Can you use infra-red goggles (or similar principle) to see through mist and fog? As per title really... fog is obviously quite opaque to visible light yet transparent to radio waves. What is the range of frequencies at which fog is opaque, and is either end of this range at a suitable wavelength where UV/IR receivers could be used to "see normally" in foggy conditions (e.g. the things you want to be able to see are not also transparent)?
 A: Contrary to what the other answer assumes, fog is not made of vater vapour, and light attenuation is not the reason why you can't see through fog.
Fog is a suspension of microscopic droplets of liquid water in air. This is the same material we know as "cloud" when it doesn't reach all the way down to the ground. The fog is opaque not because light is absorbed, but because the droplets scatter light, such that the mean free path between scatterings is less than the distance we're trying to see.
In order to escape scattering we simply need to use wavelengths that are larger than the typical droplet size. What this is varies between fogs, but a quick google search suggests that typical dimensions are between 10 µm and 100 µm. So if you're lucky enough to get a fog with particularly small droplets, typical thermal imaging systems (which top out at around 15 µm) would be of some use.
For larger drop sizes, things become more difficult. Having light to see by would be a problem. The atmosphere doesn't transmit far infrared as well as visible light, so only a small amount of 50 µm sunlight reaches the surface. Things glow with their own heat at these wavelenght, which is not a pure advantage: Unless the camera is cooled, its own thermal glow will tend to wash out the external infrared image. And at 50 µm, the intensity of the glow varies less with temperature than at 10 µm for thermal imaging, which both reduces the contrast both between different areas of the image, and between the motive and the camera's own thermal glow.
A: Here's a plot of atmospheric water vapour (fog) which is the green line.
The horizontal line is wavelength and the vertical line is attenuation. 

Liquid water is red and ice is blue.
From wikipedia:
The absorption of electromagnetic radiation by water depends on the state of the water.The absorption in the gas phase occurs in three regions of the spectrum.Rotational transitions are responsible for absorption in the microwaveand far-infrared, vibrational transitions in the mid-infrared and near-infrared. Vibrational bands have rotational fine structure. Electronic transitions occur in the vacuum ultraviolet regions. Liquid water has no rotational spectrum but does absorb in the microwave region.Ice has a spectrum similar to liquid water.
I am guessing liquid water is the nearest you will get to the small water vapour droplets that cause fog.
Wiki defines attenuation  as:
The mass attenuation coefficient or mass narrow beam attenuation coefficient of the volume of a material characterizes how easily it can be penetrated by a beam of light, sound, particles, or other energy or matter.1 In addition to visible light, mass attenuation coefficients can be defined for other electromagnetic radiation (such as X-rays), sound, or any other beam that attenuates. The SI unit of mass attenuation coefficient is the square metre per kilogram (m2/kg). Other common units include cm2/g (the most common unit for X-ray mass attenuation coefficients) and mL⋅g−1⋅cm−1 (sometimes used in solution chemistry). "Mass extinction coefficient" is an old term for this quantity.1The mass attenuation coefficient can be thought of as a variant of absorption cross section where the effective area is defined per unit mass instead of per particle.
Fairly opaque  description (sorry, couldn't  resist) of the drop in visibility versus wavelength.
Lastly, the electromagnetic spectrum to judge what frequencies are available to you.

Apologies, you will need a tablet/monitor with hi-res screen or good eyesight to make out some of it.
Hope this helps.
