AmbientWeather.com: How do I Calibrate Temperature?
Temperature errors can occur when a sensor is placed too close to a heat source (such as a building structure, the ground or trees).
Bring the sensor inside a controlled environment where the temperature is stable, like your house or facility.
To calibrate temperature, we recommend a mercury or red spirit (fluid) thermometer.
Here is an example:
Bi-metal (dial) and other digital thermometers are not a good source and have their own margin of error. Using a local weather station in your area is also a poor source due to changes in location, timing (airport weather stations are only updated once per hour) and possible calibration errors (many official weather stations are not properly installed and calibrated).
Place the sensor next to the red spirit thermometer, and allow the sensor to stabilize for a few hours. Compare this temperature to the red spirit thermometer and adjust the console to match the fluid thermometer.