radar system

Radar wavelengths range between less than 1mm to 1 meter. Long wavelengths are not affected by cloud, smoke or even some plants.

Radar remote sensing used microwave as the 'sensing agent', which is not affected by the optical brightness of objects. It is in fact classified as an 'active senser", i.e. one that doesn't rely on the environmental condition for sensing.

Moreover, the microwave used in radar remote sensing usually is of long wavelength which is not easily absorbed by water molecules in clouds, thus allowing these systems to 'see' through clouds, smoke and some vegetation.