If you walk across a nylon rug and then touch a large metal object such as a doorknob, you may get a spark and a shock. Why does this tend to happen more on dry days than on humid days? (Hint: See Fig. 21.30.) Why are you less likely to get a shock if you touch a small metal object, such as a paper clip?

Water is a good conductor of electricityand on humid days, there are water droplets suspended in the air which aids in carrying away some of the charge from the body. Also, objects have the tendency to absorb the moisture in the air or form a thin layer of humidity on their surface that also tends to conduct the charge away. In dry days, the charge build up is very easy and there is now way to conduct the charge away. Thus, there is a chance of getting shock or spark in dry days more.

The simple explanation is that bigger the size, greater is the amount of charge accumulated on the object. So, in case of paper clips, the surface area is very small and so the charge piled up is also less as compared to large metal doorknobs.