Velocity of sound increases on a cloudy day. why?

Because there's a decrease in density.

Explanation:

For fluids, the speed of sound (Vs) is given by the relationship :

, where:

Ks = coefficient of stiffness, the isentropic bulk modulus (or the modulus of bulk elasticity for gases).

d = density of the fluid.

As we can see, speed of sound is inversely proportional to density. If density decreases, speed of sound increases.

In a cloudy day, humidity is very high (in fact, when it rains, relative humidity is 100% (maximum) which is why the clouds can't hold any more water). Density is inversely proportional to humidity. So, in a cloudy (or highly humid) day, density decreases, causing an increment in the velocity of sound.

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