the rain in thailand falls mainly on...me

Remember this?

Father-in-Law, fount of information on such things as HTML, has come through for me again.

Your body is intercepting drops that are falling, and the impact area you present to the falling drops depends on your posture on the bicycle and the direction that the drops are coming at you. For example, lets say there is no wind so the rain is coming down vertically, and you are sitting upright and not moving. The impact area is then essentially the same as the area of your shadow from vertical illumination. So the rate at which you would be getting wet is less than if you were moving forward (you are presenting less impact area when intercepting vertical rain). However, you are making no progress so while the rate of wetting is low, the time of wetting is ridiculously long. You are making no progress and you will keep getting wet until the rain stops or until your reason for biking is gone.

If you were to proceed through the rain, you would likely present a bigger impact area (given an upright posture) and the speed of the drops relative to you would increase so that your rate of wetting would increase but the time would decrease. At the extreme case of going so fast that the time is negligible (essentially instantaneously moving from start to finish) the rate of wetting would be very high from both an increase in impact area (your larger horizontal shadow) and the extremely high horizontal speed of the drops relative to you (from traveling so fast). Your total wetting would be from the drops contained in the volume swept out by your horizontal impact area multiplied by the distance you bicycled (at super high speed).

One can draw intermediate diagrams that show the swept out volume of rain for different travel speeds (assuming vertical rain at a given rate and its vertical speed). If you draw these diagrams from a side view you will have a very tall, almost vertical column for very slow bike speed, a series of parallelograms for intermediate speeds, and a horizontal rectangle for super-high speed. With the usual rules for calculating volume, one finds that the higher the speed, the less the volume. You can further decrease the volume at high speed by crouching and decreasing the horizontal impact area.

This analysis was based on vertical rain. With rain not vertical, the advantage from going faster will generally still be there, but the payoff from incremental speed increases will depend on windspeed and direction.

Safety was ignored in this analysis. Clearly there is a danger from speeding in the rain. Also, going faster might kick up more water from the puddles.