While there isn’t any one major storm coming through the region, this week will be quite active with an almost daily procession of systems starting Wednesday. For the short term, Tuesday will give us a brief break in the action with a short lived high pressure ridge moving through. It will be followed by another low moving down from the northwest, bringing a slight chance of showers Wednesday, although its path is a little too far to the north to really benefit us too much. Each successive day brings a chance (though not a sure bet on any individual system) of valley rain and mountain snow right through the weekend.

I talk about the “rain shadow” a lot around here… not surprising since we are on the lee side of a major mountain range. If you have ever wondered why we are in a desert environment on this side of the mountains and things are a lot more lush (lusher?) on the west side, but never really understood why, read on.

Before we can understand how a mountain can cause a decrease in precipitation, first we need to talk a bit about what causes rain and snow in the first place. In an oversimplified nutshell, precipitation can occur if you take moist air and cool it to the point to where the water vapor in the air condenses back to a liquid (or solid…ice…but for the sake of this discussion, we’ll combine both states here.) This forms clouds, which if the condensation continues, will form droplets big enough to fall out as rain. The best way to cool air is to force it higher up in the atmosphere, where the drop in air pressure causes a drop in temperature (see the gas laws).

In fact, much of my job as a meteorologist is wrapped up in trying to figure out where the upward vertical motion is and will occur. Air moves up… you can get rain. So how does this forcing upward happen naturally? This can occur several ways. A cold front will burrow under warmer moist air, lifting it. Converging airmasses at the lower levels of the atmosphere will have nowhere to go but up, and diverging airmasses in the upper levels of the atmosphere will draw up air from the lower levels. But land can also play an important part in vertical motion of the atmosphere. The Sierra Nevada will take moist air coming in off the Pacific and lift it, greatly increasing precipitation in the mountains.

More tomorrow.