I'm not an expert but have played around with sensors on some Android devices. The absolute thermometer accuracy will vary by device. When the temperature rises by 1℃ over the course of an hour is that because it actually got warmer, or is because the user moved inside or put it in a warm jacket pocket or ran another app that caused the CPU to heat up? In theory I suppose if you had thousands of devices in a small area uploading simultaneous temperature readings you could perform a regression analysis and extract a little bit of signal from the noise. But in practice I just don't see how you could ever get enough device density in a small area to make that work. Remember that in urban areas the vast majority of smart phones spend most of their time inside climate-controlled vehicles and buildings, so any temperature readings from those are worthless for weather prediction. But they do still give useful barometric pressure data because the pressure is fairly consistent inside and outside for a given location and altitude.