Cloud Chasing is the practice, or hobby of attempting to build coils or find atomizers which produce the maximum amount of vapor from the e-liquid used. People who do this regularly (even professionally) are commonly known as Cloud Chasers.
Cloud chasing competitions are commonly held at vape shows, events, exhibitions and even in some shops. Some involve producing as much vapor as possible, while others put emphasis on the projection of vapor over a distance, usually with contestants standing back to back to more easily compare efforts. Cloud chasing competitions usually make contestants use similar devices, with similar coils and often use a particular e-liquid to make sure each contestant is on a level playing field, e-liquids used are generally very high VG, if not 100% VG to ensure maximum cloud production.