The WEATHERmeter takes some time to acclimate to varying temperature. This is true of all handheld temperature sensors. For instance, if stored indoors @ 75°F (or in your pocket where it is closer to body temperature) and then taken to an environment at a lower or higher temperature, the meter will need some time to acclimate. This time can vary from 1 to 15 minutes depending on airflow over the meter and the temperature difference between locations.
The reason for this is that the body of the meter can transfer heat to or away from the temperature sensor. The solution to this is to be sure there is plenty of air moving through the vents - as long as there is good airflow, the acclimation period takes a minute or so. If there is no wind, we recommend leaving the meter in the location you want to measure, out of the sun and away from artificial heat sources and sinks, for at least 10 minutes. To reduce the acclimation period, you can also swing the sensor through the air using the keychain attachment and a lanyard.
It's also important for taking an accurate temperature measurement not to hold the meter tightly in your hand, as your body temperature will affect the reading, especially in still air. Placing the meter in direct sunlight will also cause the case to heat up and the sensor to read a higher temperature.
The temperature sensor itself is pre-calibrated and factory tested so you can be assured that readings are accurate once the sensor has acclimated to ambient conditions.
Using the included headphone jack adapter is also good option: