Good sentence! It is grammatically correct. Here is a simple, step-by-step explanation:
- In summer, — tells when this happens (this is an introductory time phrase). The comma after it is correct and helps the sentence read smoothly.
- Mosquitoes — this is the subject (the things the sentence is about). It's plural, which is right because there is more than one mosquito.
- fly — this is the verb (what the mosquitoes do). It matches the plural subject.
- in cloudy swarms — this tells how they fly. "Swarms" means lots of mosquitoes together. "Cloudy" is a descriptive word (an adjective) that makes a picture in your mind — it means the swarms look a bit like clouds. This is a figurative use, which is okay.
Some kid-friendly alternatives that mean the same thing:
- In summer, mosquitoes fly in big swarms.
- In summer, mosquitoes fly in swarms that look like clouds.
- During summer, mosquitoes gather in cloudlike swarms.
So, you can keep your original sentence as is, or use one of the simpler versions if you want it to sound clearer.