Weather APPS are OK, but you need to know where the data is being recorded vs the actual track location. There can be notable differences at some tracks! However, it's better than nothing.

HERE'S Justin Lamb on FB answering a question about weather stations.

I'm helping someone remotely right now who is racing in Phoenix. I have two predictions running; one from an online weather source and he's sending occasional screen shots from a friend's weather station.

The numbers are definitely different between the two sources, which doesn't really matter as long as they "move together" proportionately. I've only used weather station data before, and this experience isn't changing my mind at all on that.

With that said, there was a bad run yesterday that had us scrambling a bit to know what might be wrong. It would have been real easy to see with a data recorder.

Data logger.

Air Density Online is ok, but the issue I have with it is that what it generally shows isn't quite what I see at the track, and the updates are slow during the day....The advantage to the weather station/prediction software is that you can tie your run into the weather reading AT THAT PARTICULAR time. Racing in Denver and now in Alaska, I've seen the weather swing so much between rounds that you can be off as much as a tenth and a half or more from one run to the next because of that...

Weather station. You can sort the car out without data logging. We did it for years before they were out there. Do they make it easier for sure but necessary no..

I have seen quite a bit of difference between weather stations. One I know of is about 1000 feet DA different than mine, now his works well for him because he uses same one every week. Now if you go to different tracks and use the online deal you will not be getting the same weather station and they may not all be calibrated the same.

I prefer to use same weather station even if it is a cheaper one and you do calculations yourself. Plus as someone else stated how often is it updated online? Every hour, every 15 minutes? Daily?

The problem I see with airdensity online is if you race at various tracks, the data can be off. For our purposes, precision is more important than accuracy. Let's say a computech is 5° off. Well, your computech is that same 5° off ANYWHERE on earth. You can build off that. When you're depending on a website, it could an airport ASOS/AWOS, a pro-sumer weather underground station, or whatever, so while they may have only a variance of 2°, when compared everywhere, there's variation. The exact same reported data from 2 different stations will net different results. The exact same report from your own station, in theory, ought to net the exact same results.