Summer Heat Waves -- Extreme Years

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.