Figure 2-9. Horizontal Refraction Due to Air Currents From Ravines
h. Another example of serious refraction trouble is a valley through an open plain, bordered by
bluffs. When it is impossible to avoid such a condition, place the stations on the tablelands as far away
as possible (Figure 2-10). Lines between the headlands and parallel to the valley will give the least
accurate results and should be avoided at the expense of additional signal building and stations.
Figure 2-10. Horizontal Refraction Due to Air Currents Down a Slope or Hill
i. Horizontal refraction is much greater in barren or open country than in built-up areas. The
effects of air currents are small over heavy timber. The reconnaissance chief may ignore the refraction
effects in such areas.
j. On calm nights, the atmosphere tends to stratify into layers of different temperatures and
densities, and the coefficient of vertical refraction becomes larger than normal, lifting distant objects
above the horizontal. This abnormal nighttime refraction may vary between 3 to 5 minutes. A
refraction line is a line that is normally obstructed but becomes visible at night. Directions observed
over refraction lines are not unduly affected horizontally, and their use is sometimes permissible, though
as a last resort. Stormy or windy nights may break up the air strata, and the abnormal vertical refraction
may disappear entirely. Over sloping ground, the air strata may be inclined, in which case there will be
a horizontal component of the refraction. Lines should always be kept clear of bare ground.
2-35
EN0593
Previous Page
