If the wind is light when it rains, most of the upper body is wet when walking, and the area is small.

If you run, the rain doesn't fall vertically, but forms an angle with people, so the area affected by the rain will be larger. This truth is the same as that clothes get wet faster and more in windy and rainy days.

If you want to model, you can consider something simpler, such as abstracting people into three columns, head, trunk and legs. The column of the trunk is larger than the head and legs, and then the rainfall during running (the amount of rain received per unit time and unit area V) and the angle a of rain relative to people are simulated, and the amount of rain received by the two situations in a period of time is calculated, so that the conclusion that running is rainy can be drawn. Complete the specific details yourself.