Which part of the cloud becomes positively charged, according to the passage?

A thunderstorm often begins on a sunny day. The Sun heats the air near the ground, and pockets of warm air start to rise. When they reach a certain height, fluffy cumulus clouds form. As the heating goes on, the clouds grow upwards into tall towers, one of the first signs that a storm is on its way, and in the end the top of the cloud spreads out into the flat shape of an anvil.

Inside the growing cloud, water freezes and falls. The top holds mostly tiny ice crystals; the middle, a mixture of ice crystals and small hail; the bottom, rain and melting hail. In the middle of the cloud these pieces crash into one another, and the collisions give them an electric charge. The light ice crystals become positively charged and are carried up by the rising air, while the heavier hail becomes negatively charged and stays lower down. The cloud ends up with a positive charge at the top and a negative charge in the middle and lower parts.

At first the air acts as an insulator, keeping the charges apart. But when the difference between them becomes too great, the air can no longer hold them back, and the electricity rushes across in a single giant spark. That spark is lightning.

It is also why weather experts give one simple warning: if you can hear thunder, even far away, you are already close enough to be in danger.