An Excessive Heat Warning is issued by the National Weather Service of the United States within 12 hours of the heat index reaching one of two criteria levels. In most areas, a warning will be issued if there is a heat index of at least 105 °F for more than 3 hours per day for 2 consecutive days, or if the heat index is greater than 115 °F for any period of time. Note that local offices, particularly those where excessive heat is less frequent or in areas with deserts or mountainous terrain, often have their own criteria. High values of the heat index are caused by temperatures being significantly above normal and high humidities, and such high levels can pose a threat to human life through conditions such as heat stroke, Heat exhaustion, and other heat-related illnesses.
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Danger to Athletes
Due to the extreme illnesses that can occur, athletes should be limited on the amount of physical activity that they participate in when an excessive heat warning is issued. According to the National High School Sports-Related Injury Surveillance System, High School RIO, an estimated number of 51,943 excessive heat illnesses occurred in nine of the high school sports observed between 2005 and 2011.
Example
The following is an example of an Excessive Heat Warning issued by the National Weather Service office in Wilmington, Ohio on July 20, 2011 during a severe heat wave.
URGENT - WEATHER MESSAGE NATIONAL WEATHER SERVICE WILMINGTON OH 944 PM EDT WED JUL 20 2011 KYZ099-100-OHZ026-045-046-052>056-062>065-072>074-079>082-088- 211000- /O.CON.KILN.EH.W.0002.110721T1600Z-110723T0100Z/ /O.CON.KILN.HT.Y.0004.000000T0000Z-110721T1600Z/ MASON-LEWIS-HARDIN-UNION OH-DELAWARE-CHAMPAIGN-CLARK-MADISON- FRANKLIN OH-LICKING-GREENE-FAYETTE OH-PICKAWAY-FAIRFIELD-CLINTON- ROSS-HOCKING-BROWN-HIGHLAND-ADAMS-PIKE-SCIOTO- INCLUDING THE CITIES OF...MAYSVILLE...VANCEBURG...KENTON... MARYSVILLE...DELAWARE...URBANA...SPRINGFIELD...LONDON... COLUMBUS...NEWARK...XENIA...WASHINGTON COURT HOUSE... CIRCLEVILLE...LANCASTER...WILMINGTON...CHILLICOTHE...LOGAN... GEORGETOWN...HILLSBORO...WEST UNION...PIKETON...PORTSMOUTH 944 PM EDT WED JUL 20 2011 ...HEAT ADVISORY REMAINS IN EFFECT UNTIL NOON EDT THURSDAY... ...EXCESSIVE HEAT WARNING REMAINS IN EFFECT FROM NOON THURSDAY TO 9 PM EDT FRIDAY... * HEAT INDEX VALUES...UP TO 110. * TIMING...HEAT INDICES BETWEEN 100 AND 105 WILL CONTINUE INTO THE EVENING. THE HEAT INDEX WILL RISE TO BETWEEN 105 AND 110 THURSDAY AFTERNOON AND EVENING AND THEN AGAIN ON FRIDAY. * IMPACTS...HEAT EXHAUSTION...HEAT STROKE AND OTHER HEAT RELATED ILLNESSES WILL BE POSSIBLE...ESPECIALLY IF YOU SPEND A SIGNIFICANT AMOUNT OF TIME OUTDOORS...OR ARE INVOLVED IN ANY STRENUOUS OUTDOOR ACTIVITY. PRECAUTIONARY/PREPAREDNESS ACTIONS... TAKE EXTRA PRECAUTIONS IF YOU WORK OR SPEND TIME OUTSIDE. WHEN POSSIBLE... RESCHEDULE STRENUOUS ACTIVITIES TO EARLY MORNING OR EVENING. KNOW THE SIGNS AND SYMPTOMS OF HEAT EXHAUSTION AND HEAT STROKE. WEAR LIGHT WEIGHT AND LOOSE FITTING CLOTHING WHEN POSSIBLE AND DRINK PLENTY OF WATER. TO REDUCE RISK DURING OUTDOOR WORK THE OCCUPATIONAL SAFETY AND HEALTH ADMINISTRATION RECOMMENDS SCHEDULING FREQUENT REST BREAKS IN SHADED OR AIR CONDITIONED ENVIRONMENTS. ANYONE OVERCOME BY HEAT SHOULD BE MOVED TO A COOL AND SHADED LOCATION. HEAT STROKE IS AN EMERGENCY...CALL 911. && $$