indices? means index , so a thermal index would be
oh never mind to much typing and make my head hurt.
sounds like they are talking about how fast snow would melt off of a roof and at given temp..
of course i will admit i could be wrong since i dont know what yoru reading
ok i thought i would look it up,
"About Thermal Index
A thermal index is a measure of atmospheric stability/instability. To determine for a given altitude, subtract the temperature of the lifted airmass from the measured air temperature. If the result is negative, actual air is warmer than standard and should continue to rise. If the result is positive, it indicates that the air aloft is warmer than the rising air would be at that altitude and a thermal would stop rising because it has encountered warmer, less dense air. Dry adiabatic rate of cooling is: 5.4 degress Fahrenheit per 1,000 feet. "