a thermocouple, which is basically a point measurement device. This is in contrast to all other reported measurements that I have seen referenced, which either used an infrared thermometer pointed at the surface or a meat thermometer thrust into the infill (the layer of ground up old tires used as the primary cushion on the current generation of artificial fields). The use of the thermocouple allowed me to gain an insight into the mechanisms involved in the heating of the fields.

A summary of the results follows. A complete record of the raw data is appended.

DATE / FIELD / AMBIENT TEMP / FIELD TEMP / TEMP RISE
8/3          L-S                  91                         156                   65
8/14        L-S                 78                          136                   58
8/28       CUT                79                          140                   61
9/5         CUT                71                          139                   68
9/13       CUT                67                          126                   59
9/13       L-S                  70                          145                   75
9/20      WAY               76                          142                   66

All dates are in 2007. L-S is Lincoln-Sudbury Regional High School athletic fields, Sudbury, Mass. CUT is Cutting Athletic Field, Sudbury, Mass. WAY is Wayland High School athletic field, Wayland, Mass. Ambient temps are in degrees Fahrenheit, taken in shade three feet above ground to the side of the field. Field temps are on the field surface.

Some of the most important information, however, is not captured in the quantitative data but in observations made as the data was taken.

1. Temperatures were highest right on the surface of the ground tire "infill". Pushing the thermocouple even a small fraction of an inch into the material resulted in a noticeable drop in temperature. Pushing it an inch or two further resulted in drops of tens of degrees.
2. A puff of wind would result in a noticeable drop in temperature. This could be seen because the thermocouple has negligible thermal mass and responds nearly instantaneously.
3. As can be seen in the appended raw data, measurements of nearby black asphalt surfaces showed much lower temperatures.
4. Evening visits to these sites resulted in the observation that the fields were cool (essentially ambient) while the nearby asphalt surfaces remained quite warm.

Key Observation

The above observations can be explained by noting that the ground up rubber tires have two key thermal characteristics. First, they are black, which we know from high school physics means they are excellent thermal radiation absorbers. (In fact, tires contain large quantities of carbon black). Second, they contain great quantities of air space and rubber itself is a poor thermal conductor, making the layer of rubber "infill" an excellent thermal insulator. Thermal energy will therefore be absorbed on the surface of the field but will not be dissipated into the mass of the field material. This allows temperatures to rise far higher than on other nearby black surfaces, such as asphalt. The asphalt mass absorbs the thermal energy and thus integrates the temperature rise over time, reducing it during the day but allowing it to remain high after the radiation input is removed at night. Temperatures on the rubber field surface, on the other hand, will drop almost immediately as solar input drops.

The Model

Integrating the above information into a conceptually quantitative model results in the following:

TT= AT + SI - DH
Where TT= Turf Temperature
AT= Ambient Temperature
SI= Solar Input, which is a function of time of day and time of year, minus cloudiness
DH= Dissipated Heat, which seems to be largely a function of wind strength and characteristics of the particular turf sample

Discussion

It should be obvious that the temperature of the field will be a function of the ambient temperature.

Solar energy is the fundamental input causing temperature elevation. Because of the lack of thermal mass in the field material, peak temperatures will be reached shortly after peak inputs occur. There were insufficient samples taken to determine this with any precision, but it is almost certainly within two hours of solar noon (approximately 1 PM Daylight Savings Time) and probably within an hour. Future research should narrow this range. Seasonally, peak inputs will occur in late June, but note that the highest temperature elevations in this sample were measured near the fall equinox- all other conditions were optimum, overcoming the effect of reduced seasonal solar input. Measurements taken under comparable conditions next June on these same fields should prove extremely interesting. Finally, the effect of cloud cover is critical. Field temperatures drop noticeably as a cloud passes, and then rise immediately when the sun is exposed again. Haze and high thin cloudiness also serve to reduce solar input.

In keeping with the requirements of the law of conservation of energy, energy is obviously dissipated from the fields or the temperature would increase without bound. And just as obviously, all three thermal transport mechanisms- radiation, conduction, and convection- are involved.

We know from elementary physics that black surfaces will be efficient radiators of infrared energy just as they are efficient absorbers, and that the magnitude of energy radiated will be proportional to the temperature. This is of particular interest in the case of an athletic field because the most likely objects in the path of that radiation are players. Of equal interest is the fact that players will be exposed to maximum thermal radiation from below at the same time as they are exposed to maximum radiation from above, i.e., the sun.

Conduction will occur to the material beneath the surface, to the air immediately above it, and to the shoes and bodies of players standing on or otherwise in contact with the surface. As noted above, conduction to the bulk material beneath the surface appears to be much less significant with the material of these artificial fields than with many other familiar surfaces, and this is almost certainly a major factor in allowing the surfaces to reach temperature extremes. Conduction to the air above appears to be a major cooling mechanism, as immediate temperature drops were observed after wind gusts and fields in general seemed to be cooler on windy days (further quantifying this effect may be a subject for future research). Conduction to shoes and bodies of players will certainly not be significant to cooling the field, but is of interest because of its potential to cause injury. There are reports of blisters on players' feet caused by conduction of artificial field heat through shoes, especially with metal spikes.

Finally, convection will become important on still days to transport heated air away from the surface.

Sample Characteristics

There appears to be variation from one field to another, and even from one area to another area in the same field, in thermal characteristics. This bears further investigation. Possibilities include:
1. Variations in the ratio of plastic "grass" filaments (polyethylene or polypropylene) to the total surface area. These strands may act as shade for the ground tire layer and also as heat dissipation fins. They are also a different color from the black tires. There are major differences in the coverage ratio from one field to another.
2. Slight variations in the grade of portions of the field from horizontal, and therefore angle to the sun.
3. Variations in the chemical composition of the ground tires, and consequent differences in thermal absorption and radiation characteristics. This material is salvaged from the waste stream and may be highly variable.

Other Thermally Related Issues

There are reports of outgassing of several toxic chemicals when artificial turf fields heat up. This bears further investigation, and the parameters defined above may help to choose test conditions most appropriate to investigate the phenomena. In addition, leaching of toxic liquids and dissolved solid materials has been reported and will probably be a function of temperature. Again, the information presented above may aid in this investigation.

Future Plans

Ashland State Park Beach
August 25, 2007, 2PM, Sunny
Ambient: 89
Beach Sand: 129

Sudbury Cutting Field
August 28, 2007, 11:45 AM, partly cloudy
Ambient: 79
Asphalt: 116
Turf: 140

Sudbury Cutting Field
September 5, 2007, 12:15 PM, sunny
Ambient: 71
Asphalt: 112
Turf: 139

Lincoln-Sudbury
September 8, 2007, 3:15 PM, Partly Cloudy, windy
Ambient 96
Old Field: 125
New Field: 132

Sudbury Cutting Field
September 13, 2007, 11:45 AM, clear, calm
Ambient: 67
Asphalt: 106
Turf: 126

Lincoln-Sudbury
September 13, 2007, 2:10 PM, a few clouds
Ambient: 70
Grass (green): 88
Asphalt: 116
New Field: 136
Old Field: 145
(Note: Old field measurement taken in a different section from previous measurements, near corner closest to new field. Readings from other spots were in the 130's)

Wayland High School
September 20, 2007, 12 noon, clear, calm
Ambient: 76
Grass: 93
Track: 101
Turf: 142