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:
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.)