Question

whats most reliable indicator you should use when determining if you should modify activity for heat?
local weather report
wet bulb temperature taken at your field
heat index
humidity percentage

Explanation:

The wet - bulb temperature takes into account both temperature and humidity. A local weather report may not be site - specific. The heat index is a useful measure but is more of a general measure. Humidity percentage alone does not fully account for the combined effect of temperature and humidity on the body's ability to cool. The wet - bulb temperature taken at the field (where the activity is occurring) is more reliable as it is a direct measure of the environmental conditions relevant to heat stress at that exact location.

Answer:

Wet bulb temperature taken at your field