Season 19 ends on the 30th of March 1982, while Season 20 begins on the 3rd of January 1983, 39 weeks later.
As with Season 19, 2 episodes are shown twice a week, though this time on Tuesday and Wednesday(apart from the opening episode which debuts on Monday the 3rd), condensing 26 episodes into 13 weeks. However Season 20's viewing figures are now averaging out at 7 million, meaning that the 5th Doctor's 2nd season is outperformed by every other season preceding it, with the exception of Seasons 18, 6, and 4 (with which it ties).
A closer look at individual episodes shows us that, like Season 19, there are no obvious peaks or troughs, but with viewing figures being lower - generally somewhere between 6 and 7 million, with a couple of episodes approaching 7.7 million and one dropping to 5.8.
But there is a far more interesting effect that the ratings show us here...
Starting with episode 1, every second episode appears to perform higher. For example Parts 1 and 3 of "Snakedance" pull 6.7 and 6.6 million respectively, while Parts 2 and 4 pull 7.7 and 7.4. Parts 1 and 3 of "Terminus" pull 6.8 and 6.5 million, while Parts 2 and 4 pull 7.5 and 7.4. This is repeated across the whole season, with no exceptions. Considering that Seasons 19 and 20 consist of two episodes a week, one on Tuesday and one on Wednesday, it becomes obvious that Wednesday's edition of Doctor Who has consistently more viewers than Tuesday's.
There is no way that Doctor Who could consist of alternately good and bad episodes, so the only conclusion must be that something is being shown on Tuesday nights on another channel that is successfully pulling viewers away from Doctor Who. Once more clear evidence that failing ratings are not always the fault of the programme itself.
In fact if we look back to Season 19 it is possible that the same kind of periodicity is apparent during the middle part of that season as well. But sadly Season 20 ends much the same as it begins, alternating between 6 and 7 million viewers, and comparable to Troughton during Seasons 4 and 5.
Not properly part of Season 20 (though filmed a couple of months after), the 23rd of November 1983 saw the broadcast of the 20th Anniversary special - "The Five Doctors". Although appearing 8 months after Season 20 and a month before Season 21, "The Five Doctors" didn't do much better in the audience ratings, pulling in 7.5 million viewers. Far less than the 10 million average for the 10th Anniversary "The Three Doctors"