To determine the correctness of the given statements about glaciers, we need to analyze each statement individually and see their relationship:

1. Statement I: "To the first order, at higher altitudes, the mass balance always has a positive gradient with elevation."
   • This implies that as altitude increases, the mass balance of the glacier tends to become more positive. Higher altitudes are typically colder, resulting in higher snowfall and less melting, leading to a positive mass balance. This statement is generally true for glaciers.
2. Statement II: "Where the mass balance is positive, it has snowed more than it melts in a year and vice versa."
   • This means that for a glacier to have a positive mass balance, the accumulation (snowfall) must exceed ablation (melting) over a year. Conversely, a negative mass balance indicates that melting exceeds snowfall. This statement accurately describes the concept of mass balance in glaciers and is indeed true.

Now, let's evaluate if Statement II explains Statement I:

• Statement I is primarily focused on the relationship between altitude and the gradient of mass balance. It mentions that increasing altitude directly impacts the mass balance due to changes in temperature and precipitation conditions.
• Statement II describes the result of net accumulation versus melting without considering altitude explicitly. Therefore, it does not directly explain the effect of elevation on mass balance gradients as described in Statement I.

Conclusion: Both statements are true individually. However, Statement II is not the correct explanation of Statement I because it explains the cause of a positive mass balance rather than its variation with altitude.

Therefore, the correct answer is: Both the statements are individually true but Statement II is not the correct explanation of Statement I.