What Is Infrared Satellite Imagery? Reading the Live Cloud Map

Open a live cloud map and you're almost always looking at infrared — even when it doesn't say so. It's the view that lets a single map show the whole planet's clouds at 3 a.m. as clearly as at noon, and once you know what the brightness means, it becomes one of the most readable weather pictures there is.

Heat, not light

A visible-light satellite image is a photograph: it shows sunlight bouncing off cloud tops. That's why it looks lifelike, and also why it goes black on the night side of the Earth — no sun, no picture.

Infrared works differently. Everything warmer than absolute zero glows in the infrared, and weather satellites carry a longwave-infrared "window" channel — around 10 to 11 micrometers — that senses that glow straight through clear air. What it measures is temperature: how warm or cold the top of each cloud is. Sunlight never enters into it, so an infrared map looks exactly as complete in the middle of the night as in the middle of the day. That's the whole reason the world's live cloud maps are built on it.

Cold means high

Here's the one idea that unlocks the map: in the atmosphere, temperature falls as you go up. So a cold cloud top is a high cloud top, and a warm one is low.

An infrared map paints that temperature as brightness:

• Bright white is very cold — and therefore very high. These are the towering tops of thunderstorms, tropical systems, and deep frontal cloud, punching up toward the coldest air. Bright white is where the weather is doing the most.
• Soft grey is warmer, lower cloud — the ordinary overcast, stratus decks and mid-level layers that sit closer to the surface.
• Dark, or transparent is the warm ground and sea showing through: clear skies, little or no cloud.

So when you scan an infrared cloud map, you're not just seeing where the cloud is — the shading is quietly telling you how tall it is, and the brightest patches are flagging the storms.

Why "live" clouds are usually infrared

Because infrared never blacks out, it's the natural backbone of any real-time global view. On LiveEarthViewer, the live cloud layer is NOAA's Global Mosaic of Geostationary Satellite Imagery — the longwave-infrared channel of the GOES, Meteosat and Himawari satellites blended into one seamless picture and refreshed hourly. Parked over the equator, those satellites together see the planet from about 72°N to 72°S (the poles are simply out of their view), and infrared lets that mosaic stay continuous right across the day-night line instead of fading to black at dusk.

Some products, like GeoColor, get the best of both: true color by day, then a smooth switch to infrared at night. Pure infrared trades that daytime realism for something simpler and more honest — one temperature scale, everywhere, all the time.

Infrared, radar and the basemap — three different jobs

It's easy to confuse the cloud view with two things that look superficially similar, so it's worth being clear:

• Infrared clouds (this layer) show the whole cloud field and how tall it is, day and night. It's the big picture from space.
• Weather radar shows something the satellite can't: rain and snow actually reaching the ground. Not every cloud rains, and radar is what tells you which cloud is doing something right now. Reading the two together — satellite for the system, radar for the rain — beats either alone.
• The satellite basemap is the cloud-free ground in true color: land, sea and coastlines, a base to lay the live cloud field on top of.

What infrared can't tell you

Infrared is powerful precisely because it's simple, but the simplicity has edges. Thin, high cirrus can look strikingly cold and bright while being nearly transparent, so a dramatic white streak isn't always heavy weather. Very low cloud and fog can be almost the same temperature as the ground beneath them, which makes them hard to pick out in plain infrared (this is one place specialized channels and GeoColor do better). And, as above, "cloud" is never quite the same as "rain."

Used with that in mind, though, an infrared cloud map is hard to beat: one glance shows you where the planet's weather is organizing, which systems are tallest and most active, and — in the dark patches — where the sky is clear. It's the living backdrop the rest of a weather map hangs on.