Tähtitieteellinen yhdistys Ursa

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Appendix

Separation between the 46° halo
and the 46° supralateral arc

By Jarmo Moilanen

Perhaps the most frequently appearing problem in halo identification is the separation between the 46° supralateral arc and the 46° halo. One has to remember that both of these can appear solitarily or together. The 46° supralateral arc is a surprisingly common halo form, but often it is misidentified as a 46° halo... or if both are present in the sky, it is erroneously neglected because it often does not pop out clear enough to an observer who perhaps isn’t used to expect its presence, nor familia or with the criteria that lead to its confirmation in field situations. The identification is easy only in when both forms are well developed and hence clearly profiled as separate arcs. However this tends to happen only in the big displays.

The halo in the 46° region is a 46° halo when:

• 1. The sun is higher than 32° (Because both the circumzenithal arc and the 46° supralateral arc cannot be formed when the sun is higher than 32°).
• 2. There is a clear gap between the halo and the circumzenithal arc. This gap is clearly evident for solar elevations between 0°-15° or 27-32°.
• 3. There is no 22° upper tangent arc in the display or it is clearly weaker than the 22° halo.
• 4. The sun is situated lower than 15° AND the brightest part of the seen 46° area phenomenon is in its top part, i.e., not on the sides.
• 5. The intensity of the seen halo in the 46° are is evenly distributed and its length as defined with its central angle (angle from the sun point) is over 160° and its form yet is a clear ring-shape, or it even continues below the parhelic circle level.

If lengthy brightenings are found on the lateral sides for solar elevations under 15° or a similar lenghty brightening accompanies on the top when the sun is at heights 15°-27°, then also a 46° supralateral arc is present in the display (a clearly more colourful sequence in the right area of the ring is also an identification tool, as good as lengthy brightening).

The halo in the 46° region is a 46° supralateral arc when:

• 6. The 22° upper tangent arc is very clear and the 22° halo is missing or both are of fully comparable brightness.
• 7. The halo seen in the 46° region is colourful. Clear green and blue are obvious identification tools since the genuine 46° halo will not show them.
• 8. The highest point of the observed halo touches the circumzenithal arc AND the solar altitude is 0-15° or 27°-32°.
• 9. Halo is symmetrically visible as two lateral arcs AND the solar altitude is less than 15°.

When the sun is below 15° the 46° supralateral arc touches the 46° halo at its both sides. This happens in such a way that when the sun is in the horizon (0°) the points of tangency are in the level of the parhelic circle, but move to higher positions along the 46° ring theoretical curve when the sun rises higher in the sky.

The identification is least easy when two annoying factors are combined: A. The solar elevation happens to be 15°-27° and B. Faint or mediocre downward curving arc is found in the uppermost part of the 46° region.

With the above conditions filled the two arcs elapse each other perfectly. Two distinct halos can well be present, but we see only one. And naturally there is no help of the circumzenithal arc either as an extra identification tool since at these elevations both arcs tangent it in a perfectly similar way. Often the identification will then have to be made by studying the relative intensities of the 22° halo and the 22° upper tangent arc (as described in situations no 3 and 6 above). If does not seem to ng certainty one can try to evaluate the colourfulness of the arc (as described in situation no 7). Sometimes one just can not reach the satisfactory identification.

The above discussion does not present the entire picture. The 46° infralateral arcs are not always easy to identify either. Especially for very high solar elevations (solar altitudes higher than 58°) the circumhorizontal arc becomes another error source when trying to identify between it and the 46° infralateral arc.

Idenfication map for 46° halo and 46° supralateral arc