Breakdown of natsu wa yuudachi ga ooi kara, sentakumono o soto ni dashippanashi ni suru to nurechau.
Questions & Answers about natsu wa yuudachi ga ooi kara, sentakumono o soto ni dashippanashi ni suru to nurechau.
は marks 夏 as the topic: As for summer, ... or In summer, ....
The sentence is making a general statement about summer, not identifying summer as the subject in contrast with something else. So 夏は sounds natural here.
If you used 夏が, it would usually sound like you were emphasizing summer itself, as if answering a question like Which season has a lot of evening showers?
夕立 means a sudden evening shower, often a short, heavy rain in the summer.
In 夕立が多い, the thing that is many/frequent is 夕立, so it takes が.
Japanese often uses X が多い for there are many X / X happens a lot.
So:
- 夕立が多い = there are many evening showers / evening showers are frequent
This is a very common pattern:
- 雨が多い = there is a lot of rain
- 人が多い = there are many people
- 仕事が多い = there is a lot of work
