Visible, Infrared & Water Vapor

In short: most satellite pictures in weather apps are one of three single channels. Visible is sunlight bouncing off clouds, so it shows detail and texture but goes black at night. Infrared is the heat a cloud top gives off, so it shows how cold, and therefore how high, the top is, day and night. Water vapor shows moisture several miles up, not the air you breathe. Each one answers a different question, and each has a classic misread.

Three views of the same sky

The GOES satellites carry an imager, the ABI, that takes pictures at 16 wavelengths at once. Three of them do most of the everyday work:

• Band 2, 0.64 micrometers (µm), red visible. The sharpest channel on the satellite, about 0.5 km per pixel straight below it.
• Band 13, 10.3 µm, the “clean” longwave infrared window. About 2 km per pixel. “Clean” means water vapor absorbs very little at this wavelength, so the satellite sees the surface or cloud top almost unobstructed.
• Bands 8, 9 and 10, 6.2, 6.9 and 7.3 µm, water vapor. About 2 km per pixel, each sensing moisture at a different height.

A single channel has one number per pixel, so it can be drawn in grays or put on a color scale. That is the difference from the false-color views, which mix three channels and have no scale at all.

Visible: sunlight off the clouds

Visible is the closest thing to a photograph. Thick cloud reflects most sunlight and looks bright white; thin cloud is gray; land is darker and water darkest. Because the pixels are small, visible shows things the other channels blur: fog filling a river valley, lines of new cumulus along a boundary, the bubbling top of a growing storm.

What to know:

• It is daytime only. No sun, no picture. Near sunrise and sunset the whole scene dims and goes grainy.
• Low sun is a gift. When the sun is low, tall features cast shadows. An overshooting top, a dome of cloud punched above the anvil by a strong updraft, throws a shadow across the flat anvil and stands out clearly.
• Fog and low cloud show well. They are smooth, bright, have sharp edges and follow the terrain. This is the best fog view there is, by day.
• Sun glint. Over water, near the spot where the sun’s reflection points straight back at the satellite, the sea can look bright or milky. The bright patch moves through the day; cloud does not behave that way.

In MesoRadar: Visible, in the Satellite tab under Popular channels ("Fine detail. Daytime only"). After dark the row switches to Shortwave IR (3.9 µm), and its subtitle says so.

Infrared: how cold the tops are

Everything gives off heat, and the 10.3 µm channel measures it as a brightness temperature: for thick cloud, close to the temperature of the cloud top. Air cools with height, so cold means high. Infrared color scales, often called enhancements, put their most vivid colors at the cold end so tall storm tops jump out. That invites the biggest satellite misread of all.

• Cold is not rain. Thin cirrus can be as cold as a storm top and drops nothing, while a shallow, warm-topped shower can pour. Infrared tells you how high the tops are; whether it is raining is a radar question.
• Watch the trend. Tops getting colder and spreading mean a storm still growing. Warming tops often mean it is weakening.
• Overshooting tops show as small, very cold spots inside the anvil and mark a strong updraft. Compare them with the anvil around them, not with a fixed number: how cold a top can get depends on the tropopause, which changes with season and latitude (Bedka et al. 2010).
• The enhanced-V and the thermal couplet. Downwind of an overshooting top, a V of colder cloud, or a cold spot right beside a warmer one, sometimes appears. The CIMSS quick guide ties these to storms producing, or soon to produce, damaging wind, large hail or tornadoes. They mark a strong updraft, not proof of severe weather; a study of enhanced-V storms concluded the feature should not be used on its own (Brunner et al. 2007).
• Hurricanes. A clear, warm eye inside an unbroken ring of very cold tops marks a well-organized hurricane. A ragged ring, warming tops or an eye that fills in often means a pause or weakening. Hurricanes: Structure, the Cone & Model Tracks.

Where infrared misleads:

• Low cloud and fog vanish at night. Their tops are about as warm as the ground beside them.
• Cold ground looks like cloud. On a clear winter night, snow-covered ground can read like mid-level cloud.
• It is not the air temperature you feel. On a sunny afternoon, bare ground can read much warmer than the air above it.

