Abstract: A detection system for use in an aircraft which is able to monitor atmospheric conditions ahead of the aircraft, generate an alarm signal if adverse atmospheric conditions exist ahead of the aircraft and generate information which indicates the position of the adverse conditions so the adverse conditions can be avoided. The system is able to detect the presence of volcanic ash cloud ahead of the aircraft and, if desired, can also be used to detect a high density of sulphur dioxide gas and/or clear air turbulence ahead of the aircraft. The system monitors infrared radiation received by the aircraft, generates intensity signals representative of at least one predetermined wavelength of the radiation and compares intensity signals with reference signals or intensity signals of another infrared wavelength.