Due to condensation of vapour,which came into the contact of cooling and get condensed into drops of water..

**Condensation on the Surface of a Glass**

**Introduction:**
When we keep cold water in a glass, we often observe water droplets forming on the outer surface of the glass. This phenomenon is known as condensation. It occurs due to the difference in temperature between the water inside the glass and the surrounding environment.

**Explanation:**
Condensation is the process by which water vapor in the air changes into liquid water. When we keep cold water in a glass, the temperature of the water is lower than the temperature of the air in the room. This temperature difference causes the air near the surface of the glass to cool down rapidly.

**Cooling of Air:**
When the air comes into contact with the cold glass surface, its temperature decreases. As a result, the air can no longer hold as much water vapor as it did when it was warmer. The excess water vapor in the air then condenses into tiny water droplets on the surface of the glass.

**Formation of Water Droplets:**
The water droplets form on the outer surface of the glass because the glass itself acts as a cold surface that cools down the air. The cold glass surface provides a suitable environment for the water vapor to condense and transform into liquid water.

**Effect of Temperature Difference:**
The rate at which condensation occurs is directly proportional to the temperature difference between the cold water and the surrounding air. The greater the temperature difference, the faster the condensation process will take place.

**Relative Humidity:**
Another factor that influences the formation of water droplets is the relative humidity of the air. If the air is already saturated with water vapor and its relative humidity is high, condensation will occur more quickly. This is because the air is already close to its maximum capacity to hold water vapor.

**Conclusion:**
In conclusion, when cold water is kept in a glass, the temperature difference between the water and the air causes condensation to occur. The cold glass surface cools down the air, leading to the formation of water droplets on the outer surface of the glass. The rate of condensation is influenced by the temperature difference and the relative humidity of the air.