Rounding in snow metamorphism occurs when the temperature gradient is less than 1°C per 10 cm.

• A. At least 1°C per 10 cm
• B. Less than 1°C per 10 cm
• C. No gradient
• D. Exactly 2°C per 10 cm

Answer: (B)

Rounding happens when the temperature gradient in the snow is small. The temperature gradient is how quickly temperature changes with depth in the snowpack. With a gentle gradient (less than about 1°C per 10 cm), water vapor diffuses around grains in a relatively uniform way, smoothing edges and turning grains into more rounded shapes. If the gradient is larger (1°C per 10 cm or more), vapor transport becomes directional, promoting growth of flat, faceted faces and depth hoar rather than rounding. So the statement that rounding occurs when the gradient is less than 1°C per 10 cm matches how low gradients drive isotropic growth and rounding. Higher gradients lead to faceting, no gradient means little metamorphism, and a gradient like 2°C per 10 cm would also favor faceting rather than rounding.