Despite its atmospheric importance, homogeneous freezing of aqueous drops is poorly understood. Here we provide evidence that at atmospheric pressures the conditions leading to the initiation of freezing in pure water are those for which the liquid compressibility and the corresponding density fluctuations reach maxima. This liquid-only criterion for the onset of freezing contrasts with the traditional view that freezing temperatures depend sensitively on the parameters of ice and the ice-water interface. We generalize the connection between compressibility maxima and freezing to aqueous solutions. Utilizing compressibility data, we predict solution freezing temperatures from those of pure water under pressure. These predictions are almost coincident with the parameterizations of droplet freezing temperatures in terms of water activity <Koop et al., 2000> used to predict ice formation in cirrus clouds. Our work then provides a physical basis for this link between water activity in solution and homogeneous freezing temperatures.