Infrared satellite imagery is a picture built from heat emitted by clouds and the surface. The sensor converts that radiation into a brightness temperature, an estimate of how warm or cold the emitting layer is, and the image is usually shaded so that colder targets stand out.
What brightness temperature actually is
The AMS Glossary defines brightness temperature as the temperature of a hypothetical blackbody that would emit the same amount of radiation the satellite is receiving at that wavelength. It is derived by inverting the Planck function applied to the observed radiance.
The GOES-R Advanced Baseline Imager carries ten infrared bands, including a longwave window near 10 to 11 micrometers where the atmosphere is largely transparent. That window sees down to whatever cloud top or surface is emitting, so its brightness temperature tracks the temperature of that emitting surface.
How forecasters read it
The colder the brightness temperature in a convective anvil, the higher its top has punched into the troposphere, and generally the stronger the updraft supporting it. Rapid cooling in the top of a growing cell often signals intensification, and small pockets of brightness temperature dramatically colder than the surrounding anvil are candidate overshooting tops.
Infrared loops let forecasters follow tropical cyclone structure, mesoscale convective system evolution, cold pools spreading beneath decaying convection, and low stratus formation overnight. Enhanced color palettes are chosen to emphasize the threshold ranges of interest for the task at hand.
Important limits
Brightness temperature is not the same as physical cloud-top temperature and is not cloud height. Semi-transparent cirrus lets some radiation from warmer layers below reach the sensor, biasing the temperature warmer than the true top.
Ground temperatures swing widely between day and night, which can flip the contrast between low clouds and the surface. Pairing infrared with a visible channel by day, and with water vapor and lightning products at night, produces a more honest interpretation than infrared alone.