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

Why is it not possible to cool down a room on a hot summer day by leaving the refrigerator open?

Why is it easier to charge a ballon on a dry day than on a humid day ?

Why does evaporation ause cooling?

(a) Why does sound travel faster on a rainy day than on a dry day? (b) Write the use of radioactive isotopes used in industries?