The watch has an altimiter so it knows what the pressure should be, and checks how much the pressure has changed vs. how much your altitude has changed. Any anomolies in that would point to a storm.EDIT (Jersey Hiker): No, you are not correct. I guess a lot of people don't understand that a pressure-based altimeter and a barometer are THE SAME THING. This is a drawback because you will always have to calibrate the alitmeter to a known altitude immediately before using it. Additionally, if atmospheric pressure changes during your climb (which it usually does to some extent) it will introduce an error into your reading (not usually a big error though, especially if you use a topo to recalibrate whenever possible).A GPS is an example of a non-pressure-based altimeter since it uses satellites to compute your position and altitude. With good reception, a modern GPS is reasonably accurate (USUALLY, but I've had mixed results myself - also having a standard altimeter or combined unit is preferred).For example: Say you're planning to summit a peak - if you calibrate your pressure-based altimeter in a location with a known altitude right before your hike, then reach the summit and find that the measured altitude is a few hundred feet higher than the surveyed peak height, this means that atmospheric pressure has probably dropped since you began your hike (or the survey is wrong, USGS figures are very accurate though). This also would suggest a possible storm (or low pressure system) approaching. Your altimeter can't warn you when your climbing, but (as I just demonstrated) you can get an idea of the barometric trend by yourself if you understand the basics.