Emissivity

How efficiently a surface radiates heat compared with a perfect emitter, on a scale from 0 to 1. Water and dense vegetation are very efficient; bare rock and sand are less so.

Why it matters

To turn the heat a satellite senses into an accurate surface temperature, scientists must know emissivity — get it wrong and a desert can look several degrees hotter or cooler than it really is.

Where you’ll meet it

• ECOSTRESS retrieves emissivity along with land surface temperature to study plant water stress.
• ASTER built a global library of surface emissivity from its thermal-infrared bands.
• MODIS land surface temperature products rely on emissivity estimates for each land cover type.

In plain terms

It’s like how a metal pan and a ceramic mug at the same temperature feel different to your skin — both are hot, but they give off their heat differently.