Rain has been tough to come by in the recent months for most of California. And even when it does rain the atmosphere seems to resist it at first.
You may have seen it happening as a storm rolls in, in the form of virga.
Virga is rain that evaporates before it hits the ground. It happens when rain falls through an especially dry part of the atmosphere which generally saturates from the upper atmosphere down to the surface.
You’ll generally see it as fuzzy bits extending from the bottom of storm clouds usually as a system starts to overspread the area.
As the virga continues, the evaporation does two things: one it releases more moisture back into the air, and two it cools the air in a process called evaporative cooling. As the air becomes wetter and cooler the relative humidity starts to rise to the point where it can support rain.
Usually rain can start to fall in relative humidity values around 50 percent but the drops will be very small and will evaporate quickly when they hit the ground. To get more meaningful rain the relative humidity needs to be nearer to 75 percent and when it gets to 80 or 90 percent the majority of the rain will be absorbed by the ground or run off into the streams and rivers.
Virga is more likely to happen in valleys than in the mountains for two reasons. One reason is that the ground is simply farther away from the clouds, so there is more air for the rain to fall through. The second reason is that mountains can force more rain to fall by way of orographic forcing. Essentially the mountains force the air to go up and over, which cools and condenses the moisture in the air and causes more rain to fall.