Map in the Sky - High Cloud Light Pillars in Xiamen, China

In the evening of May 10 2020, residents in Xiamen saw a strange, patterned cluster of light spots hovering over the city. Photographer YUAN Quan captured the phenomenon at its peak with his handphone.

© YUAN Quan, shown with permission. Equivalent focal length 39mm (from EXIF, unclear whether it's accurate).

Shortly after YUAN’s photo went public, LI Yahong and HUANG Tengyu from the China Sky Enthusiasts community found out that the light pattern, when flipped and rotated, perfectly matches downtown Xiamen’s map.

YUAN Quan's photo flipped and rotated 180°. Map from Apple Maps.

With the help of real night time images of Xiamen captured by Wuhan University’s Luojia-1A satellite (http://59.175.109.173:8888/index_en.html) and Chang Guang Satellite Technology's Jilin-1 satellite (https://mall.charmingglobe.com/Archive/), LI and HUANG’s finding was verified - we’re looking at a reflection of Xiamen’s city lights off the clouds.

*Special thanks to the Luojia-1A and Jilin-1 satellite teams for authorizing data usage.

Satellite images copyrighted to Jilin-1 and Luojia-1A satellite teams, shown with permission.

This phenomenon is most likely a high cloud light pillar event, similar to a previous case in Finland (http://www.thehalovault.blogspot.com/2017/03/map-in-sky.html) but on a much larger scale. The reflection captured in YUAN’s photo corresponds to an area of 200 square kilometers on the map. What’s even more interesting, the ground temperature in Xiamen during the event was 23°C -  a fairly warm night.

Due to the lack of background stars in YUAN and other people’s photos, it’s hard to do accurate cloud height calculations. A rough estimate by ZHANG Jiajie places the clouds between 6 and 7 kilometers above sea level. Sounding data from the night indicates the existence of a moist and wind-free layer in the 6 to 7 kilometer range so the height estimates may not be too far off.

Such warm night high cloud light pillar events are becoming more frequent in China - another two weaker ones were observed in the past few months in different cities. We believe the ubiquitous usage of decorative LED strips (which are very bright) on tall buildings, as well as the vast improvements in low-light photography on newer smartphones are two of the key factors leading to the surge in new observations.

Observing diamond dust halos at the Bílá ski centre

Superparhelia and sharp light pillars

Taken by Daniel Neumann

At the night between 18th and 19th January 2017 me and my friend Daniel decided to try our luck and head towards the Bílá ski centre (approx. 50 km away). Before we headed out, we checked the webcams located at the centre. All cameras showed sharp and tall light pillars extending out of every light source. Faint superparhelia were visible, too.

The crystal swarm was extending 3 km away from the centre itself. On the way there, upcoming cars spawned tall light pillars and visible parhelia.

Just as we arrived, the centre closed down and turned off the lights. We parked near the slope and noticed that it was completely overcast. The crystals were in the form of very thick fog that was hanging in the valley. More on the "weather side" later.

As we walked for a few hundred meters away from the centre, we stumbled upon a very bright lamp illuminating a local church. Only a sharp and tall light pillar was visible. But as we stayed for a little longer, the whole situation changed and superparhelia started to appear. At first they were hardly noticeable, but with each minute they were getting brighter and brighter.

We decided to try our not-so-much bright "spotlight" to see if anything interesting appeared. And yes it did. By shining the lamp towards the snowy surface, true divergence took form and divergent sub-parhelia appeared. They were eerily 3D, hanging in the air. Along with them, an extremely bright and tall light pillar was observed, too.

A multitude of light pillars

Tall light pillars and superparhelia

Circumzenithal arc along with light pillar and superparhelia 

Light pillar, sub-parhelia, parhelia, and parhelic circle

Now let's discuss the weather situation a little. The temperature reached below -16°C which was lower than other temperatures observed at meteorological stations (due to the topography). The ski centre was at the time actually located at the edge of a low hanging stratus that was just barely touching the mountain ridge. The snow guns then probably nucleated its lower layer and the crystals started to precipitate out. Below is a georeferenced image from the Suomi-NPP satellite along with the location of the centre. The microphysical product along with a high resolution of the image shows how the edge of a low stratus (yellow colour) extends towards our location. Now bare in mind that this image was taken approx. 1 hour after we left, so the stratus actually retreated a little by that time. There are two "branches" of a low stratus visible at the image. The right branch follows the exact location of a local dam called Šance. The left branch follows a river called Čeladenka.

(c) Suomi-NPP 24h microphysical product

What stuns me about this location is its abundance of diamond dust occurrences. Almost every time the snow guns are operating, a crystal swarm is created within minutes and light pillars are always visible on the webcam images. I have a theory about why that happens:

• Microclimate - the topography is in the form of a depressed valley, which allows for a cool and moist air to accumulate at the bottom. 
• Moisture - the river that flows through the village could possibly be a substantial source of moisture 
• Snow gun additives - if I have learned well, it is the additives in the snow guns that serve as nuclei on which the crystal formation takes place. In that case, they are probably using a specific additive that makes the crystal formation so abundant. 

The local relief of Bílá

Shaded relief + topographic map
(c) ČÚZK

METAR analysis shows a high pressure centre located above Czechia

(c) METAR

Temperature distribution

Light pillars in Finland

These light pillars were seen on the night of 8/9 March 2006 in Lappeenranta. The spot-like appearance is because the majority of the plate like ice crystals reflecting the lamp light are concentrated in a layer above the ground. Photo was taken by Tomppa Jurvanen. More of his scenes from the same display: 1, 2, 3

By Marko Riikonen