The liàng reading is the causative twin of liáng: not 'X is cool', but 'let X become cool.' Always used as a transitive verb with a
把 construction or with a direct object —
凉一会儿 (let it cool for a while),
凉在桌上 (let it cool on the table). Don't mix the readings: 'this water is cool' is liáng, 'let this water cool' is liàng.