There are several ways you can get your eTrex 30 to function more as a weather barometer than an altimeter: You should then calibrate the altimeter based on known altitudes. This will improve the barometric reading.

There is also Setup > Units > Pressure, in case you want to switch between Millibars, Inches of mercury, or Mm of mercury.

More info on using your barometer / altimeter to predict backcountry weather: [1]