Measured and calibrated HIRS brightness temperatures as a function of phase angle and fitted (in each channel separately) by a fifth-order polynomial. Top: for long-wavelength channels (12–15 μm); Middle: for mid-wavelength channels (6–12 μm); Bottom: for short-wavelength channels (4–5 μm). Phase curvesare asymmetric and with a 2–3° shifted peak (towards negative phase angles) at mid to long wavelengths. The inserted plots show the corresponding flux densities divided by TPM predictions, also as a function of phase angle. The lines have been fitted in each channel separately. The before-after opposition asymmetry is clearly visible for the long- and mid-wavelengths channels. At short wavelengths, there is a wavy structure and no linear fit was done.