During which process do hailstones gain weight?

Hailstones gain weight primarily during the process of making upward trips within a thunderstorm, where they encounter supercooled water droplets. As these hailstones ascend into regions of the storm that are colder, they gather additional layers of ice with each cycle through the cloud. These ice layers accumulate as the hailstones interact with the supercooled water, allowing them to grow larger and heavier until they can no longer be supported by the updrafts and eventually fall as hail. This process of rising and collecting additional ice is critical to the formation of large hailstones.

In contrast, the other options describe various phenomena related to hailstones but do not directly contribute to their weight gain. For instance, while descending through warmer layers, hailstones might melt slightly but do not gain weight; instead, they could lose mass. Collisions with other hailstones can lead to merging, but the primary weight gain comes from the accumulation of ice during upward movement. Finally, strong winds may impact their trajectory or cause merging but are not a mechanism for weight gain per se.