A recent "UFO" case turned out to be a plane leaving a contrail. This was initially a little confusing as the footage was shot with a thermal camera which showed the engines of the plane as incredibly dark spots of heat, but also showed the contrail in a similar dark color, which seemed to indicate that the trail was hot. And yet we know from the altitude the plane was flying when the trail formed (26,000 feet) that the temperature was well below freezing. We also know that some contrails can form from the mixing and cooling of superheated jet exhaust with the frigid air, and that this happens within seconds. So even if the plane was spaying something hot, it's impossible for it to stay hot more than a couple of seconds. So why does it look hot? The answer is not so much that the trail is hot (it's not, it's well below freezing), it's that blue sky appear incredibly cold in thermal imaging. I have a FLIR ONE thermal camera, which is limited to -4°F (-20°C) but still can illustrate this. Here's a blue sky with some clouds on a sunny day. The blue sky area (including areas with thin cloud) shows off-the-scale cold of < -4°F whereas the clouds show just above freezing at 33.6°F Notice how black the clouds are. Thermal cameras, like regular cameras, adjust the exposure to maximize detail in the scene. While the clouds are only just above freezing they show up as "hot" But in the "UFO" image we have a hot engine, and the trail is of similar color, how can this be? To investigate this I devised an experiment with a candle flame (1500°F) representing a jet engine, and some pieces of ice (3°F) representing the contrail. Against a normal background (33°F) the ice appears white, in stark contrast to the black of the candle flame. But against a blue sky background it appears dark grey. It's still not quite as dark as the candle flame, but that's largely because there are other objects in the scene (the candle itself, and the wooden support) and combined with the relative area of things in the photo this makes the ice seem less "hot". In the following video you can see the camera adjust, and the "temperature" of the ice trail change. Source: https://www.youtube.com/watch?v=w6zZ8iEC5GU The camera used in the UFO video seems even more aggressive in adjusting the exposure. In several places the regular clouds also appear to be the same temperature as the engines. But again it's just the exposure. The small size of the heat source (the engine) in the frame here plays a big part in the exposure. Here's another version of the candle/ice experiment, first close up: (Ignore the outlines added by the camera, the "shadow" is the actual IR image) The candle flame is black, but the ice is more grey. Now we move the camera back, so the flame is a lot smaller. Zoomed in: Now the flame and the ice are much more similar, with the ice being almost black (the outline here does not line up with the actual ice, which is the dark blob). Also notice the clouds are a lot darker - simply because the candle is smaller and so counts less towards the average exposure.