Yesterday’s drizzle

Some rare drizzle precip1 fell yesterday.  Suggests clouds were pretty “clean”, that is,  didn’t have much aerosol loading and the concentrations of droplets in them was low (likely less than 100 cm-3) Also likely, in view of the recent strong winds, some of the aerosols in those clouds might have been large dust particles2 rather than those due to just “smog” and other tiny natural aerosols.  Large dust particles can not only influence the development of ice at higher temperatures than normal (above -10 C), but is also known to aid the formation of rain due to cloud drops bumping into each other and sticking together; collisions and coalescence because large dust particles can accelerate this process by forming large initial drops at the bottom of the cloud where drops first condense. Here, drops are nearly always too small to bump together and join up unless clouds are deep, like our summer ones,  and ice is going to form anyway.

So, yesterday, was a bit of a novelty.  Some photos and story telling:

1:34 PM. Drizzling from Stratocumulus!


1:35 PM. Drizzling here. Hope you noticed and wrote it down.  I remember how excited I was in 1986 when I was in Jerusalem and it drizzled!  I did not expect to see drizzle there, and I remember how I screamed out, “DRIZZLE?” after putting my hand out the window of the modest hotel I was in.  In those days, the cloud drops were reported to be too small by researchers there to form drizzle in them.  Yes, Mr. Cloud Maven person was the first person in the world to report in a journal article3 that it DRIZZLED in Israel! One of the great things about blogging is that you can write ALL of the things that you like to read about yourself, and this one is no exception.  I am really enjoying today, reliving past efforts and accomplishments since there don’t seem to be too many ahead….
The late Jack Russell, engineer, listening to Art tell another cloud investigation story.
The late Jack Russell, flight engineer, listening to Art tell another cloud investigation story.


2:43 PM.
2:43 PM.  Cumulus humilis field over Saddlebrooke.
3:06 PM.  Drizzle precip just a memory.  These clouds too shallow to rain.
3:06 PM. Drizzle precip just a memory. These clouds too shallow to rain via collisions, and too warm to form ice.


Looking ahead….

Mods paint dry weather for the next 15 days, and so yesterday’s disappointing “trace” (don’t recall here that Mr. Cloud Maven person had predicted at least 0.02 inches!) may be it for October.  Phooey.


1Drizzle: Fine (size range, 200-500 microns in diameter drops) close together, that nearly float in the air.  Very difficult to bicycle in drizzle even with a cap or big hat.   Fallspeeds, just a few mph.  Smaller sizes can’t make it out of the cloud, or evaporate within a few feet almost if they do.  Even true drizzle occurrences, you can’t be too far below the base of the clouds or those tiny drops won’t make it down to you.

2What is a “large” dust particle in a cloud?  Oh, 1-10 microns in diameter, real rocks compared with the other stuff normally in them.  So’s you get a drop that’s already pretty large as soon as condensation takes places.  And, if the updrafts are weak at the bottom, then only them big ones might be activated, keeping the whole cloud’s droplet concentrations low!  Happens even in places in the middle of huge land masses where in Riyadh, Saudi Arabia, we saw this happen on a dusty, moist day in shallow Stratocumulus clouds.  They developed some drizzle drops. I was with the National Center for Atmos. Research on a field project then.

31988:  Rain from Clouds with Tops Warmer than -10 C in Israel (Quart. J. Roy. Meteor. Soc.)

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.