High pressure systems create clear, sunny weather as sinking air warms up, reducing condensation. Low pressure systems produce cloudy, rainy weather as rising air cools, allowing water vapor to condense into clouds. When warm and cold air meet, the denser cold air pushes under the less dense warm air, forcing it to rise and cool, creating clouds and potentially rain or snow if the air cools enough. As a depression passes overhead, it brings first a cold front with rain, then a warm sector with some showers, and finally a warm front with more widespread rain.