Why does a desert cooler cool better on a hot dry day?

Boiling of a liquid takes place at

Why is ice at $273 \ K$ more effective in cooling than water at the same temperature?

What is the effect of a change in pressure on the physical state of matter? Explain with an example of a gas.

Sameer once went to his granny's home with his family and he saw his granny serving water from an earthen pot and was surprised to see that and asked granny why she has kept that earthen pot for storing water. His granny then answered him that it keeps the water cool during summers,he then smiled and thanked her.
$(a)$ According to you,how does water kept in an earthen pot become cool during summers?
$(b)$ What kind of qualities are shown by Sameer?

Once Ram's mother Sunita went to her friend's home along with Ram. On visiting there,Ram saw a wet khus-khus screen hung at the door of the room. He asked his mother why they had hung this screen,and his mother answered that it keeps the room cool. Ram was satisfied and smiled.
$(a)$ According to you,what qualities are possessed by Ram?
$(b)$ How does a wet khus-khus screen hung at the door keep the room cool?