6. Decreasing Atmospheric Pressure
A fall in atmospheric or barometric pressure is a surefire way to know that the weather will soon turn bad within 12 to 24 hours. However, not everyone carries a large and cumbersome barometer around in their pockets. They can carry an altimeter on a GPS unit, which will also warn you of a rise in barometric pressure and an impending high-pressure system moving in.
If you are standing stationary and the altimeter shows an elevation change, then you know the pressure is changing. If it shows a rise in elevation and barometric pressure, then an impending high-pressure system is moving in. To maintain accuracy, calibrate the altimeter at ground level and always recalibrate it whenever you can.