Question

why do water drops appear on the outer surface of a glass containing ice cold water

Answer 1

since the water is cold, the glass gets cooled and then gets cold. when the water vapour of the atmosphere gets in contact, the water vapour condenses to form water droplets 

Answer 2

Water vapour present in the atmosphere,the water vapour comes in contact with the cooled surface of the glass and get condensded. That is why water drops are seen on the outer surface of a glass containing ice cold water.