Two adjacent surfaces in a thermal image have the same temperature, but one (a lake) appears warmer than the other (a metal roof). What physical property explains this?
AThe lake is actually warmer due to thermal inertia
BEmissivity -- water has high emissivity (~0.98) and emits nearly as a blackbody, while polished metal has low emissivity (~0.2-0.5) and emits less radiation at the same temperature
CThe metal roof reflects thermal radiation from the sky, making it appear cooler
DAtmospheric absorption is stronger over metal surfaces
A thermal sensor measures radiance, not temperature directly. Radiance depends on both temperature AND emissivity. At the same temperature, a high-emissivity surface emits more radiation than a low-emissivity surface. If emissivity is not accounted for, the sensor-derived temperature will be wrong. This temperature-emissivity separation problem is central to thermal remote sensing.
Question 2 True / False
Thermal remote sensing is an active sensing technique because the sensor detects energy emitted by the surface.
TTrue
FFalse
Answer: False
Active sensing requires the sensor to transmit its own energy. Thermal remote sensing is passive -- it detects radiation naturally emitted by the surface due to its temperature. The surface itself is the energy source, not the sensor.
Question 3 Short Answer
Explain why urban areas typically appear warmer than surrounding rural areas in nighttime thermal imagery.
Think about your answer, then reveal below.
Model answer: The urban heat island effect results from several factors: (1) Urban materials like concrete and asphalt have high thermal inertia -- they absorb solar energy during the day and release it slowly at night. (2) Urban geometry traps longwave radiation, reducing radiative cooling. (3) Reduced vegetation means less evapotranspiration. (4) Waste heat from buildings and vehicles adds energy. Nighttime thermal imagery isolates heat retention because reflected solar radiation is absent.
Nighttime thermal imagery is particularly diagnostic of urban heat islands because it reveals differential heat storage.