по мере того как

• IPA^(key): [pɐ‿ˈmʲerʲe tɐˈvo kak]

по ме́ре того́ как • (po mére tovó kak)

1. as (in line with, proportionally to)

   По ме́ре того́ как идёт снег, земля́ стано́вится холодне́е.

   Po mére tovó kak idjót sneg, zemljá stanóvitsja xolodnéje.

   The ground is becoming colder as it snows.

Both of the events in the main clause and the subordinate clause, with the conjunction по мере того как, describe actions that happen simultaneously as well as showing a gradual progress. Since both verbs of the main clause and the subordinate clause indicate two sets of gradual action, they must be imperfective.

• не успе́л, как (ne uspél, kak)