Brown rain
Almost certainly just a bit of light rain fall with some dust added to it. Lost count fthe number of times I've seen clumps of grass and bits of newspaper carried up a couple of thousand feet by summer thermal activity. And lost count of number of times I've flown in gusty conditions and come down with filthy aircraft covered in dust that was picked up into atmosphere. What you had was caused probably by the dust making just a few water droplets heavy enough to fall while otherwise it wasn't raining.
(Did you taste it?)