Q. Explain why the moisture gathers on the outside of a glass tumbler containing cold water?

Ans. Because the water vapours present in the air get cooled and appear as droplets of water on coming in contact with the cold surface of the glass tumbler.

Q. Explain why in winter evenings and mornings, fog or mist collects in valleys?

Ans. This is because in winter evenings and mornings the temperature of the atmosphere is sufficiently low so as to cause the condensation of water vapours present in the atmosphere. The condensed water vapours being heavy, appear as fog or mist and tend to collect in valleys.