Why are the base of lightning clouds negatively charged?

Question

In elementary explanations of lightning, it's generally stated that the clouds are negatively charged, the Earth is positively charged, and lightning is a discharge between the two. E.g. this source says:

As ice crystals high within a thunderstorm flow up and down in the turbulent air, they crash into each other. Small negatively charged particles called electrons are knocked off some ice crystals and added to other ice crystals as they crash past each other. This separates the positive (+) and negative (-) charges of the cloud. The top of the cloud becomes positively charged with particles called protons, while the base of the cloud becomes negatively charged.

Okay, but why does the base of the cloud become negatively charged? Why can't it be all the base that's positively charged?

I'm surprisingly unable to find an explanation for this. It's commonly-taught as a fact, without an explanation given.

Answer 1

According to this video:

Water vapour condenses into cloud droplets. Some of these cloud droplets freeze and collide with non-frozen cloud droplets to make graupels. These large graupels aren't carried up the cloud's updraft as easily as the frozen cloud droplets are. These frozen droplets collide with the graupels as they rise, resulting in the graupels obtaining a negative charge, and the frozen droplets obtaining a positive charge. These frozen and positively charged droplets will continue to rise to the top of the cloud. This makes the lower parts of the cloud net-negative and the upper parts of the cloud net-positive. I've only learned this all today so I may be wrong but it sounds about right.