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Mirage was last updated: 20091228 2045

Mirage
A mirage is a reflection of the heat through layers of air at different
temperatures and densities as seen on a warm, bright day. With the
telescope, the shooter can see a mirage as long as there is a difference in
ground and air temperatures. Proper reading of the mirage enables the
shooter to estimate wind direction with a high degree of accuracy. The shooter
uses the spotting scope to read the mirage. Since the wind nearest to
midrange has the greatest effect on the bullet, he should try to determine
velocity at that point. He can determine the amount in one of two ways:

(1) Focus on an object at midrange, then place the telescope back on to the
target without readjusting the focus.

(2) Focus on the target, then back off the focus one-quarter turn
counterclockwise. This movement makes the target appear fuzzy, but
the mirage will be clear.

As observed through the telescope, the mirage appears to move with
the same velocity as the wind, except when blowing straight into or away
from the telescope. Then the mirage gives the appearance of moving straight
upward with no lateral movement. It is then called a boiling mirage. A boiling
mirage may also be seen when the wind is constantly changing direction; for
example, a full-value wind blowing from 9 to 3 o'clock suddenly changes
direction. The mirage will appear to stop moving from left to right and will
present a boiling appearance. When this image occurs, the inexperienced
observer may direct the shooter to fire with the "0" wind. As the shooter fires,
the wind begins blowing from 3 to 9 o'clock and causes the bullet to miss the
target. Therefore, firing in a "boil" can hamper shot placement. Unless there
is a no-value wind, the shooter must wait until the boil disappears. In general,
changes in the velocity of the wind, up to about 12 mph, can readily be
determined by observing the mirage. Beyond that speed, the movement of the
mirage is too fast for detection of minor changes. In general, when the waves
of the mirage are shallow, its velocity and resultant wind speed are fast.
Mirage will disappear at wind speeds above 15 mph.

The shooter can determine the true direction of the wind by traversing
the telescope until the heat waves appear to move straight up with no lateral
motion (a boiling mirage).

A mirage is particularly valuable in reading no-value winds. If the
mirage is boiling, the effective wind velocity is zero. If there is any lateral
movement of the mirage, it is necessary to make windage adjustments.

Another important effect of mirage is the light diffraction caused by
the uneven air densities, which are characteristic of heat waves. Depending
on atmospheric conditions, this diffraction will cause a displacement of the
target image in the direction of the movement of the mirage. Thus if a mirage
is moving from left to right, the target will appear to be slightly to the right of
its actual location. Since the shooter can only aim at the image received by his
eye, he will actually aim at a point that is offset slightly from the center of the
target. This error will be in addition to the displacement of the bullet caused
by the wind. Since the total effect of the visible mirage (effective wind plus
target displacement) will vary considerably with atmospheric conditions and
light intensity, it is impossible to predict the amount of error produced at any
given place and time. It is only through considerable experience in reading
mirage that the shooter will develop proficiency as a "wind doper."

Before firing, the shooter should check the mirage and make the
necessary sight adjustments or hold-off to compensate for any wind.
Immediately after firing, but before plotting the call in the scorebook, he
again checks the mirage. If any changes are noted, they must be considered
in relating the strike of the bullet to the call. The above procedure should be
used for each shot.