This was a great question posed a while back:

I received a Kestrel 2500 Weather meter a number of years ago and was starting to look more closely at it. I had a question about one of the features.I noticed that once I turn on the meter, it has the temperature of the last time I used it. It then progresses slowly towards the actual temperature and sometimes oscillates around that one temperature until it reaches a steady-state. Does the same hold true for the barometer (i.e. the pressure change is the past 3 hours of operation and not simply the last three hours of being idle)?

Tech answer:

The 2500 when off, will periodically "wake up" and take a pressure reading and store that in the pressure trend field. This is the only measurement though that the unit will do this for, so that is why the customer is seeing the temperature from prior to turning it off. So the pressure trend will be measuring the last three hours of being idle. The pressure measurement should be close to accurate when turning the unit on as well (i.e., not the last value before turning it off).