The position of the observer, marker, and reflection affect the visibility of the thermal markers. An ideal situation appears here. There is a clear sky, reflecting its cold temperature off the numbers on the roof of the vehicle.
However the image on the left, a building’s temperature is being reflected. With the building in the way, the letters will not look as cold, which will make them less visible.
In the image on the right the helicopter has adjusted his position so that the sky temperature is reflected again. Trees and buildings are the most likely objects to cause this problem.
Another position issue involves the Sun. When the observer happens to be positioned so the Sun’s temperature is reflected, the markers will appear very hot. In this case the roof may appear hot as well and the markers may get washed out. On the other hand the thermal markers may be very visible, appearing hot on a cooler surface. Moving slightly will fix this problem.
The sky temperature is LOWER when the sky is clear as shown here. The lower sky temperature reflects on the Vehicle Markers, resulting in a greater ΔT (temperature difference) between the marker and the roof of the vehicle. The greater ΔT makes the thermal markers more visible on the roof.
Dark, thick clouds INCREASE the sky temperature. The higher sky temperature reflects on the Vehicle Markers, resulting in a lower ΔT between the marker and the roof of the vehicle. The lower ΔT makes the markers less visible on the roof.