It's air temperature according to a linked article (https://www.theguardian.com/environment/2018/feb/25/mass-mortality-events-animal-conservation-climate-change

It's air temperature according to a linked article (https://www.theguardian.com/environment/2018/feb/25/mass-mortality-events-animal-conservation-climate-change https://www.theguardian.com/environment/2018/feb/25/mass-mor...): > They concluded that a rise in temperature to 37C and an increase in humidity above 80% in the previous few days had stimulated the bacteria to pass into the bloodstream where it caused haemorrhagic septicaemia, or blood poisoning.