Extreme environmental temperatures and high relative humidity can have serious negative effects on animal production at the farm level, but less is known about environmental changes during live transport of domestic animals to slaughter. Although upper temperature limits have been established to transport pigs in Europe, few indices include relative or absolute humidity maxima or mention appropriate enthalpy ranges. In this study we measured temperature, humidity and calculated air enthalpy (kg water kg dry air −1) on commercial farms, during seven long distance (>24 h) journeys and at an abattoir. There was an approximate overlap of data points on the psychrometric charts for each location (farm, transport and abattoir). However, the temperature time derivative (°C s −1) and enthalpy time derivative (kg water kg dry air −1 s −1) were up to ten times higher during transport than the corresponding derivatives on the farm or at the abattoir. Post-transport observation of pig behaviour also suggested that journeys with higher temperature or enthalpy time derivatives were more stressed (evaluated as the amount of time they spent resting or drinking). In conclusion, times derivatives of temperature or enthalpy could be used as non-invasive welfare indicators during transport and appear to be much more sensitive than absolute values of temperature or relative humidity.