How do clouds form over a low pressure center?
A low pressure center is a region of rising air. The rising air cools by isentropic expansion, which causes the water vapor to condense, forming clouds. The latent heat release from the condensing water vapor strengthens the vertical motion, lowering the surface pressure, drawing more moist air into the low, resulting in more condensation ... you get the picture. (Easier answer) Low pressure areas exist because air is rising, leaving less pressure behind. When it rises, it reaches its level of saturation. Water condenses, forming a cloud.