This time of year, a frequent feature of weather reports is the “heat index.” The heat index is a combination of the temperature and relative humidity. It is an attempt to indicate how we will perceive the outdoor environment. When there is high temperature coupled with high relative humidity, there is a high heat index.
Because the human body cools itself through evaporation, factors that slow evaporation reduce the effectiveness of our natural cooling system. When temperatures are high, the cooling system works harder to maintain a desirable temperature level. That’s why you often hear the phrase “It’s not the heat; it’s the humidity.” At 105 degrees F and 30 percent humidity, the heat index would be 114 degrees F; at 105 degrees F and 10 percent humidity (as might be seen it the desert Southwest) the heat index is 100 degrees F, cooler than the actual air temperature.
Animals are also impacted by heat stress. The USDA Animal Research facility has a website that measures a cattle heat stress index. The URL is:
It notes that heat stress can be aggravated by various local conditions, including saturated soils (as from a leaking tank), nearby irrigated crops, lack of air movement, night temperatures above 70 degrees F , and conditions in the danger zone or worse for more than 2 consecutive days. As with people, the stress is cumulative.
The K-State Mesonet web site has a special page that tracks the current heat index at
The data updates every five minutes when you refresh the page, and is available for all 55 stations.
Christopher Redmond, Mesonet network manager
Mary Knapp, Weather Data Library
Busby, D., and Loy, D. 1996. Heat Stress in Feedlot Cattle: Producer Survey Results. Ames, IA: Iowa State University. A.S. Leaflet R1348
Hungerford, L. L., Buhman, M. J., Dewell, R. D., Mader, T. L., Griffin, D., Smith, D. R., and Nienaber, J. A. 2000. Investigation of heat stress mortality in four Midwest feedlots. 430-433: ISVEE.
Mader, T. L., Hungerford, L. L., Nienaber, J. A., Buhman, M. J., Davis, M. S., Hahn, G. L., Cerkoney, W. M., and Holt, S. M. 2001. Heat stress mortality in Midwest feedlots. J. Anim. Sci. 79: Suppl. 2:2.