What caused the exceptional mid-latitudinal Noctilucent Cloud event in July 2009?

► Nocltilucent Clouds were observed from unusually low latitudes during the summer of 2009. ► The combination of satellite observations and a microphysical model demonstrates that Noctilucent Clouds at middle latitudes are mainly generated locally. ► Due to a strong dependence of the local temperature, NLCs will mainly occur during early morning hours.