One of the simplest variables that we often look at to predict the weather is the atmospheric pressure. In and of itself, pressure can tell us a lot about what kind of weather to expect. For instance, I'm sure most of you have seen barometers like this one (taken from the barometer Wikipedia page...):
You've seen that the pressure scale is often accompanied by weather descriptions (and in this case, little pictures). Rain is associated with lower pressure, "change" or sometimes "partly cloudy" is often observed with average pressure (around 1000mb or 29.5 inches of mercury) and "fair" or "sunny" is associated with higher pressures. In general this holds true, though each location is different. However, let me show you a curious phenomenon that occurred the the other day at the weather station at my house. The following image is a meteogram of weather variables over a 24-hour period.
But look more closely at the pressure pattern (the middle, brown-colored panel). While the pressure did fall throughout the day, something curious happened right when it started raining--the pressure actually increased a bit and stayed slightly higher until the rain had stopped. Higher pressures associated with rain?
But this makes perfect sense when we think of the air movements that are associated with rain. To get such large precipitation rates that we were seeing there, there probably were some sort of convective motions going on to help produce extra-heavy rain. Most convective processes in the atmosphere boil down to two parts--updraft motion helping to feed moisture into the storm and also downward motion accompanying the precipitation.
Rain falls with cold air exiting the thunderstorm, and this colder air is more dense than the surrounding air. Furthermore, the raindrops themselves exert a drag on the air as they fall, pulling air down with them. The net result is a slight increase in downward pressure at the surface underneath areas of heavy rain. Colder, denser, sinking air combined with downward drag on the air as the rain falls combine to increase the pressure at the surface. So, this little "blip" of higher pressure actually makes sense.
You can see this effect a little bit in the temperature curve (the red line in the top panel) as well. As the rain accompanies cooler air down to the surface, you can see that the temperature suddenly begins to decrease rather sharply once the rain begins. It continues to cool at this steady rate until the rain stops. At the same time, the dewpoint temperature (the green line in the top panel) also shows a significant rise while it was raining. Some of that liquid water falling to the surface evaporates into the air (notice that the air temperature was well above the dewpoint, so the air was not saturated). This extra evaporation due to the presence of so much liquid water dramatically increased the dewpoint temperature over a short period of time.
This was a bit of micro-meteorology here, but it just illustrates why you can't just say, "lowering pressure means rain". The falling rain itself can cause the pressure to increase, too...