When a cold air mass advances on a warm air mass, the cold air, being denser, slides underneath the warm air and forces it to rise rapidly. This rising warm air cools and condenses, often leading to the formation of cumulonimbus clouds and potentially heavy precipitation or storms. This situation is known as a cold front
. Before the cold front arrives, the weather is usually characterized by the presence of the warm air mass. This warm air is less dense and tends to be moist, often leading to relatively warmer temperatures and sometimes light precipitation or clouds ahead of the front. As the cold front approaches, the pressure typically begins to drop
. In summary, what usually comes before a cold air mass advancing on a warm air mass is the warm air mass itself, often accompanied by gradual changes such as increasing cloudiness and falling pressure, signaling the impending arrival of the cold front
