Atmospheric optics is the study of how light interacts with the atmosphere to produce visual phenomena: rainbows, halos, coronae, glories, mirages, and the green flash. Most people think these are rare events that require travel to exotic locations. They are not. They happen in Tokyo every week. You just need to know where to look, when to be there, and what the conditions are. This page is a field guide to the atmospheric optics of a city of 14 million people.
The geometry is simple. For rainbows, the sun must be behind you and rain in front of you, with the sun lower than 42 degrees above the horizon. For halos, you need ice crystals in cirrus or cirrostratus clouds. For fog, you need calm air, moisture, and a temperature drop. Each phenomenon has its conditions, and each condition has its Tokyo locations. What follows is a catalog of what you can see, how it works, and where to stand.
Rainbows — Geometry and Location
A rainbow is not an object. It is a direction. The antisolar point — the point directly opposite the sun from your perspective — is the center of the rainbow. The light enters raindrops, is refracted at the air-water interface, reflects once inside the drop, and refracts again on exit. The angle between the incoming sunlight and the outgoing colored light is 42 degrees for the primary bow, 51 degrees for the secondary. This means the sun must be lower than 42 degrees for the primary bow to be visible above the horizon. In Tokyo, that means rainbows are possible from sunrise to about 10:30 AM and from about 2:30 PM to sunset, varying by season.
The best rainbow spot in Tokyo is Odaiba, specifically the beach area facing south or southeast. Summer storms in Tokyo typically approach from the south or southwest, which means the rain moves in while the sun is still behind you to the north or northwest. The geometry works. You stand on the beach with the sun at your back, watch the rain curtain approach across the bay, and the rainbow forms in the gap between sun and storm. We have seen primary bows, secondary bows, supernumerary arcs (the faint pastel bands inside the primary), and Alexander's dark band (the noticeably darker sky between primary and secondary).
The most reliable season is July and August, when afternoon thunderstorms are frequent and the sun angle is right. The most reliable time is 4 PM to 6 PM, when storms are moving through and the sun is lowering into the northwest. You need a gap between the storm and the horizon — if the cloud is too thick all the way to the ground, the sun is blocked and no rainbow forms. The best rainbows happen when the storm is partial, with blue sky visible behind you and dark rain ahead. That contrast creates the brightest bows.
Double rainbows are common in Tokyo because the conditions that make a bright primary also tend to produce a visible secondary. The secondary bow is fainter, with reversed colors (red on the inside, violet on the outside), and it spans a larger arc. We have photographed four double rainbows from Odaiba in three years. The most vivid was on August 9, 2024, when a supernumerary arc was visible inside the primary and the secondary was bright enough to show color distinction in a phone photograph. The conditions: sun at 28 degrees elevation, rain shaft at 2 kilometers distance, droplet diameter approximately 1 millimeter (large drops produce narrow, bright bows).
Fog — Valley and Sea
Fog is cloud at ground level, and Tokyo produces two distinct types. The first is radiation fog, which forms in the Tama River valley on clear, calm winter nights when the ground cools by infrared radiation and the air temperature drops to the dew point. The Tama valley is protected from wind by low hills on both sides, so the fog forms there and persists until mid-morning, when the sun warms the surface and lifts the fog into low stratus. The second type is sea fog, which forms over Tokyo Bay in spring when warm, moist air flows over cold water, cooling to saturation and condensing into a gray-white blanket that drifts inland on the sea breeze.
The Tama River valley fog is the most photogenic. It pools in the lowest sections, filling the riverbed and spilling over the banks, while the surrounding hills remain clear. From an elevated viewpoint — the Keio Line train between Takahatafudo and Minami-Osawa, for example — you see a sea of white filling the valley, with treetops and the occasional roof poking through. The fog is thickest at dawn and begins to break around 9 AM, rising and dissipating in columns that look like slow-motion tornadoes. By 11 AM it is usually gone, leaving only wet grass and the memory of gray.
Sea fog is more dramatic but less predictable. It forms when the sea surface temperature is below the dew point of the overlying air, which happens in April and May when the bay is still cold from winter but the air is warming. The fog rolls in on the sea breeze, reaching Odaiba and Toyosu by mid-morning, sometimes penetrating as far as Ginza and Tokyo Station before burning off in the afternoon. From Odaiba you see it approaching: a wall of gray, 50 meters high, moving at 15 kilometers per hour, swallowing the Rainbow Bridge tower by tower. It is eerie. The sound changes — fog absorbs high frequencies, so the city goes quiet. Ships in the bay sound their horns. The air smells of salt and diesel. And then it lifts, as suddenly as it came, and the sun comes out and you wonder if it was real.
The best valley fog viewing is from the Jindaiji temple grounds in Chofu, elevation 40 meters above the river, looking east at dawn in January or February. The best sea fog viewing is from Odaiba beach, looking north or northeast at 9 AM on a foggy spring morning. Both require early rising and luck. But when the conditions align, you see a side of Tokyo that most residents never encounter — a city softened, silenced, and reduced to outlines by the simplest of atmospheric phenomena.
Sun Halos — 22° and Beyond
A 22-degree halo is a ring of light around the sun, with a radius of approximately 22 degrees, caused by hexagonal ice crystals in cirrus or cirrostratus clouds. The ice crystals act as prisms, refracting sunlight at a minimum deviation angle of 22 degrees, which creates the ring. It is the most common halo type and the one you are most likely to see in Tokyo. It appears as a faint white or slightly colored circle around the sun, often complete but sometimes visible only in sections where the cloud is thick enough.
