Festival Lighting and Skyglow: Measuring Diwali's Impact on India's Night Sky

When an entire subcontinent turns on decorative lamps and ignites aerial bursts on the darkest night of the Hindu calendar, our skyglow spikes by orders of magnitude — here is what the physics and satellite sensors tell us.

Step out onto a rooftop in West Delhi, suburban Bengaluru, or north Kolkata on the night of Amavasya in the month of Kartika. Astronomical wisdom tells you that this should be the darkest night of the lunar month. The moon sits precisely between Earth and the Sun; its illuminated face points away from us, leaving behind a sky pristine enough to reveal the subtle dust lanes of the Milky Way.

Instead, the sky overhead glows a dull, luminous saffron-grey. Shadows fall across terrace floors without a single celestial light source in view. The horizon hums with diffuse, scattered radiation.

For five days every autumn, India stages what is arguably the largest coordinated illumination event on the planet. Millions of households string multi-hued LED fairy strips across balconies, light earthen diyas, wash temple facades in architectural floods, and launch aerial pyrotechnics that erupt hundreds of metres into the boundary layer.

At SkyQI, our central goal is tracking artificial light at night (ALAN). While routine light pollution creeps upward across decades, Diwali offers something unique to atmospheric physicists and citizen observers alike: an enormous, pulsed experiment in temporary skyglow.

This post examines what happens to India's night skies when festival lighting peaks. We look at the physics of how festival illuminations escape into the atmosphere, compare the optical signatures of traditional oil lamps versus high-intensity LEDs, dissect the aerosol trap that compounds the glow across the Indo-Gangetic plain, and review what satellite radiometers see from 800 kilometres overhead.

Amavasya: The Inversion of the Darkest Night

To understand the optical transformation wrought by Diwali, one must first look at the calendar. The festival’s primary night is deliberately anchored to the new moon of the Hindu lunisolar month of Kartika.

For millennia, the timing made complete aesthetic and practical sense. The Vedic astronomers whose traditions inform works like the Surya Siddhanta mapped planetary motions and lunar stations (nakshatras) across skies unmarred by electric infrastructure. Celebrating the return of light on the darkest night of the month yielded the highest possible visual contrast. A single mustard-oil flame against an unlit village horizon in ancient Ayodhya or medieval Ujjain stood out brilliantly against the dark ground, framed by a pristine sky stretching down to magnitude 6.5.

Today, modern urbanisation has inverted this relationship. On the night when the heavens should naturally register a Sky Quality Meter reading of 21.6 magnitudes per square arcsecond or darker, metropolitan skies across northern and central India drop down into Bortle Class 8 and 9 conditions.

The natural darkness of the new moon provides the ultimate dark canvas. Because there is no natural lunar skyglow to mask the change, every extra lumen pumped upward by city infrastructure becomes immediately detectable. If Diwali fell during a full moon, the festival’s optical footprint would blur into natural lunar backscatter. On Amavasya, however, every upward-directed photon belongs squarely to human celebration.

The Photon Budget: Diyas, Incandescent Strings, and Solid-State LEDs

Not all light produces equivalent skyglow. The impact of Diwali lighting depends heavily on the spectrum, shielding, and total lumen output of the light sources being deployed. Over the last two decades, India’s domestic festival lighting has experienced a massive technology transition.

To see how the optical budget has changed, consider the three primary light sources present during the festival:

The physics of atmospheric scattering explains why this shift matters. When light travels upward into the atmosphere, it scatters off nitrogen and oxygen molecules via Rayleigh scattering. The intensity of Rayleigh scattering is inversely proportional to the fourth power of the wavelength.

Short-wavelength blue light around 450 nanometres scatters nearly six times more aggressively than long-wavelength red or amber light around 650 nanometres.

A traditional diya produces a tiny amount of light, nearly all of it concentrated in the deep red, amber, and infrared parts of the spectrum. Even if millions of oil lamps burn simultaneously across Varanasi's ghats or Jaipur's old city, their collective contribution to atmospheric skyglow remains modest. The photons are red, their scattering cross-section is low, and the clay rims direct light upward only at broad angles.

The ubiquitous decorative LED string tells a fundamentally different story. Over the past fifteen years, inexpensive solid-state decorative lighting imported and manufactured at massive scale has replaced both clay lamps and low-efficiency incandescent bulbs. These strings frequently employ white LEDs driven by a sharp, highly energetic blue pump peak at roughly 455 nanometres, or single-colour royal blue, violet, and bright green emitters.

