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The rain-drops arrive at the surface of earth after they condense from the cloud at a fairly high altitude. As they move down under the action of the earth’s gravity, their velocity increases.

However, friction or drag force offered by the atmospheric air tends to slow it down. Under the concurrent action of these two opposing forces, the drops reach the earth with some velocity depending on the size of the drop and the wind velocity.

Any moving object carries a momentum which is the product of its mass and its velocity. This bears a relative velocity with respect to another moving object.

The relative velocity of a moving body with respect to another is the sum of the two velocities if the two objects are moving against each other; it is the difference in the velocities if they are moving in the same direction.

When we move perpendicular to the vertically falling rain drops, the relative velocity is the velocity at which we move towards the falling drops.

The product of the relative velocity and the mass of the rain drop is the relative momentum. This momentum gets transferred on to our body when a drop hits. A localized application of such a force causes the sensation of getting pricked by the drop.

When we are walking, our motion is slow and the force exerted by the drop on our body is low and we feel no pricking.

But when we are moving on a bike at a high velocity and rain drops fall at an angle (on the front side), the relative velocity of the drop with respect to us is the velocity of our motion which is high. The pricking force is experienced due to the high relative momentum that gets transferred onto us.

It may be noted that this prick like experience is only on the forward side of the body rather than on the back side although rain drops do fall on the back side of our body while we are moving forward on a bike. This is so, because the relative momentum in such an event is practically zero and no force is experienced.

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