During the journey
The vehicle, travel enclosure and any container used should be adequately ventilated at all times. Heat and moisture can quickly build up inside unless there is a good airflow through it.
Animals should never be left in vehicles in direct strong sunshine and/or high temperatures as it is difficult to ensure sufficient ventilation to keep them cool. Unless animals are fully acclimatised overheating, distress and suffering are likely when the temperature exceeds 25 degrees Celsius for more than a few minutes. The temperature in a car in full sun on a hot day can rise to double that outside of the vehicle in a short time, leading rapidly to distress for any animal in the vehicle.
See also the section on Heat stroke in Dogs.