Question

in humid atmosphere copper vessels get covered with green layer

Answer 1

Copper metal when exposed to air turns green in colour due to corrosion. Thus, when copper vessels are exposed to air in rainy season for sometime, the metal reacts with gases and moisture present in air to form a mixture of copper carbonate and copper hydroxide [(CuCO3). Cu(OH)2]. This gives a green colour to the vessels. 

Answer 2

Copper Vessels turn greenish black with the passage of time as the copper is oxidized to copper oxide and most of it reacts with carbon dioxide in the atmosphere to form basic copper carbonate which is green in colour.
The following equation may help you....
$2cu + {co}^{2} + {h}^{2} o + {o}^{2} - - - > \: cuco3. \: cuoh2$

Hope it helps you...