Why do my windows have drip lines under the frames?

Start with where the mark runs: down the pane itself, or down the stucco, paint or masonry under the sill. The possibilities below come from a window maker's care instructions, a glass association and a federal science agency, and all of them speak to the window; none speaks to the wall.

A mark on the wall below a window is a painter's or a mason's question, and the last section here says why this page stops at the sill.

PGT, CGI and WinDoor's homeowner instructions name this directly: “Rinse and dry any run-down quickly to prevent the cleaning solution from leaving a residue on your glass.” A line that traces to a wash is residue the rinse did not reach.

USGS explains where a raindrop starts: “For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus, binding them together.”

When a drop runs down a pane and dries, it leaves behind what it carried and what it picked up on the way, which is one way a line can appear after rain; no source read measures that deposit. Whether rain makes clean windows dirty works through the same idea for the whole pane.

PGT, CGI and WinDoor rule out one explanation first: “Faulty windows do not cause condensation. Glass is usually the first place you notice condensation because glass surfaces have the lowest temperature of any surface in a house.” The same guide names the cause: “The moisture in the air causes condensation.”

Fog on the glass and fog between the two panes of a sealed unit are different things, taken apart under whether foggy windows can be cleaned.

PGT, CGI and WinDoor ask owners to “routinely inspect and clean your water drainage (weep systems) to maintain proper drainage.” A weep system drains water out of the window, so water leaving it and running down below is one of the possibilities.

The National Glass Association's quick-reference list, written about cleaning, says “DO NOT allow water or cleaning residue to remain on the glass or adjacent materials.” Water that leaves a frame and dries on the glass or frame below it can leave behind what it carried; that is reasoning from NGA's rule, not a published finding about weep systems. The weep-system line is the maker's homeowner maintenance item, and the window cleaning page's section on frames and drainage quotes what that guide asks an owner to do.

None of the sources above describes what happens once water leaves the window and reaches stucco, paint or masonry. A dark line on that surface is a question for a painter or a mason, not for a glass association or a window maker's guide, and this page stops at the sill. On the glass, a dried line is a streak, and streaking has its own entry.

A dull film across a whole pane, rather than a line, is a separate subject under what the film on my windows is. The parts of a window a line can run past have glossary entries of their own, among them muntins.