Summer Heat Waves - Extreme Years

Authors: Nienaber, John; Hahn, George; Brown-Brandl, Tami; Eigenberg, Roger

Submitted to: American Society of Agri Engineers Special Meetings and Conferences Papers
Publication Type: Other
Publication Acceptance Date: 5/10/2007
Publication Date: 6/20/2007
Citation: Nienaber, J.A., Hahn, G.L., Brown Brandl, T.M., Eigenberg, R.A. 2007. Summer Heat Waves - Extreme Years. In: Proceedings of American Society of Agri Engineers Special Meetings and Conferences, June 17-20, 2007, Minneapolis, MN. Paper No. 074084. Available: http://www.asae.frymulti.com/techpapers.asp?confid=mid2007.

Interpretive Summary: Cattle in feedlots can be strongly affected by hot weather, especially when humidity is high, wind speed is low, and no clouds or shade are available. There are several index values that represent dangerous hot weather conditions. Four heat stress indices were compared for known events using weather data from several locations. Each index correctly identified all heat events at each location. Comparisons showed differences in performance among the models that might be confusing if several are used together. Each model had its strengths, depending on the person using the predictions. Models were compared for one known fatal heat wave. Conditions at the two closest weather stations did not appear to be bad enough to create the losses. It was concluded that small areas of extreme intensity can be created, but no conclusive data were available. Weather stations located at feedlots would document conditions and may provide warnings.

Technical Abstract: The performance of four heat stress indices was compared for response to known events. A 12-yr period of weather data was analyzed for occurrence of heat wave events at each of three locations--Grand Island and Concord, NE and Rockport, MO. Numerous events were detected at each location. The Temperature-Humidity Index (THI) was used to show the duration of events and characterize the intensity in terms of temperature and humidity. Three additional indices were used to provide a similar depiction, based on additional weather factors of solar radiation and wind speed incorporated in the index. Each index correctly identified the events at all locations based on the analysis of hourly records of temperature, humidity, solar radiation and wind speed. Comparisons among the three index values with the original THI value showed that the index computed by the adjusted THI model was the most sensitive for all events, giving emergency category warnings most frequently. The respiration rate (RR) model was generally the least sensitive in comparison, giving the least number of emergency warnings and estimating the greatest recovery time. The Accumulated Heat Load Units (AHLU) model generated an accumulative index value which generally maintained the emergency warning, for the very severe events, after the heat wave event had passed. In a comparison of the models and indices for a known severe heat wave that was responsible for the loss of about 5000 cattle in northeast Nebraska, it was apparent that dangerous heat waves exist beyond the coverage of the weather station network. Death losses indicated that the local conditions were more severe at the affected feedlots than at two nearby (35 km) weather stations that also had feedlots in the immediate area without cattle deaths, stressing the value of an on-site weather station. Future efforts to compare indices should focus on sites having both weather data and known records of livestock losses.