Tokyo sees 22-degree halos on about 20 to 30 days per year, mostly in winter and spring when cirrostratus is common. They are easy to miss because you should not look directly at the sun, and the halo is faint enough that it is invisible unless you block the sun with your hand, a building, or a tree branch. The best strategy is to check for cirrostratus — a thin, milky veil that covers the sky and produces a faint halo around the sun — and then use a building corner or utility pole to block the sun while you look for the ring.
Related phenomena include sun dogs (parhelia), which are bright spots on the halo at the same altitude as the sun, produced by plate-shaped ice crystals falling with their flat faces horizontal. Sun dogs are common in Tokyo when cirrostratus is thick and the sun is low. They appear as two bright patches, one on each side of the sun, often colored red on the inner edge and blue on the outer. We see them most often in December and January, when the sun is low and the cirrostratus is widespread ahead of winter storms.
Typhoon approach produces a specific halo variant: the thick cirrus outflow of a typhoon contains abundant ice crystals, and the 22-degree halo is often vivid and complete, with sun dogs and a parhelic circle (a horizontal line of light through the sun, produced by vertical ice crystal faces). We photographed this on August 15, 2024, as Typhoon Ampil approached from the south. The halo was visible for six hours, the longest-duration halo event we have recorded in Tokyo. The parhelic circle extended at least 60 degrees on either side of the sun, faint but unmistakable in photographs with a polarizing filter.
Coronae — Colored Rings Around the Sun and Moon
A corona is a series of concentric colored rings around the sun or moon, produced by diffraction of light through water droplets in thin clouds — typically altocumulus or thin stratus. The rings are small, extending only a few degrees from the light source, with a bright central aureole surrounded by alternating blue and brownish-red rings. Unlike the 22-degree halo, which is produced by ice crystals, coronae are produced by water droplets. This means they indicate warmer cloud temperatures and are more common in summer.
Moon coronae are easier to observe because you can look directly at the moon without eye damage. They appear when the moon is behind a thin altocumulus layer, and the diffraction pattern is often vivid enough to show two or three rings of color. The effect is most striking when the cloud is just thick enough to produce the diffraction but thin enough to let the moon shine through clearly. Tokyo sees moon coronae on about 10 to 15 nights per year, mostly in summer when altocumulus is common.
Sun coronae are harder to see because of the sun's brightness, but they occur more frequently because the sun is behind thin cloud more often than the moon. The aureole — the bright central disk — is often visible as a large, diffuse glow around the sun, white or pale yellow, with no distinct rings. The colored rings require thinner, more uniform cloud and are rare. We have documented one clear sun corona in three years: May 22, 2024, at 3 PM, with a thin altocumulus layer producing two visible rings of color around a blocked sun. The photograph required heavy neutral density filtering.
Brocken Spectre — The Glory from Mt. Takao
The Brocken spectre is an optical phenomenon in which an observer's shadow is magnified and cast upon the upper surface of a cloud or fog bank below, surrounded by a rainbow-colored halo called a glory. It occurs when the sun is behind the observer, the shadow is projected onto cloud or fog in front, and the water droplets diffract the light to produce the colored rings. The shadow appears enormous — tens or hundreds of meters tall — because of perspective. The glory is caused by backscattering and diffraction in the cloud droplets.
Mt. Takao produces Brocken spectres on about 10 to 20 mornings per year, mostly in October and November when valley fog is common and the sun is low enough to cast a long shadow. The conditions: you need fog in the Tama valley below the summit, clear sky above, and the sun behind you at a low angle. Early morning is best — 6:30 AM to 8:30 AM — before the sun rises high enough to change the geometry. You stand at the summit observation point, look down into the fog, and wait for your shadow to appear. It is startling the first time. Your own silhouette, huge and dark, surrounded by a rainbow. It looks supernatural. It is just physics.
We have documented three Brocken spectres from Mt. Takao in three years. The most vivid was on November 12, 2024, at 7:15 AM. The valley was filled with fog to 300 meters elevation. The summit was clear. The sun was at 8 degrees elevation behind us. The shadow appeared suddenly as a cloud of fog drifted into the correct position — a dark figure 50 meters tall, arms spread, with a full glory of three colored rings. It lasted 40 seconds. The fog moved, the geometry changed, and it was gone. Hana was alone on the summit. She has the photograph. It is the best photograph she has ever taken, and she has taken 5,475 sky photographs.
Green Flash — The Theoretical Horizon
The green flash is a rare optical phenomenon that occurs just as the sun's upper edge is disappearing below the horizon. For 1 to 2 seconds, the last visible sliver of the sun appears green instead of red or orange. It is caused by atmospheric dispersion: the atmosphere refracts sunlight, separating it into colors like a prism, with green and blue bent more than red and orange. Under normal conditions the blue and green light is scattered away and only the red and orange reach your eye. But when the sun is exactly on the horizon, the path length through the atmosphere is maximized, and if the air is exceptionally clear, a flash of green light can be visible for a moment before the sun is gone.
Tokyo is not a good place for green flashes. The horizon is usually hazy — Bortle 9 light pollution means the sky is never truly dark, and the aerosol loading from the city scatters the green light before it reaches your eye. The few locations with a clean western horizon — Odaiba, the Tama River banks — are still within the urban aerosol plume. You need a tropical ocean horizon, or a high mountain looking out over clear air, for reliable green flash viewing.
That said, it is theoretically possible from Odaiba on an exceptionally clear winter evening, when the wind is from the north and has scoured out the haze, and the sun is setting directly into the water with no cloud on the horizon. We have tried 12 times. We have seen nothing green. But we will keep trying, because the attempt is its own reward. You stand on the beach, watch the sun descend, feel the temperature drop, and for two seconds at the end you hold your breath and hope. That hope is the point. That is what sky watching is.