Furthermore, these strings are strung vertically along apartment balconies, draped along compound walls, or hung across the faces of high-rise developments in Gurgaon, Whitefield, or New Town Kolkata. Unlike streetlights, which often possess cut-off shielding designed to direct lumens downward toward the asphalt, festive facade lighting emits photons sideways and directly into the upper hemisphere with zero shielding.

When you combine a 500 percent increase in raw luminous flux with unshielded horizontal orientation and a spectrum heavily weighted toward the blue scattering peak, the total skyglow potential rises dramatically. The modern festive cityscape acts as a gigantic, diffuse uplight directed straight into the lower atmosphere.

The Aerosol Multiplier: Why North India's Skies Trap Light

If Diwali occurred during late May under dry, turbulent atmospheric conditions, or in July when monsoon rains scrub the troposphere clean, the skyglow spikes would dissipate relatively quickly. But the festival falls in mid-to-late autumn, during a meteorological phase transition that creates the ideal conditions for severe light entrapment.

Across the Indo-Gangetic plain—stretching from Punjab through Haryana, Delhi, Uttar Pradesh, and Bihar—late October and November bring post-monsoon atmospheric stagnation. The ambient temperature drops, winds drop to calm or negligible speeds, and an inversion layer forms within a few hundred metres of the surface.

This thermal blanket traps local particulate matter: vehicle exhaust, industrial emissions, construction dust, and smoke from seasonal agricultural residue burning across the agricultural belt.

The skyglow equation shifts fundamentally once high aerosol optical depth (AOD) enters the picture. When particles exceed the wavelength of light—such as airborne soot, nitrates, and sulphates measuring 2.5 microns across—Rayleigh scattering is superseded by Mie scattering.

Mie scattering does not favour blue wavelengths as steeply as Rayleigh scattering does, but it possesses another characteristic: forward scattering. Instead of redirecting photons equally in all directions, aerosol particles scatter light predominantly in directions close to the photon's original trajectory. When unshielded urban decorative lights throw lumens upward into a dense blanket of suspended particulate matter, these photons encounter multiple forward-scattering events.

The aerosol layer ceases to be transparent air. It becomes an optical diffuser.

Consider the contrast between two locations during festive peaks:

• If your horizon is Hanle in Ladakh (elevation 4,500 metres), the aerosol optical depth is exceptionally low, typically below 0.05. Even if the local village lights up, the air column above lacks the particulates required to bounce those lumens back down to an observer. The upward light escapes largely unhindered into space.
• If your horizon is Delhi, Lucknow, or Patna, the post-monsoon aerosol optical depth routinely surges beyond 0.8 or 1.2. The atmosphere directly overhead becomes a semi-opaque ceiling. Upward-bound lumens from millions of decorative lights do not escape into the cosmos. They collide with aerosol droplets, scatter, bounce sideways, and reflect back downward toward the observer's eye.

When pyrotechnic displays ignite on the festival's peak evening, this aerosol trap undergoes a massive positive feedback loop. Fireworks detonate between 50 and 300 metres above the surface, releasing vast plumes of potassium nitrate, strontium, barium compounds, and elemental carbon. They generate millions of lumens of direct, unshielded omnidirectional light at the precise altitude of the inversion layer, while simultaneously flooding the immediate air column with dense, fresh hygroscopic aerosols.

The sky no longer requires distant city lighting to produce skyglow. The atmosphere itself turns into a self-illuminating, light-scattering fog bank, slashing limiting magnitudes and suppressing ground-to-space transparency.

The Orbiting View: What VIIRS and Satellite Sensors Detect

Ground-level observers see a brilliant dome of skyglow, but what does the planet's orbital monitoring infrastructure see looking down from space?

The premier tool for tracking Earth's nocturnal radiance is the Day/Night Band (DNB) of the Visible Infrared Imaging Radiometer Suite (VIIRS), carried on board the Suomi NPP and NOAA-20 satellites. The VIIRS DNB operates from a sun-synchronous polar orbit roughly 824 kilometres up. Crucially for light-pollution researchers, its overpass occurs around 01:30 AM local solar time.

A satellite overpass at half past one in the morning presents both opportunities and distinct observational quirks during Diwali:

First, by 01:30 AM, domestic firecracker activity has tapered off considerably compared to the 20:00 to 23:00 IST peak. Second, many residential households switch off their decorative string arrays when retiring for the night, though commercial complexes, major streets, and high-rise facades frequently stay illuminated until dawn.

