The NERC Co-ordinated Airborne Studies in the Tropics (CAST) project was a collaborative initiative with NASA's Airborne Tropical Tropopause Experiment (ATTREX) programme to study the Tropical Tropopause Layer (TTL) over the Pacific Ocean and South East Asia.

Coordinated flights of the FAAM (Facility for Airborne Atmospheric Measurements) BAe-146 aircraft, deployed in Guam in Jan/Feb 2014, and NASA's Global Hawk unmanned aerial vehicle (UAV) made corresponding in situ measurements in the TTL over the West Pacific. These aircraft measurements were interpreted in conjunction with ground-based and balloon-based measurements of very short-lived halogen species and ozone, using a complementary group of regional high resolution models, global composition models and a global cirrus model to address fundamental questions related to atmospheric composition, radiation and transport.