Think of thermal as just another color we can't see. Thermal imagers see that color.
Objects that are relatively warm or hot will emit wavelengths in that band, just as something really hot will emit wavelengths in the visible spectrum. You can heat up a large ball to 2000 degrees so it glows brightly, but you can't see what's in the middle, because light can't get out of a solid ball - you can only see the light emitted by the surface, or perhaps beneath it if the material is transparent to that color. Just like our sun is super-hot in the inside - 27 million degrees. That kind of heat would kill us through UV radiation if we saw it, but we can't because that radiation is absorbed by the sun again long before it ever reaches the surface. On the surface it's just around 5500 degrees - so that is the color of the sun we see.
Some things like glass act like mirrors. You don't see the glass like you don't see a mirror. You see what's reflected in it. Thermal colors are reflected by glass, so we can't measure the temperature of glass with a thermal meter. Likewise some metals just reflect light, so you can get a better heat measurement from them by putting some tape on them.
Likewise, a polished metal pot might be red hot but we tend to see reflected light in it, while a dirty or rough metal pot heated to the same temperature on the stove might appear to be glowing.
How much this "glow" appears to how much a perfect radiator glows is called emissivity. Shiny stuff has low emissivity. You can't measure the heat of a shiny aluminium pot, but you can measure the heat of an anodised aluminium pot with a thermal sensor.
Many materials reflect some thermal light to some extent. A car's hot exhaust may appear to be heating the ground to a thermal imager, but when the car moves, there is no real heat absorption from the ground - it's just reflecting some of that back at us.
Air does distort thermal colors just like it distorts visible light colors. It can bend thermal light too, but air is mostly transparent to thermal light. You can't see very hot air just like you can't see warm air. However some gases do absorb different colors of light and this is why some gases can be detected by thermal - it's just how a yellowish gas might appear yellow because it absorbs blue light, but it only appears that way because there is white light behind it and around it. If you used blue filters, it would be much easier to see. If you used red filters, you wouldn't see it at all. Same with thermal light.
People who use thermal sensors in their work need to learn how to avoid misreading temperatures because of all of these factors - and sometimes they still do make mistakes.
But generally, thermal colors are just like visible colors - only they have really long wavelengths in comparison - more than an order of magnitude longer.
David.