Despite this post-peak timing, nighttime satellite imagery across India registers massive radiance increases during the festival period compared to seasonal baselines taken two weeks prior or subsequent:

The data reveal an intriguing geographic pattern. In saturated urban centres like Mumbai or central Delhi, where street lighting and commercial boards already pump intense radiance upward year-round, the festival produces roughly a two-fold increase.

The most pronounced percentage jumps consistently occur in Tier-2 cities, peri-urban ribbons, and rural districts across Uttar Pradesh, Bihar, and Rajasthan. In these areas, baseline public lighting is relatively modest. When entire residential belts drape themselves in unshielded fairy lights, the relative change in upward radiance jumps by 300 percent or more.

There is an important calibration caveat that spaceborne sensors highlight. VIIRS DNB has an optical filter bandpass spanning 500 to 900 nanometres. It has essentially zero sensitivity to short-wavelength blue light below 500 nanometres.

Because modern decorative strings are increasingly reliant on cool-white LEDs with their dominant 455-nanometre emission spike, VIIRS is actually blind to a significant fraction of the blue-spectrum light injected into the sky during modern festivals. If anything, orbital datasets underestimate the true optical surge that human eyes and ground-based silicon photometers record beneath the dome.

The Spatial Gradient: City Core to Dark Sky

Skyglow does not remain neatly confined over the pin-drop coordinates where lights are turned on. Thanks to multiple scattering inside the aerosol-laden boundary layer, an intense urban light dome can propagate dozens of kilometres into rural zones.

To map how the festival footprint expands, consider a cross-sectional transect moving outward from a metropolitan core into a surrounding rural fringe.

In the deep core of an Indian metro—such as Connaught Place in Delhi, T. Nagar in Chennai, or the Andheri belt in Mumbai—the normal baseline sky already sits firmly in Bortle Class 8 or 9. The zenith brightness might routinely hover between SQM 17.0 and 17.8.

During the peak festive hours, raw atmospheric skyglow pushes this down into the low 16s. A human observer looking straight up notices relatively few subtle stellar changes simply because the faint stars were already invisible. Only Jupiter, Venus, Sirius, and perhaps Capella remain discernible.

The real devastation happens in the buffer zones.

Consider an amateur astronomer located 40 kilometres outside Bengaluru near Savandurga, or 50 kilometres outside Pune toward the Sahyadri foothills. Under standard October conditions, these locations offer decent Bortle Class 4 or high Class 3 skies. The summer Milky Way's tail or autumn targets like the Andromeda Galaxy (M31) can be seen with the naked eye or basic binoculars, provided you point away from the distant city light dome.

On Diwali night, the effective footprint of the metropolitan light dome balloons outward. Because the city's total upward lumen output doubles while boundary layer aerosol concentrations climb, the physical radius of the visible light dome expands by roughly 30 to 60 percent.

The observer in the foothills, who typically enjoys an SQM of 20.8 overhead, suddenly watches their zenith degrade to 19.1. The horizon dome, normally a flat glow spanning 15 degrees in the direction of the metro, surges up toward 45 degrees of altitude.

Even distant dark-sky enclaves are not entirely immune if wind patterns carry aerosol smoke plumes across hundreds of kilometres downwind. While high-altitude sanctuaries like Hanle remain protected behind the Himalayan rain shadow, lowland nature reserves, wildlife corridors, and rural farming tracts throughout central India experience a total loss of their normal nocturnal darkness.

The Ecological Ripple: A Transitory Optical Shock

Light pollution discussions often focus on chronic, permanent illumination. Road networks, security floodlights, and industrial plants emit light every single night of the year, permanently altering ecosystems. Diwali, by contrast, acts as an acute optical pulse.

Does a high-intensity burst of light lasting only a few nights actually matter to ecological networks?

Biologists and sensory ecologists increasingly argue that it does, particularly given the timing. In India, October and November mark a major seasonal transition. The southwest monsoon has retreated, temperatures are dropping, and numerous fauna are initiating autumn mating rituals, seasonal movements, or nocturnal foraging routines to build winter reserves.

Avian Disorientation
The collision of explosive concussions, high-altitude flare lighting, and dense skyglow disorients diurnal birds resting in tree canopies. Flocks of common urban birds, such as parakeets, mynas, and pigeons, take flight in complete panic in the middle of the night. In clear rural darkness, birds navigate using magnetic fields, polarized sky patterns, and stars.

