What Is a Layer Swipe? (And Why It Beats Stacking Two Maps)

A map layer is one picture of the world: the clouds right now, the roads, the temperature, last week's flooding. Put two of them on the same screen and you hit an immediate problem — the top one covers the bottom one. The usual fix is to make the top layer partly see-through, but a half-transparent layer is a compromise that satisfies no one. Colours wash into muddy in-between shades, fine detail disappears, and you end up squinting at a blend instead of reading either layer properly.

A layer swipe solves it differently. Instead of mixing the two layers, it splits the screen with a divider you drag. To the left of the divider you see one layer at full strength; to the right, the other. Drag the divider across the part of the map you care about and you wipe one layer away to reveal the other, cleanly, exactly where your eye already is. Nothing is faded, nothing is blended — the only place the two layers touch is the divider line itself.

Why the swipe reads better

The reason a swipe works is that comparison is a local act. You're almost never comparing two whole maps at once; you're comparing them at a spot — is this city under cloud, is that coastline clear, did this field flood. A swipe lets you put the divider on that spot and rock it back and forth, watching the two layers alternate under a fixed point. Your eye does the comparison across a single sharp edge, which it's very good at, instead of trying to separate two overlaid signals, which it's very bad at.

It's the same trick as a before-and-after photo slider, and it works for the same reason: a hard edge that moves is far easier to read than a soft blend that sits still.

Reading the clouds-versus-ground swipe

The flagship comparison on LiveEarthViewer holds two very different views of the same planet against each other:

• Live clouds. An infrared mosaic from the world's geostationary weather satellites — the American GOES, the European Meteosat, the Japanese Himawari — blended by NOAA into one seamless global picture and refreshed every hour. Infrared reads the temperature of the cloud tops, so cold high cloud glows bright and clear sky reads dark, and it works just as well at night as in daylight.
• Satellite ground. High-resolution imagery of the actual surface — land, sea, cities, mountains, ice. This is imagery, not a live feed, so it doesn't change minute to minute; it's the fixed stage the weather moves across.

Drag the divider and you're peeling the current weather back to see what it's covering. A spiral of cloud resolves into a storm sitting over a named coastline. A dark gap in the cloud turns out to be clear air over open ocean. A straight edge of cloud reveals itself hugging a mountain range. The clouds are live; the ground is constant; the divider is where the two meet.

Swipe or blend?

Both have their place, and they answer different questions:

• Swipe when the two layers show the same thing and you want the difference — clouds versus the ground under them, a place today versus the same place last month, one data source checked against another.
• Blend (a see-through overlay) when the two layers show different things and you want the relationship — wildfires laid over wind direction, or shipping lanes over sea-surface temperature, where the whole point is to see them interact at once.

A swipe isolates. A blend combines. Knowing which question you're asking tells you which one to reach for — and on the live map, the divider is right there waiting for you to grab it.