If only I had seen this thread yesterday, or this morning! I rode in pouring rain and ended up with a nice dirt line, much like yours, plus a splattering of larger mud blobs. Fortunately I wear black shorts all the time, and no white shirts!

Perhaps tonight I'll take a picture. It's supposed to pour rain here all day.

PS - The ultimate in road-dirt collecting: Sun block + a 97-degree humid day. Dirt, bugs, pollen, small dogs...everything sticks. It's really disgusting.