The ghosts like the rain. Whether it’s the water or the cast of the sky, something about wet weather makes the faded signs seem more vivid. You see details of an ad that’s been pitching flour since Woodrow Wilson was president, a sign for a store that was a memory when 35W carved up the neighborhood.

By contrast, the new billboards wipe themselves clean every few seconds, swapping one bright ad for another. Pull the plug and they’re dark; reprogram the chip and a new set of pictures blares bright. The old billboards were made to last longer, and they did.

A century of wind and rain hasn’t erased them. A hundred years of sunshine couldn’t fade them completely. All that strength and endurance — and they could be gone in an afternoon if someone got industrious with a brush.

There are several kinds of ghost signs.

• The Eternal. Intact, brilliant, crisp, hardly ghostly at all. In other words, fake. Or restored. Sometimes a building owner or a city will decide that the signs lend atmosphere to the street, provide a reminder that this isn’t a strip-mall where “history” means “Radio Shack was in that spot until last month.” It’s nice, and better than erasing them, but they lose something when they’re brought back to life. It’s a bit like watching Gloria Swanson smile in “Sunset Boulevard.”

• The Rip Van Winkle. Often the destruction of an old building reveals a sign on the adjacent property’s wall, and you wonder if the guy who paid for it got anything to compensate for the sign’s obliteration. These signs can be startling. They blare out a brand long gone with the same bright cheer, even if it’s been unseen for 50 years. A local example would be the Gethsemane Episcopal Church sign on 4th Avenue S. and 10th Street in downtown Minneapolis. It was covered by the Leamington Motor Lodge, and when that battered flophouse was knocked down in 2008, the pristine church sign was revealed again.

Sometimes these signs are doomed by a new building. At the shopping center across Nicollet from Richfield’s Hub, a renovation stripped away some cheap spray-on spackle and released a perfect Warner Hardware sign. It lasted longer than the old Warner ghost on Hennepin Avenue S. and 5th Street, which advertised all the locations of Warner stores, but it, too, was obscured again. It’s not like Warner was paying the owner any money, you know.

• The Palimpsest. So named for ancient documents that have been erased and written over. The palimpsest ends up looking like abstract art, with pictures and words overlaid on words and pictures, Coke over coffee, coffee over tobacco.

They pose a mystery: If the paint is scraped away by the elements, how can several ads still exist in the same place? Perhaps because you’re not seeing paint. Thank Mr. Lead, which made the paint bond to the brick better. (Obligatory: Don’t lick old ghost signs.)

In some cases the paint doesn’t rest on the surface of the bricks, but has soaked into the wall like ink into tattooed skin. When you see a building with multiple signs overlapping, that’s because the oldest images inhabited the brick, and when one layer faded, the one below it appeared. What the lead did to the men who painted them, we can only guess.

They were sometimes known as “wall dogs,” a term that might have suggested their place in the order of artists — above the guys who slapped on a coat and called it a day, but below the Whistlers who daubed in a nice warm studio.

• The Stubborn Survivor. On the side of the Electric Fetus record store at Franklin and I-35W, there’s a sign for Knudson’s Department Store. It is fading so slowly it’s likely it will be around when the sun goes nova. Two mysteries: what the tiny department store was like, and why no band ever named itself after it.

• The Puzzler. These are ads so degraded it’s almost impossible to tell what they were, unless you know the commercial language of the era.

On one side of the Masonic Temple downtown is a huge sign, faded to almost nothing But you can see the G and the O, and if you wonder who was spending money for signage like that 90 years ago, well, of course: Gold Medal Flour. Likewise the contours of the Coke script, or the general design for Mail Pouch Tobacco signs. But without enough words, you’re stumped.

The Lumber Exchange bears a sign that says Direct From Chicago to St. Louis route, and there’s a sign above that must be a railroad logo. From a distance you’re sure it’s “Burlington Route,” but the more you look, the more the words elude you.

As with all ghost ads, the closer you get, the less you see.

They’re reminders of time’s patient eraser, the long, slow strokes that dismantle the things we see. Change in the city is often fast: Buildings stand until they’re gone, trees tower until the workmen saw them to chunks. But the average street is a river of cars, pedestrians, storefronts, always moving, the same from day to day, different from year to year. A ghost sign is like a bell, struck hard, fading into imperceptibility so slowly you cannot gauge the pace of its diminution. All you know is that someday it’ll be gone, and you’ll probably forget it was there.

Until the rain comes, that is. Then the sign appears for a while, a face pressed against a cloudy window.