Balloonborne measurements of ozone and particle size distributions at Kiruna, Sweden indicate that on 6 February 1990 a major cooling event occurred over northern Scandinavia, reaching temperatures as low as -93 ¿C at 22 km. Both nitric acid trihydrate and water ice clouds formed in the 19 to 23 km region. The temperature of the appearance of the water ice clouds is consistent with about 5 ppmv of water vapor at 22 km. The particle size distributions suggest that a sizeable degree of denitrification had occurred in the 19 to 23 km region in this air mass. Although ozone levels may have been reduced by a small degree of chemical depletion, the ozone ''minihole'' of 165 DU observed by TOMS was probably an artifact caused by ice clouds at 22 km since two ozonesonde measurements on that day indicated total ozone values of about 285 DU. ¿American Geophysical Union 1990