Water vapor imagery is a satellite view built from infrared wavelengths where atmospheric water vapor absorbs and re-emits radiation. It reveals moisture and flow patterns in a broad middle- to upper-tropospheric layer, day and night.
How the channel senses moisture
The AMS Glossary describes water vapor imagery as data taken in a water vapor channel, for example 6.7 or 7.3 micrometers. Water vapor absorbs strongly in these wavelengths, so the outgoing radiation the satellite receives comes mostly from the highest moist layer that is not blocked by even more moisture above it.
The GOES-R ABI carries three water vapor bands centered near 6.2, 6.9, and 7.3 micrometers, each weighted toward a different vertical layer. Colder brightness temperatures in the image mean moisture is being detected high in a cold part of the atmosphere. Warmer brightness temperatures mean the sensor is seeing deeper into a drier column and picking up warmer air from lower down.
How forecasters read it
A water vapor loop shows the mid- and upper-level flow directly. Forecasters trace jet streams, jet streaks, shortwave troughs, and closed upper lows even when no clouds are present, because the moisture pattern itself flows with the wind.
Dark, dry slots wrapping into a developing extratropical cyclone often signal a strengthening system. Cold, moist plumes streaming off a tropical air mass mark ascending branches. Watching where moisture is being pulled into a region gives context to what surface-based instability and shear will have to work with.
Where it misleads
Water vapor imagery is not a moisture profile. The channel effectively sums moisture across a deep layer, and where in that layer the strongest signal originates shifts depending on how wet the column is. A very dry column lets the sensor see lower and warmer than a saturated one, and features can appear to sit at a different height than they actually do.
Water vapor imagery also cannot see through opaque thunderstorm anvils, and it is a poor proxy for low-level moisture that drives severe convection. It complements a surface-to-mid-level analysis rather than replacing it.