Aerogels are the world's lightest (least dense) solids. They are also excellent thermal insulators and have been used in numerous Mars missions and the Stardust comet particle-return mission. The focus of this video is silica aerogels, though graphene aerogels are now technically the lightest.
The first direct observers of the clouds were aeronauts, rising in balloons (ie, from the end of the XVIII century). They noticed that, according to their structure, clouds form into two groups - clouds consisting of water particles in liquid form and clouds consisting of small ice crystals.
Today we will look at an unusual sight - cities in clouds and fog.