Fig. 4. Seasonal mesospheric OH airglow winter temperature trend over Svalbard 1980 - 2001. The daily mean temperatures are plotted as small dots. The monthly mean temperatures are plotted for December (triangles) and January (circles). The seasonal averages (filled circles) have the standard deviation plotted as error bars. The solid line represents the linear trend. The dotted line is the estimated linear trend using weights equal to number of days per season divided by the standard deviation.