Slight confusion with "Es sind 20 Grad."

I understand that some plurals in English are singular in German (e.g. Die Hose ist..., Meine Brille ist...), but in one of my Babbel lessons they used, Es sind 20 Grad.

That caught me off guard, since I thought it would be Es ist 20 Grad (e.g. Das Wetter is 20 Grad.)

Or maybe since there is more than one degree, it would be Sie sind 20 Grad.