Question

why clouds float in atmosphere

Answer 1

clouds are made up of lighter gases such as water vapour etc..
Maybe clouds “float” for the same reason that helium balloons float. Maybe they are lighter than air. That can't be true because we know that clouds are made of water. Water is not lighter than air - water does not float.

Answer 2

$\small\mathcal\red{Why\:clouds\:remain\: suspended\:in\:the\: atmosphere?}$

Clouds are formed by the condensation of water vapour around the fine dust particles in the atmosphere. The water droplets so formed are less than 0.001 cm in dimension. This is why clouds float in atmosphere.