High-pressure areas are surrounded by areas with lower pressure, When air cools, molecules draw together, creating denser air that sinks toward the surface of the planet in a process called subsidence. As the air nears the surface, it spreads outward toward lower pressure areas. Subsidence also causes evaporation of water, resulting in a tendency toward clear skies, higher temperatures and lower winds. Due to lack of cloud cover, warm air tends to escape at night, leading to colder night conditions.
In a low-pressure area, warm air with lower density rises, cooling in the process. A thermal low occurs when air is warmed as sun energy is reflected back from the Earth's surface. Low pressure can occur at the boundary of a warm air mass and cold air mass, where the warmer air will flow over the colder air. The cooling of air as it rises causes condensation, often resulting in cloud formation.
Weather fronts form at the boundaries between air masses of different densities. A cold front develops as cold air moves beneath a warm air mass, lifting it and creating a low-pressure environment. The result is often showers or thunderstorms. With a warm front, the warm air overcomes a receding cold air mass. The arrival of a warm front usually brings heavy cover of stratus clouds and local warming. After a warm front passes, typically skies clear and air pressure rises.
Rain and snow generally form in areas where air pressure is decreasing. Warm air has greater capacity to hold water vapor and has a tendency to rise. In low-pressure areas, moist rising air cools, causing condensation as the air loses capacity to hold water vapor. In many cases, rain will form, but will simply re-evaporate due to the strength of low pressure updrafts. If the weight of water or ice exceeds the power of the updraft, precipitation will result at ground level.