To evaluate the relative importance of different mechanisms responsible for the formation of polar lows, we have developed a three-layer, two-dimensional, quasi-geostrophic model, which includes both the effects of latent heating and baroclinity. Latent heating was parameterized for both stable precipitation associated with moist baroclinic processes as well as for convective precipitation associated with Conditional Instability of the Second Kind (CISK). Seven case studies and other observational studies are used to demonstrate that CISK or dry baroclinity does not individually provide the necessary forcing to allow the instability to grow to the observed wavelengths at the observed rates. It is found that moist baroclinic processes alone may explain the origin of Pacific polar lows, while moist baroclinity and CISK are essential in the genesis of Atlantic polar lows. Thus, there appears to be two types of polar lows. Sensitivity studies reveal the importance of low-level shear for the Atlantic ... Abstract To evaluate the relative importance of different mechanisms responsible for the formation of polar lows, we have developed a three-layer, two-dimensional, quasi-geostrophic model, which includes both the effects of latent heating and baroclinity. Latent heating was parameterized for both stable precipitation associated with moist baroclinic processes as well as for convective precipitation associated with Conditional Instability of the Second Kind (CISK). Seven case studies and other observational studies are used to demonstrate that CISK or dry baroclinity does not individually provide the necessary forcing to allow the instability to grow to the observed wavelengths at the observed rates. It is found that moist baroclinic processes alone may explain the origin of Pacific polar lows, while moist baroclinity and CISK are essential in the genesis of Atlantic polar lows. Thus, there appears to be two types of polar lows. Sensitivity studies reveal the importance of low-level shear for the Atlantic ...