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As the 3 or 9 o’clock wind rises to a light breeze of 4 to 7 miles per hour, the heat waves will
make a greater angle with the vertical and will have the appearance of crossing from 8 o’clock
to 2 o’clock or 4 o’clock to 10 o’clock. This gives the third classification, is a medium mirage
The medium mirage will also require both elevation and windage corrections of different
amounts than for slow mirage. Changing the angle from 30 degrees to 60 degrees because of
the increased inclination, the corrections become:
V = D cosine 60 degrees = 0.50 D
H = D sine 60 degrees = 0.87 D
Going back to the total displacement of 1 ½ minutes caused by the heavy mirage, the vertical correction for a heavy-medium mirage will be 0.75 of 3/4 minute and the horizontal correction will be 1.31 or 1 ¼ minutes. Again the rear sight is lowered and the windage correction is added to that required for wind drift.
Notice the mirage effect with the same heavy mirage during a wind increasing from one to 3 m.p.h. (slow mirage) to 4 to 7 m.p.h. wind is applied, the rear sight must be lowered 1 ¼ minutes and 3/4 minute windage added for horizontal component of the mirage. During the string, the mirage picture changes from heavy-slow to heavy-medium which signals the increase in wind velocity. The necessary wind correction is made and an additional ½ minute applied to give a total horizontal correction of 1/1/4 minutes for mirage. Adjusting for the vertical component of the mirage is
accomplished in a similar manner. The sight was originally lowered 1 ¼ minutes and the mirage picture now indicates that only 3/4 minute is needed, so the sight is raised the difference, which is ½ minute. The beginner shooter should practice mentally so that he is able to make the necessary changes without becoming confused.
A fast mirage, the fourth classification, will be visible when the wind velocity reaches the
gentle breeze stage of 8 to 12 M.P.H. from 3 o’clock or 9 o’clock. The heat waves will move
horizontally across the face of the target (Fig.6) and the apparent target displacement will be
only horizontal. The fast mirage thus requires only additional windage correction. Wind
velocities over 12 M.P.H. are indicated by the heat waves having the appearance of being
stretched straight, and letups can be detected by slight waves beginning to appear on the
Boiling with lateral movement
The last and most difficult mirage picture to identify is the “boiling mirage with lateral left or
right secondary heat waves” ( Fig.7), which is an indication of the wind coming in from
between 9 and 12, 9 and 6, 3 and 12, or 3 and 6 o’clock. An easy recorded notation is the word “boiling” with an arrow drawn through it to indicate the direction of the secondary heat waves. The primary and secondary heat waves are caused by the 2 components of the wind, a 6-12 o’clock and a 3-9 o’clock component, the latter being the more important in detecting wind shifts. The secondary heat wave lines should be read in the same manner as the previously mentioned classes, with the secondary lines indicating that both vertical and horizontal corrections will be necessary to compensate accurately for the mirage displacement. The new shooter should be on the lookout for the appearance of these secondary waves while firing in the other mirage classes.
Mirage can also be used to determine the true wind direction. Traverse the scope until a “boil” is seen, then the wind is parallel to the axis of the scope. Turning the scope through 90 degrees will be equivalent to observing a 3 or 9 o’clock wind. The mirage classification gives the wind velocity up to about 12 m.p.h.
The beginning shooter can possibly keep abreast of the mirage changes, by plotting them in a book. You can draw one or 2 wavy lines through the circle to indicate the mirage classification. A category change can easily be shown by putting the initial L, I, or H in the circle. However, do not spend to much valuable time at this. It should not require more than 3 to 4 seconds to note a mirage in this manner and the target will, no doubt be gone during that time. If more time is being consumed, additional practice is necessary and several trips into the country, between hunts, with a spotting scope or binos can be beneficial.