Not an expert, but my understanding is, yes, clouds cool the earth during the day, compared to no clouds, and they warm it at night. But on net water vapor is a

Not an expert, but my understanding is, yes, clouds cool the earth during the day, compared to no clouds, and they warm it at night. But on net water vapor is a greenhouse gas: it holds onto heat, and it absorbs the wavelengths emitted by the earth disproportionately more than those emitted by the sun, so it keeps more heat in than out.