(6) Winds have different effects on the thermal presentations of targets. All small
thermal emitters on a snowy surface may be erased by a wind of 1 or 2 miles per hour (mph),
while targets such as roads or manmade objects may not be affected by winds as high as 10
Seasons of the year also influence IR surveillance results and the appearance of the
data on the film.
(1) In the summer the terrain generally is subjected to good weather conditions
and terrain features generally show up well. Vegetation is more abundant and ground
temperatures are higher, which is evidenced by darker images.
(2) During the winter ground temperatures are much lower and overcast days are
more prevalent and most of the summer vegetation has disappeared. A large body of water
that appeared slightly cooler (lighter) than its surroundings during a summer day now generally
appears warmer (darker) than its surroundings. Snow covered terrain also affects IR
surveillance results. Larger temperature differences between heated materials and the
background occur. Because of this, much larger contrasts in received IR energy may be
expected. Careful adjustments of gain, contrast, and level must be made in the IR
surveillance system during such a time to prevent exaggerated effects. Generally, In snow
conditions, manmade heated objects are easy to detect, but general terrain features may be
obscured because snow coverage tends to give all materials the same emissivity level of the
(3) Spring and fall are simply transition periods of summer and winter, and they
follow the trends of one or the other depending on the general weather conditions. Spring
tends to be an extension of winter, while fall tends to be an extension of summer in the
collection of IR data.
Time of day. Care must be taken during interpretation as to the time of day,
especially during the transition or crossover time.
Filters are used to exclude certain regions of the electromagnetic spectrum or to
pass a specific region.
Equipment control settings. The control settings of the components of the IR
surveillance system also affect tones:
(1) Gain. A control which determines the amount of signal made available for
recording and display control purposes.
(2) Contrasts and level. A fine gain control that regulates the intensity of the
printed data representing the received energy.