The gradient of this quantity is misaligned from the density gradient in an instability simultaneously discovered by Eady and Charney. This quantity is plotted against temperature on emagrams, which can be transformed into skew-T diagrams by rotating isotherms 45 degrees, making them more suitable for plotting radiosonde data. In hydrostatic equilibrium, the gravitational force balances a force named for a gradient in this quantity, (10[1])which also balances the Coriolis force in geostrophic balance. It’s not a direction, but high and low values of this quantity differentiate anticyclones and cyclones. Drops in this atmospheric quantity are associated with inclement weather. For 10 points, name this force measured by barometers. ■END■