Cirrus bands tend to parallel the jet stream. When bands occur, 50 per cent of the time they are found within 370 km (200 n mi) of the jet core, on the warm side and in the entrance or neutral areas. Sharp northern edges of cirrus sheets are generally found about 100 km south of the jet core. Cirrus is often found on both sides of the jet when the jet is weak or when the jet is south of another jet. The high-level temperature discontinuities tend to “wrap around” the jets from about 4 km below the jet core to the level of the core 50 to 400 km to the north. The observed cirrus bands have been classified into three categories according to average spacings between bands: large scale, 185 km apart, major bands, 4.2 km apart, and minor bands 0.37 km apart. Known maximum band lengths are 650 km, 148 km, and a few kilometers, respectively. Cloud-cell spacings along the major bands average 1.3 km. The major bands are created between parallel horizontal vortices through which the air flows in a helical motion. The vortices rotate in opposite directions and cloud forms where the combined motion is upward. The resulting clouds indicate conditional instability in all cases, and their form apparently depends on the steepness of the lapse rate and prevailing vertical shear. Maximum lateral components of motion measured across the tops of the vortices ranges from 0.6 to 6.0 m sec−1. When these components are combined with the forward speeds, a median angle of 4.4 deg from the large-scale wind direction is found across the tops of the vortices.