In MesoRadar: Infrared, under Popular channels ("Cloud tops, 24/7"), is Band 13 colored by temperature, with a temperature scale under the map. Drag along the scale and the pill names the temperature under your finger. On GOES-East it is free on a short loop; the longer loop is Pro. IR Sandwich, under Color products, lays the cold tops over visible texture by day.

Water vapor: moisture aloft

The water vapor channels are infrared too, at wavelengths water vapor absorbs strongly. The satellite cannot see through the moisture, so it senses heat from the moist air itself, several miles up. Moist air high up is cold and reads bright, or blue and white on most color scales. Where the upper air is dry, the channel sees deeper, to lower and warmer air, and reads dark, or brown and orange. So this is a map of moisture in the middle and upper atmosphere, not a cloud picture, and it says little about the air near the ground.

• Three heights. Band 8 (6.2 µm) senses highest, Band 9 (6.9 µm) in the middle and Band 10 (7.3 µm) lowest, down to roughly 500 to 750 hPa (about 18,000 to 8,000 ft) in clear air. The layers are not fixed: in a dry atmosphere each band sees lower, in a moist one higher. Plots of where a band is seeing are called weighting functions.
• Dry slots. A dark tongue wrapping into a storm system is dry air sinking from aloft, often where a low is strengthening.
• Jet streaks. A long, sharp edge between moist and dry air often lies along a strong jet stream.
• Shortwaves. Small swirls and dark patches moving through the flow mark disturbances that can set off storms.
• Dry air over the tropical Atlantic. In summer, broad dark areas on the lower-level channel off Africa often mark the dry, dusty Saharan Air Layer, which can choke developing tropical storms.

Where water vapor misleads:

• Dark is not clear. Fog, low cloud and humid air at the ground can sit under dark water vapor; the lowest few thousand feet are mostly out of reach.
• Thick cloud blocks the view. Over a storm, the channels see the cloud top, much like infrared.
• The edge of the view reads colder. The path through the air is longer there; the quick guides put it at up to about 8 °C (14 °F), which makes the air look moister than it is.

In MesoRadar: Water Vapor under Popular channels is Band 9, the mid level ("Mid-level moisture, jet stream"). Under More channels, the Water Vapor heading adds Upper-level (Band 8) and Lower-level (Band 10). All three are part of Pro.

The common misreads

The misreads above share two fixes. Loop it: snow and terrain hold still, cloud moves with the wind. Check another view: a cold top with no radar echo under it is probably cirrus. For fog at night, use Night Microphysics.

In MesoRadar: to tell snow from cloud by day, the Snow/Ice channel under More channels, Visible and near IR, shows snow on the ground darker than water cloud.

Parallax: why tall storms look displaced

GOES sits about 22,236 miles above the equator and sees North America at a slant, so a tall cloud top is plotted beyond where the storm really is. A study of GOES imagery put the shift for a 50,000 ft (15 km) top viewed from GOES-East at about 9 km (6 miles) over Florida and more than 40 km (25 miles) over the Pacific Northwest (Bernal Ayala et al. 2023).

In practice: on GOES-East, tall tops over the US sit north and somewhat west of the radar echo; on GOES-West, north and east. Low cloud barely shifts. Over radar, expect the cold top to sit a little off the core, more so the farther north you are.

Scan times and the two satellites

In its normal mode, each GOES imager scans:

• The full disk, the whole visible Earth, every 10 minutes.
• The US sector every 5 minutes. GOES-West’s is centered farther west, over the Pacific coast and the ocean beyond.
• Two small mesoscale sectors, about 1,000 km (620 miles) on a side, every 60 seconds each, or one of them every 30 seconds. Forecasters move them onto hurricanes, severe weather and fires.

GOES-East is GOES-19 at 75.2 degrees west; GOES-West is GOES-18 near 137 degrees west. Use the one that looks at your area more directly: East for the central and eastern US and the Atlantic, West for the western states, Alaska, Hawaii and the Pacific. A more direct view means smaller pixels, less parallax and less cooling at the edge of the view.

In MesoRadar: the GOES-East and GOES-West pills on the Satellite tab, each showing its US sector every 5 minutes. Under each pill, a full disk Infrared view (every 10 minutes, Pro): GOES-East's covers the whole Atlantic, South America and West Africa, and GOES-West's covers Hawaii, Alaska and the Pacific. GOES-West is part of Pro.

Which view, when