Monday, May 5, 2014

How can you tell visually whether rain is falling out of a cloud?

It is a useful skill to be able to tell visually when rain is falling out of a cloud and BEFORE it reaches the surface.

That way you can have a bit of warning to find that umbrella, duck under cover, or end that outdoor adventure before getting soaked.

So let's provide some hints on how to read the sky for rain.

To begin with, look at the base or underside of the clouds; if you see distinct structure or texture, it is probably not raining.  Like this:
 or this:

But when the clouds start to fuzz up and become diffuse looking, there is a good chance rain is falling out of the clouds.  Like this:
or this.

Now if you have the right angle and a good view of the sky, one can sometimes actually see the rain fall as tendrils of precipitation extending downwards :  this is called virga.  Some examples:

As shown by the virga pictures above, sometimes rain does not not reach the surface:  it can evaporate in the intervening air if it is dry enough and the rain is light.  Or it will evaporate before reaching the surface until the air below becomes saturated, which can take up to 30 minutes, depending on the situation.

And yes, there is another sign of incipient rain:  if suddenly the temperature falls and the winds increase.   The cooling is due to the evaporation of precipitation falling out of the clouds and the wind occurs because the cooler air is more dense/heavy than warmer air and can accelerate downwards until it reaches the surface, producing wind.

To be explained in a future blog.


Greg Johnston said...

One thepbeloCool post. I've come to know rain approaching just by being outdoors in the NW a lot, the pics and tips are spot on.

Unknown said...

Love this post, Cliff. I drank it in with my morning cup of coffee. I have a wide open view of the sky from my office and I'm going to start training myself to see the rain before it gets here.

jeb Thurow said...

I can't wait to hear your take on the climate change report that came out today! I read the NW Chapter, There are some interesting play on words used....

Unknown said...

I always figured that native western WA/OR people recognize what clouds look like when raining, and can also smell rain coming, in the right conditions.