FLIR Navigator II Operator's Manual page 24

conditions however, atmospheric moisture and dust scatter can absorb some of
the radiated energy before it reaches the imager.
the overall thermal signal and shorten the range at which you can detect it.
e weather can impact more than just the range at which the Navigator II
can detect a specific object – it can also affect an entire scene's thermal contrast
and affect overall system performance.
Cloud cover affects the diurnal cycle in two ways:
First, cloud cover decreases the amount of solar radiation allowed to strike the
earth's surface, keeping days cooler and nights warmer.
Second, clouds form a layer of insulation over the earth, prevent heat from
being radiated back into space at night.
Like clouds, humidity tends to reduce contrast and wash out the effects of the
diurnal cycle. While humidity doesn't block out solar radiation during the day,
it does tend to keep nights warmer.
Rain acts differently because water tends to cool the surfaces it touches.
Remember that thermal imagers only detect differences in thermal energy
radiated from an object's surface; therefore, rain can markedly reduce a
scene's contrast. While rain reduces contrast between objects with no heat
source, it will allow objects with a heat source (like, people, animals, running
vehicles, some structures) to show up with even more contrast to their
now-cooler surroundings.
Conclusion
Tired? Confused? No problem. If you see something through your Navigator
II that looks suspicious, don't get too hung up on trying to figure out why it
looks the way it does. Just remember: if something is in your way, play it safe
and steer clear!
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e effect of this is to weaken