Under an intense, scattered artificial light dome with zero visible stars and heavily reduced ground-to-sky contrast, these birds lose visual bearings, flying directly into building glass, transmission towers, and high-voltage cabling.

Disruption of Pollinators
A substantial portion of terrestrial pollination is carried out not by diurnal bees, but by nocturnal moths, beetles, and small bats. Night-blooming flora rely on precise, low-luminance ambient light conditions to attract their specialized partners.

When ambient surface illuminance surges from normal new-moon darkness (under 0.001 lux) to levels exceeding 1.0 to 10 lux beneath festive facade arrays, nocturnal insect behaviours freeze. Moths abandon flight paths, circling high-kelvin decorative LEDs until exhaustion or predation claims them, suppressing the pollination rate of autumnal night-blooming plants across urban and suburban gardens.

Circadian Phase Shifts
In human populations and urban domestic animals, exposure to high-intensity cool-white light strings late into the evening suppresses melatonin synthesis. When living rooms and bedroom windows are bathed in decorative uplight from neighboring balconies until past midnight, internal biological clocks register extended daytime conditions.

While humans can draw blackout curtains, peri-urban wildlife inhabiting adjacent green belts cannot shield their retinas from the ambient glow.

What This Means for SkyQI Readings

If you are an active SkyQI contributor, the five-day Diwali window represents one of the most scientifically interesting periods of the entire year to upload measurements.

Our platform does not discard readings simply because they are contaminated by festive illumination. On the contrary, these anomalous data points help calibrate our models:

Separating Static Infrastructure from Human Activity
By contrasting a user's reading on Diwali night against their baseline reading taken at the same coordinates three weeks earlier, SkyQI can calculate the difference between fixed municipal lighting (streetlights, highways, permanent architectural arrays) and elastic cultural lighting (temporary decorative strings and festival events).

This delta provides municipal researchers with a baseline tool to estimate how much unshielded lighting private citizens actually own and deploy.

Documenting the Extinction Gradient
When uploading a photo or meter reading during the festival period, your metadata provides invaluable local atmospheric context. If your reading shows a plunge from your neighbourhood's average SQM 19.5 down to 17.8, while the local air monitoring station registers a spike in PM2.5 to 400 micrograms per cubic metre, you are providing ground-truth data on how aerosols and lighting interact in the real world.

To make your measurements as scientifically valuable as possible during the festival window, we recommend three protocols:

1. Take an exact baseline: Record a reading from your observation point (terrace, balcony, garden) exactly seven days before the festival begins, at 21:00 IST on a moonless or low-moon night.
2. Log the peak pulse: Record a reading from the identical physical spot, oriented straight up toward the zenith, on the primary Amavasya night between 20:30 and 22:30 IST.
3. Capture the recovery: Take a third reading three to five days after the festival concludes, once firecracker emissions have settled and decorative facade strips have been unplugged.

When our processing pipeline ingests these three values, it can construct an optical recovery curve for your specific pin code. If your post-festival reading stays degraded for an extra week, it confirms that your local skyglow is being kept alive by lingering aerosol suspension rather than continuing light emissions.

A Final Reframe

It is tempting to view light pollution purely through a lens of ecological loss and observational frustration. When an astronomer steps onto a terrace expecting the deep, velvet blackness of a new moon and finds a sky washed in glowing haze, the initial instinct is naturally one of disappointment.

Yet cultural lighting carries profound human meaning. The impulse to gather, celebrate the close of the harvest, and defy the encroaching darkness of the northern hemisphere's winter is as old as civilization itself.

The purpose of measurement is never to demand an end to celebration. The purpose of measurement is awareness of our physical footprint.

Physics shows us that we can alter the consequences of our light without losing a shred of its cultural richness. A single clay diya or warm-spectrum lamp directed downward onto a doorway or family courtyard honours the festival's roots without firing photons needlessly into the upper troposphere.

It is the switch to unshielded, high-intensity, cool-blue solid-state LED arrays and high-altitude fireworks that transforms a domestic festival into an atmospheric event visible from interplanetary orbit.

Tonight, step outside and look straight up. Notice the colour of the sky. Check whether the stars of Orion's shield or the faint outline of Pegasus can pierce the dome.

Then take out your phone, capture an exposure for SkyQI, and add your data point to India's map of the night. Every measurement you make helps us document not just where our darkness is disappearing, but the exact physics of how we might one day bring it back.