Cirrus Cloud Study Shows Laser Beam Impact Published Dec. 12, 2006 By Plans and Programs Directorate AFRL/XP WRIGHT-PATTERSON AIR FORCE BASE, Ohio -- AFRL scientists completed a study on the potential impacts that cirrus clouds have on airborne defensive laser systems. These laser systems have envisioned potential both in missile defense and in the engagement of theater ballistic missiles during their boost phase. However, high-energy lasers are subject to optical turbulence that may diffract the beam, causing it to spread and thus reducing the power directed towards the target. Another atmospheric effect is the absorption and scattering of laser light by ice crystals in cirrus clouds. The AFRL study quantified the effects that observed cirrus layers might have on laser transmission from a high-altitude laser source. AFRL scientists conducted an 11-month field campaign at Hanscom Air Force Base, Massachusetts, to collect cirrus cloud measurements. Ground-based cloud profiling radar and light detection and ranging (lidar), radiosondes, satellite imagery, and a surface observer collected data throughout more than 26 different 3-hour cirrus episodes occurring between February and December. Radar and lidar data specified the top and base altitudes of the cirrus layer, and satellite data provided an estimate of effective ice particle size and ice water content. The research team found that radiosonde relative humidity soundings must supplement satellite imagery to ensure accurate top and base height estimates, with respect to radar and lidar measurements. Additionally, the research confirmed that radiosondes can infer the presence of very thin cirrus layers that escape satellite detection. Scientists also used the cirrus altitudes and crystal properties in laser transmission models. They computed the transmittance, or fraction of initial laser power, incident on a hypothetical missile as a function of ascent altitude. The results indicate that cirrus clouds can have a profound impact on a propagating laser beam. This study suggests that passing through even thin cirrus will result in a reduction of power. State-of-the-art radiosondes can detect the presence and altitude of cirrus layers of varying optical thicknesses. The radiosonde can provide critical support to high-altitude laser system field tests when complemented by satellite imagery for aerial coverage to avoid cirrus clouds. This will greatly reduce the cost of cirrus detection over expensive shipment and operation of ground-based radars and lidars.