We measured the velocity V step of isolated elementary spiral steps and the distance L eq between adjacent equivalent spiral steps on ice basal faces by advanced optical microscopy. We determined the step kinetic coefficient β from V step measured under various supersaturations. We performed similar experiments under various temperatures T, and determined the temperature dependence of β of ice basal faces, for the first time, in the temperature range of −26.0 to −2.7 °C. When −6.2 ≤ T ≤ −2.7 °C, the value of β decreased significantly with decreasing T. In contrast, when −15.0 ≤ T ≤ −6.2 °C, the value of β increased with decreasing T, and had the maximum at T ≈ −15 °C. When −26.0 ≤ T ≤ −15.0 °C, the value of β decreased monotonically with decreasing T. Such complicated temperature dependence of β strongly implies the existence of unknown phenomena in the temperature range examined. To obtain a clue to the complicated behavior of β, we also measured dependence of L eq on surface supersaturation Δμ surf. When −13.0 ≤ T ≤ −3.2 °C, plots of L eq vs 1/Δμ surf satisfactorily follow the spiral growth model. However, when −26.0 ≤ T ≤ −15.0 °C, the L eq vs 1/Δμ surf plots do not follow any model: this temperature range agrees with the temperature range in which β decreased monotonically with decreasing T.

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