What meteorological phenomenon forms when cold air sinks and spreads out?

The formation of a high-pressure system is characterized by cold air sinking and spreading out. In meteorology, high-pressure areas occur when cooler air, which is denser than warm air, descends toward the Earth's surface. As this cold air sinks, it compresses and warms slightly, leading to a situation where the air spreads out from the center of the high-pressure area.

This descending air inhibits cloud formation and results in clearer skies and more stable weather conditions, typically associated with fair weather. It’s important to understand that high-pressure systems are pivotal in influencing weather patterns, as they are often linked to calm, dry conditions.

Other options do not describe the phenomenon related to sinking and spreading of cold air. Low pressure systems typically involve rising air, which can lead to cloud formation and precipitation. Thunderstorms result from rapid upward air movements, not downward sinking air. Warm fronts involve the movement of warm air over colder air, which is again contrary to the processes occurring in high-pressure systems.