| Summary: | An increase of air traffic over the North Pacific during the last 30 years has been accompanied by an increase in cirrus coverage. To help alleviate the uncertainty in the contribution of air traffic to the cirrus increase, an analysis of linear contrail coverage over the region has been initiated using afternoon NOAA-16 AVHRR data taken during 4 months in 2002 and 2003. Manual evaluation of the automated contrail detection method revealed that it misclassified, on average, 32 % of the pixels as contrails and missed 15 % of the contrail pixels. After correction for detection errors, the contrail coverage over the domain between 25° and 55°N and between 120° and 150°W varied from a minimum of 0.37 % in February to a maximum of 0.56 % in May, respectively. The annual mean coverage, after correcting for the diurnal cycle of air traffic, is 0.31 %, a value very close to earlier theoretical estimates for the region. Contrail optical depths for the 4 months average 0.24 resulting in a mean unit contrail longwave radiative forcing of 14.2 Wm−2. The contrail optical depths are twice the mean value expected from theoretical estimates.
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