Pileus clouds form on sunny afternoons when the heat of the summer sun causes cumulus clouds to boil upwards. Roiling toward the sky, cumulus clouds push layers of moist air above them where they cool and condense to form droplet-rich cloud caps or 'pileus' (Latin for cap).
Sometimes, as in Mutare on Jan. 6th, pileus clouds form very quickly. In such cases their water droplets tend to be all the same size--the perfect condition for iridescent colors.
Lowenstein took four pictures over a period of just three minutes. "They show the cloud appearing, then changing shape and color," he says. "One minute later it had disappeared behind the summit of the growing cumulonimbus cloud."