Aufice, Why is it blue? Options

[BJ]

Hello, I live near the Rocky Mountains in Colorado and was just amazed by your great pictures of
the aufice (s?).

When ice forms here, it's more a dingy, yellowish color; not the beautiful blue as seen in your pictures.
To what would you attribute the difference - is it the pristine waters in Alaska or perhaps a cleaner atmosphere?

I'm really enjoying the journals.

BJ