Diurnal crossover
IR signature by aspect
Diurnal temperatures — skin vs terrain background
LWIR thermal contrast ΔT = skin − background
Terrain background
Aircraft skin
ΔT (contrast)
Crossover (ΔT = 0)
Diurnal model
Background swing
18 °C
Skin swing
6 °C
Skin phase lag
1.0 h
Daily mean T₀
15 °C
Baseline (type-along)
Desert day
Overcast / low swing
Background peaks ~14:00 (8 h shift); skin lags it by the phase-lag hours and swings less.
Crossover 1
—
≈ 1 h after sunrise
Crossover 2
—
≈ 1 h after sunset
Peak +ΔT
—
warmest vs ground
Peak −ΔT
—
coldest vs ground
—
A well-characterized IR signature still depends on the clock: twice a day the airframe vanishes in the LWIR.
IR signature components — stacked by aspect and band
Plume core
Hot metal (nozzle)
Airframe skin
Aero heating
Reflected (sun/sky)
Sensor band
MWIR (3–5 µm)
LWIR (8–12 µm)
MWIR is dominated by the hot plume and nozzle metal — an ingress timed on the LWIR crossover still shouts here. LWIR lives on skin contrast, which is exactly the term the crossover kills.
Stern
—
tail-on
Beam
—
side
Nose
—
head-on
—
You do not get to disappear in every band at once — hide the skin and the plume still gives you away.