Pittsburgh’s weather had a tropical flavor this month with a chance of thunderstorms predicted every afternoon.

On clear days I could see individual thunderheads form and sometimes hear them rumble in the distance.  This late morning cloud on August 13 had some interesting features. Was it going to rain or thunder?

It was already a two-part cloud: Cumulus is the bright white top, nimbus is the darker lower section.

While I watched, two updrafts rapidly pushed moisture higher in the sky. If I’d made a video you would see both columns expanding upward and growing fast. The small cloud on the left dissolved moments after I took this photo. The big one kept growing.

The nimbus section looked threatening with dark lumpy areas (photo retouched to add contrast).  I’m not sure what the lumps mean but they probably weren’t good.

I watched this cloud as I walked home and, sure enough, it thundered after I took the photo.  Fortunately I was indoors before it rained.

(photos by Kate St. John)