Which factor is most significant in the formation of clouds in the atmosphere?

The most significant factor in the formation of clouds in the atmosphere is the ascent of parcels of air. When air rises, it expands due to lower pressure at higher altitudes, which leads to cooling. This cooling causes the moisture within the air to condense as it reaches its dew point, forming water droplets or ice crystals, depending on the temperature. This process is crucial because without the upward movement of air, known as convection, the conditions necessary for cloud formation—namely the cooling of air and subsequent condensation—would not occur effectively.

In contrast, the descent of air parcels generally leads to warming and drying, as descending air compresses and heats up, reducing the likelihood of condensation. Temperature inversions can prevent air from rising, thus inhibiting cloud formation, while low pressure systems can contribute to rising air; however, they are not the primary driver of cloud formation. It's the actual ascent of moist air that plays the decisive role in forming clouds.