What "food" said was dead on. Think of your altimeter as a barometer. Or the barometer as an altimeter....they share the same circuitry but with revered readouts.
I use my trusty Thommen mountaineering altimeter as both a weather predictor as well as a navigation tool. They are only really useful were you might have 200 foot elevation changes on your hike. Mine is in 50ft resolution but in my part of Texas, I'm only at 681 ft....not too useful. In the mountains, I'll hike that elevation change every few hours! Once you know exactly where you are on the map, re-calibrate your altimeter to the correct altitude, based on what the map says....several times during a day's hike. If the altitude is correct, the barometric pressure will be also, and you can tell what kind of weather might be headed your way.
You use your altimeter in concert with map and compass to help identify where you are....in the middle of a long series of switchbacks, for instance.


So, when you see your altitude below where it should be, the barometric pressure of the surround air is higher than normal. Clear or clearing skies. With the big hurricane coming your way, you should be showing higher altitude than where you are. Low pressure, cloudy skies, storm front heading your way.