Shadow Bands
A phenomenon sometimes seen briefly several minutes before and
after totality as rapidly shimmering, irregular bands of shadow on the ground
and walls. (A white surface helps make them more visible since shadow bands
have low contrast.)
Bands may be a few centimeters (1/2 to 2 inches) apart,
up to a meter (a few feet) apart and travel a few meters per second (about 10
feet per second). The cause of shadow bands is probably
the refraction or distortion of light from the thin solar crescent by the
Earth's atmosphere. Even veteran eclipse observers do not always see them. They
are also difficult to photograph. (Use fast film and a fast lens with short
exposure times—1/250 second or less.) Most intense bands seem to occur over
dry, warm areas rather than from coastal and shipboard areas. (Water surfaces
may help to thermally stabilize air layers that would otherwise cause the
bands.)
Appearance.
Refer
to Figure 3.3. Shadow bands vary considerably in both width and separation, but
range most frequently between 2 and 5 cm (0.75 and 2 in) in width and are
separated from one another by 5 to 25 cm (2 to 10 in).
Their
direction of motion across the ground seems to depend upon where an observer is
located along the eclipse path and whether the bands are observed before or
after totality. Their velocities vary most often between 1.5 and 3 m (5 and 10
ft) per second.
In short :
- The flying shadows consist of undulating, moving bands or shades
of light and shadow of some centimetres to some decimetres in width. The
intervals between light and dark are alike and holes of some meters can come
between the pattern.
- The bands are perpendicular to the edge of the moon's shadow
further away from totality, and parallel with the edge of the shadow near
totality. The band orientation runs parallel with the sun's crescent. The band
orientation is the same near the central line before and after totality. Near
the extremities of the totality zone, the band orientation has shifted up to 90
degrees.
- The turbulence, which is mainly the cause of the shadow bands,
is under 2 kilometres altitude. Turbulence in the tropopause
would have no impact on the shadow bands 2 to 3 seconds away from totality.
- Although they sometimes seem stationary, they can reach a speed
of some meters per second. One shadow band can appear and disappear so fast
that it moves individually over more than one meter before it dissolves.
- Shadow bands are visible from some seconds to various minutes
before and after totality. During partial and annular solar eclipses, this
could even be more. The bands themselves have the biggest contrast when there
is space between them, and the least when the sun's crescent is at it's smallest.
- The contrast between light and shadow is bigger when wavelengths
are shorter. So the shadow bands must be seen the best in blue light and the
worst in red light. The band gap on scale (band spacing) is the square root of
the wavelength near totality.
- Longer solar eclipses would have shadow bands with more contrast
and linearity.
-
Because the turbulence and the winds are not predictable, the explicit shadow
bands can not be predicted. But with the meteorology they can be better
understood.
Shadow Bands - Sicily 1870