[volkerp]

The BME280 is not particular suited for ambient temperature measurements. It's for accurate and fast pressure and humidity sensing. Quote from the datasheet about the temp sensor: Its output is used for temperature compensation of the pressure and humidity sensors and can also be used for estimation of the ambient temperature. You will get the temperature of the sensor rather than that of the environment. The tiny BME280 is thermal coupled to the board and also heats itself up during repeated measurements. For accurate ambient temperature you could use a DTH11 or DHT22. These come in plastic housings to isolate them from the surroundings and provide better airflow.