The four mechanisms are as follows:
1) Orographic lifting: Air is forced to rise over a mountainous barrier
2) Frontal wedging: Warmer, less dense air is forced over cooler, denser air along a front
3) Convergence: Pileup of horizontal air flow resulting in an upward flow
4) Localized convective lifting: Unequal surface heating causes localized pockets of air to rise because of their buoyancy.
