Altostratus: a misunderstood cloud and for good reason

Yesterday afternoon the clouds thickened and dimmed the sun, and our high temperature struggled only into the mid-50s.  What cloud was that?  Here it is, with Twin Peaks on the horizon.

Our names for clouds, originating with English pharmacist, Luke Howard, are based on visual attributes from the ground.  Here, “Altostratus”  (As) does RESEMBLE its lower namesake cloud, Stratus, a low fog-like cloud with little definition often found in summer along the West Coast.  See a rare example of Stratus (St) hereabouts below.  Note that it is topping the Tortolita Mountains to the west, it is that low.

However, about the only thing that these clouds have in common is that they are both relatively smooth looking clouds.  Inside them, they are totally different. Also, St is a shallow cloud usually less than 1 km (3,000 feet) in depth, while As is normally 2-3 km  6,000 to 10,000 feet) in depth.  In Stratus, you just have cloud drops and maybe, as below, a few drizzle drops (mist-like)  falling out.  OK, once in awhile in cold locales you have a few ice crystals falling out, but drops rule!  On the other hand, in Altostratus, if you were flying in them with a 1998 version of the Stratton Park Engineering Company’s Cloud Particle Imager ($130,000 or so–I’ve added a link in case some of you want to go shopping now),  you would find nothing but ice crystals for the most part.  Water droplet clouds are sometimes found in them, and, oddly, if the top is not too cold (warmer than about -30 C), at cloud top, the coldest place!  So, it is not unusual to see, even in journals, a thin layer cloud consisting of drops called, As.  Makes sense really.   (A name change of As to “Altonimbostratus” would be helpful to emphasize its internal ice and falling snow particles.)

An example of the kinds of crystals found in a As clouds is shown below, collected over Barrow, AK, in a 1998 project called FIRE/ACE/SHEBA.

These typical crystals, having grown on the way down from simple plates or tiny columns, or sphere-like  “germs”, are called “bulett rosettes.”