August 25, 2011

Mid-Level Convection

Yesterday was quite a meteorological treat, with interesting clouds and an amazingly colorful sunset.   But it was also a good example of mid-level instability--when the mid-levels of the atmosphere starts to break out into cumulus-like convective features.

Yesterday, a weak upper level disturbance was approaching, with upward motion aloft, producing an unstable layer  above the surface.

One sign of the upward motion was the development of cirrus, include some impressive fallstreaks (made of ice crystals).  Several of you sent me pictures you took of this beautiful feature and several others were shown on the web (see example below from George Tanaka of Bainbridge Island--courtesy of Scott Sistek's blog):

The curved ice crystal "tails" falling from the cirrus are often called "mare's tails."  Why are they curved?  The reason is that the wind speed change with height--usually decreasing below the main body of the cirrus--thus the curve.

But as the day progressed the air at mid-levels started to convect into small cumulus, actually altocumulus.  Some of these cumuli developed tower or turret shaped features--they are known as altocumulus castellanus.  Now that is name that will impress your friends!  Just say that casually at some party and it will turn some heads.

Here is a great video from the UW web cam that shows the destabilization of the mid-levels and the development of the convection, with some of it deep enough that rain started to fall out:

Click for video

And you can catch the marvelous sunset.The instability clouds were apparent on the satellite pictures during the afternoon and afternoon:

An offshore band was particular impressive.

And an added features---Mt. Rainier developed a nice "cap" during the afternoon...a sign of moisture and lift aloft.

It is now clear that Hurricane Irene is going to savage the East Coast--a really serious storm. But for us, just the opposite.  The warm, perfect weather will continue through Sunday, with perhaps a slight cooling on Saturday.

And a reminder---I will be teaching Atmospheric Sciences 101 this fall if anyone is interested.  UW students, of course, and others who want to take it as non-matriculated students.  Retired folks can get in for practically nothing.


  1. Is it me or has the humidity been unusual? Where is the dry northerly flow commonly associated with our summers. With showers in the area last evening my station was reading 72 w/ 58% RH at 8 PM. Right now I'm showing 68 w/ 62% RH.

  2. I'm only leaving a comment because I have a curiosity in the subject, its weird.

    But they always call it elevated convection. And its a common form of lightning activity I use for photography here in E. Wash.

    I just read its parcels rising from the top of a stable layer or inversion. Triggered by vort maxes, jet streaks, etc. Makes sense now why its mostly at night. And why it dissipates during the day over land but holds more over the ocean.

    My camera was hoping for some action tonight but I just stepped out and there's not one altocumulus castellanus. Besides one morning outbreak that really hurt some crops, its been an extreme thunderstorm free summer.

  3. "Retired folks can get in for practically nothing." How? Do you have a link?

  4. My dad's beach house at the east end of Emerald Isle is likely to get pummeled by Irene. Luckily, there are some hardened weather stations, including this one about eight miles from his place. This is an interesting way to watch a hurricane in real time, from afar, from a weather perspective. Morehead City station

  5. This was amazingly beautiful. The appearance of the clouds where I saw them in Whatcom County, was as if something had been thrown at a window.
    A few more photos


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