Over the last week, we’ve seen some nice examples of mid-level convection, technically called altocumulus castellanusor less technically known is castle in the sky.
The sky on Saturday night north of Seattle was beautiful.
And earlier that day, one could watch the cells form several thousand feet above the surface (image from the Seattle PanoCam).
Here is a good example of such clouds near UW taken by cloud expert Art Rangno decades ago.
The Altocumulus castellanus clouds provide powerful hints about what is happening in the atmosphere, as well as useful information for short-term forecasting.
Let me tell you about it.
Altocumulus castellanus clouds typically form between 6000 and 20,000 ft and represent an average instability: the tendency of the atmosphere to mix vertically, due to the large difference in temperature with altitude.
We call that instability convectionand if there is enough moisture, the upward movements caused by instability will create cumulus clouds.
If you’ve ever prepared hot cereal, you’ve seen convection in your pot, with the oatmeal moving up and down due to the heat at the bottom.
The large-scale upward motion associated with an approaching weather system can help initiate and support moderate convection–so the existence of cumulus clouds provides a hint. strongly suggest future weather activity.
High-altitude cumulus clouds also show moderate humidity, which is also helpful for thunderstorm formation. Furthermore, as convection combined with surface warming extends upward to reach a layer of castellanus clouds, deeper, more intense convection and thunderstorms can occur.
Midwest storm chasers often look for castellanus altocumulus clouds as a favorable sign of future severe thunderstorm development, and many northwest thunderstorm events have been heralded by “long-term radio in the sky”.
But either way, such clouds are often unusually beautiful.
More like a subtle impressionist painting than a meteorological phenomenon.
_________________
