Primary rainbow or 1st order rainbow, is an arc that appears in the colors of the spectrum and is created when light is refracted and reflected in raindrops in the sky. Its outer color is red and its inner color is blue / purple.
The main rainbow is visible on the opposite side of the sky from the light source that causes it. A typical light source is the Sun, but extremely rarely the Moon can also cause a rainbows. The lunar rainbows have an own category in Skywarden.
One or more supernumerary bows may occur inside the main rainbow.
The fully developed main rainbow is at the point of the shadow of the observer's head, i.e. the ring surrounding the so-called antisolar point. Usually the phenomenon is interrupted by the horizon, but from the air, the rainbow can be seen as a perfect circle continuing below the horizon.
The rainbow gradually turns into a fogbow as the droplet size decreases. The smaller the droplets are, the smaller the rainbow is in size. Rainbows caused by the smaller droplets tend also to be thicker and the white color begins to dominate them. Usually, rainbow and fogbow are clearly separate phenomena, but sometimes intermediate forms can occur.
Colors can be used as the primary guideline for distinguishing different bow-like phenomena. If the bow shows colors of the spectrum and is not white, it is a rainbow. If, on the other hand, the arc is mainly dominated by white and some of the colors in the spectrum are missing, it is a fogbow.
Anomalies can sometimes be seen in the appearance of rainbows. They are reported in Skywarden by clicking the box for an anomalous rainbow. One anomaly is a point of discontinuity where the radius of the rainbow changes. The second is the division of the rainbow into two separate arcs.
Rainbows of four different orders have been observed. A secondary rainbow, i.e. a 2nd order rainbow, occurs often outside the primary rainbow. 3rd and 4th order rainbows are very rare and occur in the direction of the light source (eq. on both sides of the Sun). Each order of rainbows in Skywarden has its own phenomenon identification tag/button.
Primary and secondary rainbow. Image by Eetu Saarti.
Primary (on the left) and secondary rainbow (on the right). Image by Vesa Vauhkonen.
The primary rainbow is often seen as just a band of colors close to the horizon. Photo by Eetu Saarti.
When the Sun is close to the horizon, the colors of the rainbow turn red. Primary rainbow can be seen on the left side of the picture along with a faint secondary bow on the right. Image by Vesa Vauhkonen.
Primary rainbow from plane. Also a faint secondary bow is visible. Photo by Jouni Finnilä.
Primary rainbow with supernumerary bows within the main arc. Photo by Arja-Sisko Airila.
Strange weather condition with the primary rainbow in the horizon (Sun is at the height of 42 degrees). A faint secondary rainbow is also visible in the sky. Image by Jouni Matula.
In dense rain, a rainbow may appear against nearby buildings or the forest. Here the main rainbow stands out weakly against the forest behind the apartment building. Photo by Matias Takala.
This slightly divided main rainbow (at the top) is probably a sign of flattened water droplets. Image by Jaakko Kuivanen