Why is that sky appears white sometimes when you view it in certain directions on hot days?

Our atmosphere contains atoms and molecules of different sizes. According to their sizes, they are able to scatter different wavelenghts of light. For example, the size of the water molecule is greater than the size of the $N _{ 2 }$ and $O _{ 2 }$ .It acts as a scattering centre for other frequencies which are lower than the frequency of blue light.
On a hot day, due to rise in the temperature water vapour enters into atmosphere which leads to abundant presence of water molecules in the atmosphere. These water molecules scatter the colours of other frequencies (other than blue).
All such colours of other frequencies reach your eye and the sky appears white.