Fig. 3From: A statistical study of convective and dynamic instabilities in the polar upper mesosphere above TromsøVariations of square of Brunt–Väisälä frequency, and Richardson number. Variations of a square of Brunt–Väisälä frequency, b Richardson number, and c modified Richardson number. Data with N2 < 0 are denoted by black rectangles. In c, data with 0 ≤ Ri < 1/4 are denoted by red rectangles, and stable regions are denoted by green rectanglesBack to article page