UC Berkeley researchers presented evidence for the hypothesis that convective instability limits annual maximum surface air temperatures over midlatitude land. They provided a theory for the corresponding upper bound of midlatitude temperatures, which accurately describes the observed relationship between temperatures at the surface and in the mid-troposphere. They showed that known heatwave drivers shift the position of the atmospheric state in the phase space described by the theory, changing its proximity to the upper bound. This theory suggests that the upper bound for midlatitude TXx should increase 1.9 times as fast as mid-tropospheric temperatures at the time and location of peak surface temperatures. Using empirical mid-tropospheric warming, they project that the upper bound of peak surface temperatures over Northern Hemisphere midlatitude land (40N to 65N) will increase about twice as fast as global mean surface air temperature, and the peak surface temperatures will increase faster than this bound over regions that dry on the hottest days.