I wanna consider some cases and make sure if im right , if i have fixed volume balloon filled with helium , and let balloon fly in air , 1) if air temperature increases , ( without increasing temp of helium ) that will decrease density of air , so buoyancy force decreases. 2) but if helium temperature increase , volume is constant so pressure increases then helium density is constant , and density of air decreases so the buoyancy force decreases . 3) but if volume of helium increases with constant pressure , helium density decreases , air density decreases so no effect . So buoyancy force for immersed body contained contained fluid decreases with increasing temperature if temperature of internal fluid is insulated or constant volume , buoyancy force will not change with changing temp if volume change . [ ] ive read this answer here Why do helium balloons rise and fall? but i see different cases when volume of helium constant , as air density decreases so buoyancy force will decrease