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Wind causes changes!
How do the four (4) different classes of mirage come about? Wind blows the heat waves as they rise, causing them to move vertically, obliquely, or laterally across the target. The amount that they vary from the vertical depends on the cross wind velocity, within limits from zero (0) to about twelve
(12) miles per hour (m.p.h.). As each mirage class is described, you must keep in mind that the description also applies to each category, and to a wind from right (3 o’clock) or from the left (9 o’clock).
The first class to be considered is the boiling mirage, with heat waves rising vertically as in
(Fig 3). A boiling mirage is present when there is no measurable wind, and when the wind
blowing from the shooter directly toward the target (6 o’clock wind) or from the target toward the
shooter (12 o’clock wind). This mirage class condition requires that the rear sight be lowered, to
center the shot group within the target. The amount of correction will depend on the category
(density) of the mirage and vision of the individual shooter. An accurate record of mirage category, mirage classification, wind, temperature, light, sling tension, etc., combined with knowing the rifle’s no wind zero and the zero for that distance, will enable the beginning shooter to refine the values given below to fit his particular location.
Although the boiling mirage presents little problems in itself, the near 6 o’clock wind or near 12 o’clock wind which appears as a boiling mirage to the inexperienced shooter can present quite a problem when it is fishtailing from about 11:30 to 12:30 or 5:30 to 6:30.
Wind correction is generally rated as a fraction of the wind’s 3:00 or 9:00 o’clock effect, with the 11:00, 1:00, 5:00, and 7:00 o’clock winds being half value. The 11:30, 12:30, 5:30, and 6:30 o’clock winds could be rated as eight value each, while the wind fishtailing from one side of 6-12 o’clock line to the other gives a combined effect of quarter (1/4) value. A small change in wind direction is very difficult to detect by feel, but secondary heat wave lines will begin to show up in the mirage and have the appearance of just leaning away from vertical, signaling the shooter that a change in wind direction has occurred.
A Slow Mirage
A slow mirage; the second classification, exist during a light air of one to 3 m.p.h. blowing from
3 o’clock or 9 o’clock. Heat waves will be slightly inclined as they move across the target from
7 o’clock to one o’clock of from 5 o’clock to 11 o’clock (Fig 4).
Target displacement with the slow mirage requires both an elevation and windage correction. As with the boiling mirage, the rear sight is lowered to correct for vertical component of the apparent displacement. The horizontal component requires a wind correction which will be into the wind, in addition to the correction for wind drift of the bullet.Since the heat waves are crossing from 7 o’clock to one (1) o’clock or 5 o’clock to 11 o’clock, they are making an angle of 30 degrees with the vertical, and their vertical and horizontal components.
can be computed in relation to the total apparent displacement:
p = total apparent displacement
v = vertical component of D
H = horizontal component of D
V = D cosine 30 degrees = 0.87 D, and
H = sine 30 degrees = 0.50 D
Experience has shown the total displacement due to heavy mirage amounts to be â‰ˆ 1 ½ minutes. The vertical correction due to heavy-slow mirage will be 1.31 or 1 ¼ minutes, and the horizontal correction will be 0.75 or 3/4 of a minute.