1st April: Fools Day Cloud?

1144BST on 1st April 2020

At about 1140 on April 1st I was busy self-isolating by sitting on the jetty with a cup of tea. It was a sunny morning and small fair weather cumulus clouds were beginning to develop. To my surprise, one of the small cumulus appeared to have developed a pileus cloud.

Now pileus or “capping” clouds are usually associated with large vigorously growing cumulus clouds (“cumulus congestus”) and not the small clouds that were developing that morning in a light (~2m/s on the jetty) northeast breeze.

1147 BST on 1st April 2020

Indeed at first I thought it might be an optical illusion and that I was viewing some form of alto-stratus wave cloud behind the low level cumulus. However as I watched it was clear that it really was a pileus cloud which was rapidly disappearing as such clouds tend to do.

Curious as to what might have enhanced the convection, from the photographs I estimated the bearing of the cloud to be about 265° to 270°.

Looking at the map I wondered if the cloud might be over the Fawley refinery which sometimes triggers the formation of small cumulus clouds. However that would be 9 km or more away and the cloud seemed much closer. Indeed if the cloud were at around 1km above the ground, then I estimated it being around 2.5km to 3km away. That could place it above the Woolston waste water treatment works but I wouldn’t have expected that to trigger enhanced convection.

So I don’t know why the pileus cloud formed – maybe the sky was playing an April Fools’ Day joke!