Other heat resources
Click on the following links to see risk assessment tools for football specifically and for activity outside in-general, using relative humidity and environmental temperature only (there is no accounting for radiant heat in these tools, but they are still good rough gauges of risk.

Related Documents:

How Bad Does It Feel the Heat Index.pdf
Here is a chart that tracks relative humidity versus environmental temperature and estimates heat risk for outdoor activities in general.
Heat Index1.pdf
Here is a graph from a medical journal article that looks at the environmental conditions above which most football players can no longer keep their core body temperatures from rising uncontrolled. The three lines represent (from lower left to upper right) practice in full uniform, practice in helmet and pads, and practice in t-shirt and shorts. Simply take the day's relative humidity and temperature and intersect the two to see if the line intersects above or below the line that applies to you.