Megan, to elaborate further - the sensor on the IR element of the camera is cooled to around -200 deg C and is very sensitive to received emissions in the appropriate band.
The level of radiated emission of an object is the total of what it transmits, what it reflects and what passes through it which is why it is too simplistic to talk about the temperature of an object.
If the total emissivity of foreground and background objects is the same then you can't tell one from the other - this is called thermal crossover and can really hamper searches.
Rain and the moisture in clouds attenuate the amount of radiation travelling from the object to the sensor so degrading the picture by reducing contrast.
Hope that helps.