CLow pressure, causing air to rise, cool, and form water droplets.
Basically, storms form due to differences in pressure in the atmosphere. High pressure means drying and sinking air (good, hot weather) while low pressure means rising, cooling air (rain! storms!). During high pressure, air is constantly getting pushed down and then moving away from the high pressure area, then getting replaced by more air, while during low pressure, air is forced to converge and because it can't just crash into the surface, it moves upward, forming water droplets as it cools and rises.
