Figure 3-18: Change in cirrus occurrence frequency
between 1987-1991 and 1982-1986 (in %) computed from surface observations
over the North Atlantic Ocean averaged over grid boxes with fuel consumption
above 8 km greater than indicated on the x-axis (open circles). Data are from
surface observations of Hahn et al. (1996) and are averaged over 3� x 3� grid
boxes in the region 99�W to 21�E, 0�N to 72�N of the Atlantic Ocean. Only
cloud reports satisfying the illuminance criterion defined by Hahn et al.
(1995) are included in the statistics. The bold line indicates the number
of grid boxes used for the average (adapted from Boucher, 1999).