Quick answer: 1) drops too small to coalesce and form ones bigger ones ; 2) no ice in ’em, for the most part.
Read below if you want a LONG discussion about yesterday; dull photos way below
Let’s talk about it, though probably more than you want to. You’re probably a little down because it didn’t rain yesterday, hour and hour though it looked like it should, except for a couple of “sprinkles-its-not-drizzle” drops. You probably had to use your headlights in the middle of the day like people in Seattle do. It began to clear up some, gradually, in the afternoon. Here are a few of scenes below, beginning with the morning overcast, with the last two shots between 4 and 5 PM AST as the clearing was underway.
So, what kind of clouds are they? Well, Stratocumulus in the first shot, in the second shot a higher layer of Altocumulus or water-topped Altostratus1 is underlain by Stratocumulus and Cumulus clouds (when the bases are more isolated, we call them “Cumulus”; when they are more connected together, we hedge the name toward “Stratocumulus”.)
The third shot, showing Stratocumulus looks particularly ominous, probably the darkest part of the daytime was here around noon AST. A shot toward the mountains next shows the underlying Stratocumulus and Cumlus below the higher layer of Altocumulus/Altostratus. These clouds can’t be “nimbo” this or “nimbo” that because there is no rain to speak of falling from them. (“Nimbus” means rain in Latin.) Note the good visibility under all of the clouds; no precip there.
Finally, with the breakup of the overcast, shown in the last two shots, we can get an idea of the thickness of the lower clouds, at least at that point, about 2,000 to 3,000 feet at most. Also in those last shots you will notice that the higher layer has moved away or dissipated, and with a bit more heating, the clouds are tending more toward Cumulus rather than Stratocumulus.
The higher layer, located at Altocumulus level, about 12,000 feet above ground level, was actually the cloud layer producing the sprinkles, and was a key player in how dark it was; two layers, naturally, stacked on top of each other, will make it darker looking than just one, especially, in this case, when they are both pretty shallow. And with a top at about -18 C, you can almost be assured that the top was composed of mostly droplets, not ice crystals. A droplet cloud reflects much more sunlight back into space than ice crystal clouds like Cirrus and Cirrostratus.
Anyone still reading? I’m doing my best here…
That last photo demonstrates that in spite of having a little rain overnight, and even during the day, there was a lot of haze/smog in the air. It wasn’t washed out by rain. And, the more clouds get bunged up with aerosol particles on which drops can form on, the higher the concentrations of cloud droplets are in them, and the smaller they are as a result. Smaller drops cause more sunlight to be reflected back into space, and when that happens, the bases look darker. In Seattle, in our airborne studies, it was usually the case to have darker based clouds downwind of the city, and light gray clouds near the coast and offshore, even when both cloud layers were about the same depth. However, there are natural sources, like volcanoes that can also affect clouds this way. For example, “VOG” (volcanic smog) in Hawaii darkens clouds there because VOG has particles that can form drops in clouds. I seen it myself and I know a dark, polluted cloud when I see one!
What happens when you get smaller drops in clouds, as smog produces in them? It makes it harder for something to fall out the bottom in two ways.
First, in smog filled shallow clouds, drops don’t get big enough to collide and stick together to form larger drops (something that happens when they get to 30-40 microns in diameter (about a third of a human hair in width). But, even in the event that could have happened yesterday, drops got that large, the result would have been only TRUE DRIZZLE, fine, close-together drops that go under your umbrella if there is a breeze of any kind. Very tough on people who bicycle and wear glasses.
The more important key to not raining clouds, was that the clouds did not have, IN GENERAL, cold enough tops to form ice crystals. The lower ones seem to have topped out around -5 to -7 C (23-20 F), temperatures at which smoggy clouds with itty bitty drops cannot produce ice.
The higher layer, seen in the second and fourth shots, was just cold enough, about -18 or so at top from the morning TUS sounding, to form a few ice crystals. Also, being higher, it was probably not impacted as much by smog.
Quitting here, brain exhausted. Hope this is somewhat comprehensible.
1The smoothness of that higher layer is due to ice crystals falling out the bottom, obscuring an Altocumulus-like cloud from which they are originating. Sometimes this has been called the “upside down” storm because the top is liquid like Altocumulus clouds where it is COLDEST, but underneath is all the ice, where the temperatures are higher. (Man, this is getting way too complicated to comprehend!)