Season 13 commences on the 30th August 1975, only 16 weeks after Season 12 ends (10th May). The stated reason for this short break is to move the broadcast of Doctor Who from the beginning of the year (traditionally January to June) to the end of the year (September to March/April). This was also the reason for the shortness of Season 12, as the proposed final story "Terror of the Zygons" was held over to become the opening story of Season 13.
Regardless of whether this decision is actually taken to gain higher audience figures by showing it in the winter, or to enable location filming to take place during the longer days of summer, the average ratings for Season 13 are almost identical to those for Season 12.
This does further bely any supposed seasonality in Doctor Who's audience, and it can also quickly be seen that the lower average figures are down to a drop in viewers for the opening 4-parter. It is quite probable that this was due to some viewers being unaware that Doctor Who was now starting in August instead of the usual January (it certainly happened to me. I was on holiday in a boarding house with no television when I found out that Doctor Who would be starting the next Saturday. I promptly persuaded my parents that we needed to come home a day early so I could watch it!) Remember that this was in the days before Iplayer, BBC3 repeats, TIVO, Sky+, or even vHS recorders. If you were away from a TV and you missed the start of Doctor Who, you missed it for good.
After week 4 the rest of the season quickly improves on the 8 million opening and sees no episode getting less than 9 million viewers, 16 of the 26 episodes getting over 10 million, and 10 of them getting over 11.
At an average of 10.1 million viewers, Season 13 has just beaten Season 12 as the highest-rated season of Doctor Who since Season 2.