A pileus accessory cloud, named from the Latin for a felt ‘cap’, forms when moist crosswinds are lifted by rising air currents within a rapidly building cloud, like this Cumulus congestus spotted by Paul Seabrook (Member 47,300) over Tuggeranong, Australian Capital Territory, Australia. Accessory clouds, generally, are ones that form on or near one of the ten main cloud types. In the case of pileus, the lifting of the airflow passing over the towering Cumulus’s rising summit can cool the air enough for some of its moisture to condense into droplets. These are what appear as a fetching cloud hat. Fetching, but fleeting. The Cumulus will continue its vertical growth, pushing right up through this gossamer hat, and in no time, the pileus, like any other accessory du jour, will be so last season.