May through September (75 or greater when conditions are too hot and humid):

((Maximum dry-bulb temperature + corresponding wet-bulb temperature) X 0.4) + 15

October through April (65 or lower when conditions are too cold and dry):

((Minimum dry-bulb temperature adjusted for wind chill + corresponding wet-bulb temperature) X 0.4) + 15

The white zone is ideal (66 - 74).