An excessive heat warning is a notice issued by the

National Weather Service The National Weather Service (NWS) is an Government agency, agency of the Federal government of the United States, United States federal government that is tasked with providing weather forecasts, warnings of hazardous weather, and other weathe ...

of the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

within 12 hours of the

heat index The heat index (HI) is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is als ...

reaching one of two criteria levels. In most areas, a warning will be issued if there is a heat index of at least for more than three hours per day for two consecutive days, or if the heat index is greater than 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 Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...

being significantly above normal and high

humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...

, and such high levels can pose a threat to human life through conditions such as

heat stroke Heat stroke or heatstroke, also known as sun stroke, is a severe heat illness that results in a body temperature greater than , along with red skin, headache, dizziness, and confusion. Sweating is generally present in exertional heatstroke, ...

heat exhaustion Heat exhaustion is a severe form of heat illness. It is a medical emergency. Heat exhaustion is caused by the loss of water and electrolytes through sweating. The United States Department of Labor makes the following recommendation, "Heat illness ...

, and other heat-related illnesses.

Danger to athletes

Due to the extreme illnesses that can occur, athletes should limit their physical activity 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, 2011, and 2020.

Example

The following is an example of an excessive heat warning issued by the National Weather Service office in

San Antonio, Texas ("Cradle of Freedom") , image_map = , mapsize = 220px , map_caption = Interactive map of San Antonio , subdivision_type = Country , subdivision_name = United States , subdivision_type1= State , subdivision_name1 = Texas , subdivision_t ...

on June 11, 2022 during a severe heat wave.


692
WWUS74 KEWX 111801
NPWEWX

URGENT - WEATHER MESSAGE
National Weather Service Austin/San Antonio TX
101 PM CDT Sat Jun 11 2022

TXZ171>173-191>193-203>208-219>222-120900-
/O.UPG.KEWX.HT.Y.0005.000000T0000Z-220613T0000Z/
/O.NEW.KEWX.EH.W.0001.220611T1801Z-220613T0000Z/
Llano-Burnet-Williamson-Hays-Travis-Bastrop-Uvalde-Medina-Bexar-
Comal-Guadalupe-Caldwell-Frio-Atascosa-Wilson-Karnes-
Including the cities of Llano, Burnet, Georgetown, San Marcos,
Austin, Bastrop, Uvalde, Hondo, San Antonio, New Braunfels,
Seguin, Lockhart, Pearsall, Pleasanton, Floresville,
and Karnes City
101 PM CDT Sat Jun 11 2022

...EXCESSIVE HEAT WARNING IN EFFECT UNTIL 7 PM CDT SUNDAY...

* WHAT...Air temperatures ranging from 103 to 107, as well as heat
  index values up to 105 to 110 expected.

* WHERE...Portions of south central Texas.

* WHEN...Until 7 PM CDT Sunday.

* IMPACTS...Extreme heat and humidity will significantly
  increase the potential for heat related illnesses,
  particularly for those working or participating in outdoor
  activities.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Drink plenty of fluids, stay in an air-conditioned room, stay out
of the sun, and check up on relatives and neighbors. Young
children and pets should never be left unattended in vehicles
under any circumstances.

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 lightweight and loose fitting clothing when
possible. 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 9 1 1.

&&

$$

References

External links

National Weather Service
{{SevWea nav Weather warnings and advisories