I wrote this a few years ago, but it still seems to fit, even if today wasn’t a January day:

The January rain came down in sheets fine enough to drift and cling to every surface; persistent enough to penetrate even your bones. The sky, a thick blanket of leaking cloud, was so dark that throughout the day the street lights had stayed on, the drizzle swirling in front of them like effervescence in a murky pint of bitter. Outside, the tarmac hissed with passing car tyres. The throb of an idling vehicle rattled my studio windows, making me stop painting. I looked out.
Facing me was the top of a double decker bus, its glass translucent, running with condensation. The sodden occupants only vaguely visible within, save for a shoulder or arm pressed through the veil of precipitating steam and fog breath.

One of the rear panes showed some sign of activity as a fingertip became visible from the other side, travelling across the surface to make an arc headed by a pink dot. The dot vanished only to reappear in a different part of the glass and form another line, a puncture, a zig zag. Each time a new mark was made, not only more of the image came into being but more of its author. A crude face appeared, evanescently painted onto the glass dripping droplets and runs. The drawing itself disclosed the bored countenance of a drenched adolescent, finally revealed, as, with a wipe of his now visible hand, the picture was obliterated and the bus moved off.