Official meteorological station data underestimate the heat stress on dairy cattle in a tropical environment

ABSTRACT The values of air temperature, relative humidity and Temperature Humidity Index (THI) of one pasture-based milk production farm and from the nearest official weather station were confronted. At the farm, data readings were recorded by an automatic weather station, every five minutes, from February 2015 to September 2016. Then, the hourly average and hourly THI were calculated. The minimum, average, and maximum daily values were selected. The same was done with data from the official hourly weather station data. The minimum and mean temperature in the meteorological station were higher than on the farm throughout the experimental period and in 16 months, respectively. The maximum temperature of the rural property was greater than that of the station in six months. Meanwhile, the minimum, average, and maximum relative humidity of the meteorological station were lower than those of the farm. The minimum THI was higher at the official station and the maximum THI was higher at the farm in every month. Finally, the number of days with a maximum THI equal to or greater than the critical THI was greater on the farm. Thus, official weather station data underestimate the heat stress.摘要 本研究对某放牧型乳品生产农场的气温、相对湿度及温湿度指数（Temperature Humidity Index, THI）数据，与邻近官方气象站的对应观测数据展开对比分析。该农场的自动气象站（automatic weather station）于2015年2月至2016年9月期间，以每5分钟一次的频率记录监测数据。随后计算得到逐小时平均气象参数与逐小时温湿度指数，并提取每日的最小值、平均值与最大值。官方逐小时气象站数据亦采用相同的处理流程。整个实验周期内，官方气象站测得的最低气温均高于农场；而在16个月的观测周期中，官方气象站的平均气温亦高于农场。另有6个月的观测期内，牧场的最高气温高于官方气象站。与此同时，官方气象站测得的最低、平均及最高相对湿度，均低于农场监测点的对应数值。逐月统计显示，官方气象站的最低温湿度指数更高，而农场的最高温湿度指数在每个月均高于官方站。最终，农场出现最高温湿度指数达到或超过临界温湿度指数的天数多于官方气象站。由此可见，官方气象站数据会低估牧场面临的热应激（heat stress）程度。