GPS running watches may be built with weather measuring devices that can be useful to runners wherever they happen to be. Thermometers are, of course, essential for keeping yourself warm or cool and planning properly. Altimeters and barometers are also tools commonly found in GPS running watches, and knowing how to best utilize them is important.
A Barometer:
To put it basically, barometers measure air pressure. Air pressure is used to understand not only wind and its movement but the movement of air fronts. Air fronts are heavily tied to the weather in an area, so when the measurements on your electronic barometer suddenly rise or fall, it means the current weather is about to change.
You can use this to know when precipitation, rain or snow, is about to stop or start. You can plan your runs around this information without having to seek out another source, or just know when its best to take a break and avoid the rain.
An Altimeter:
Altimeters measure your current altitude or current height of where you are, a useful tool, when combined with a barometer, in predicting changes in weather. Essential to the runner who is inclined to trail running in mountains or hills because changes in altitude go hand in hand with changes in air pressure.
Temperature and precipitation are two factors heavily affected by altitude, which are the two things that runners should be most concerned about in terms of weather. GPS running watches with an altimeter can help you best know the nature of your route and plan ahead.