The Earth’s “limb” is the glowing arc of Earth’s upper atmosphere against space, Limb-scanning infrared (IR) spectrometers or “limb sounders” point tangentially toward the horizon, so the optical path traverses the atmosphere nearly horizontally at a chosen tangent altitude, scanning the atmosphere layer by layer. This trades horizontal resolution (typically 300–500 km along-track) for excellent vertical resolution of 1–3 km.
