Erica's Answer:
Atmospheric pressure is literally the weight of the atmosphere on an object, such as the Earth's surface, your skin, the table in front of you, and so on. As a pure number (for instance, a barometer reading of 29.94 inches), the barometer reading doesn't mean very much. What's more important is whether the pressure is rising or falling. If the pressure is rising, it means the pressure is getting higher, which is an indication of more sunshine and fewer clouds. If the pressure is dropping, the direct opposite is true: more cloud cover is apt to develop, and precipitation becomes more likely. A rise or fall in pressure is really only relevant if the trend has existed in 3 hours or more, since small pressure variations are always occurring in our atmosphere.- Erica
