The American College of Sports Medicine has produced guidelines regarding how a race should consider reacting to various weather conditions. The ACSM has purposely avoided creating a strict cancellation standard, stating that it does not want to handcuff a race director; rather it offers recommendations about the course of action that should be taken in various weather conditions. The temperatures in the ACSM guidelines are all WBGTs (wet bulb globe temperatures). The WBGT is an index that takes into account heat (dry bulb), humidity (wet bulb), and the ambient temperature (black bulb, which factors in additional variables such as reflected heat and wind). A WBGT thermometer can be purchased from a medical supply house for about $100. The ACSM guidelines are as follows:
WBGT >82° F (>28° C) Black Flag: Cancel or recommend voluntary withdrawal. (Races that are always held in these conditions should acknowledge the extreme heat risk to the poorly acclimated and non-resident competitors in the pre-race literature and the pre-race announcements)
WBGT 73° F - 82° F (23° C - 28° C) Red Flag: Recommend participants at increased risk for heat collapse withdraw from race and others slow pace to match conditions.
WBGT 65° F - 72° F (18° C - 22° C) Yellow Flag: Recommend participants at increased risk for heat collapse slow pace. Warn entrants of increased risk of heat collapse.
WBGT <65° F (<18° C) Green Flag: Collapse can still occur. Decreased risk of hyperthermic and hypothermic collapse.
WBGT <50° F (<10° C) White Flag: Increased risk of hypothermic collapse.