Which factor primarily affects a diver's ascent speed?

Ascent speed is crucial for diver safety, particularly in preventing decompression sickness, and the primary factor that affects this speed is the depth of the dive. As a diver ascends from greater depths, the surrounding water pressure decreases. This reduction in pressure allows dissolved gases—such as nitrogen and helium, which are absorbed by the body under pressure—to come out of solution and form bubbles.

If a diver ascends too quickly from depths where these gases have been accumulated, the risk of forming large bubbles increases, which can lead to decompression sickness or "the bends." Therefore, divers are trained to ascend at a controlled rate, typically around 30 feet per minute, and to perform safety stops during ascent when diving deeper than a certain threshold, often around 15-20 feet.

While other factors, such as the diver's experience level and the amount of air in the tank, may influence aspects of diving, they do not primarily dictate ascent speed in the same way that depth does. Water temperature may affect comfort or gear selection, but it does not have a significant effect on ascent speed. Hence, depth is the critical element that governs how a diver should manage their ascent.