The race between dust and rain
Soiling is the layer of mineral dust, traffic particulate, pollen, salt and bird droppings that accumulates on module glass and blocks light before it reaches the cell. Unlike the heat penalty of part 01, it is not a property of the technology: it is a race between deposition and cleaning, and in most of the world the cleaner is rain. The literature converges on a threshold of roughly 1 to 5 millimetres in a single event to wash a lightly soiled array, with less sometimes sufficient (Kimber's modal modelling value is 3 mm; the IEA PVPS basic model resets at 1 mm; about 0.5 mm cleaned lightly soiled Atacama arrays), and with two documented complications: light rain on heavy dust can bake on a muddy residue rather than clean, and Moroccan red rain deposits dust instead of removing it.
Priced across the 40-market baseline, the soiling term spreads wider than any single global assumption can cover:
| Market | Annual loss, no cleaning | Dry-period rate %/day | Dominant soiling | Confidence |
|---|---|---|---|---|
| Masdar City / Abu Dhabi, UAE | 20%+ | 0.6 | Desert dust, humidity cementation | Medium |
| Riyadh, Saudi Arabia | 20%+ | 0.5 | Desert dust | Medium |
| Antofagasta (Atacama), Chile | 18% | 0.31 | Desert dust | Medium |
| Jakarta, Indonesia | 8% | 0.06 | Traffic-industrial PM | Medium |
| Santiago, Chile | 7% | 0.6 | Urban pollution PM | High |
| Dubai, UAE | 6% | 0.3 | Desert dust | Medium |
| Perth, Australia | 5% | n.a. | Dust, coastal salt, bird | High |
| Phoenix, Arizona, USA | 3% | 0.25 | Dust, monsoon haboobs | High |
| Shanghai, China | 1.7% | n.a. | Traffic-industrial PM | High |
| Berlin / London / Paris / New York | 1.5% | <0.1 | Urban PM, pollen | High |
Selected from the 40-market soiling baseline. Abu Dhabi and Riyadh carry the NREL Middle East lower bound (20%+, post-storm peaks to 80%); Atacama is the Iquique transect point (range 3-39%); Perth is the Murdoch University field midpoint (4-6%); the temperate 1.5% rows are the NREL regular-rainfall band midpoint. Full 40-row table, with sources, windows and the reason behind every blank, in the public dataset.
Seventeen of the forty markets carry no annual figure at all, by design. For markets like Cairo, Lagos and Nairobi the strongest available studies report optical transmittance or seasonal losses that cannot be annualised without inventing a number; for Cape Town, Kabul, Lhasa and Reykjavik no credible field study exists. Sub-Saharan Africa and South and Central Asia are blank across the board despite carrying measured dry-period rates near 0.44 per cent per day, among the highest in the set: the largest data gap in this series.
The rate and the level
The accumulation rate during rain-free periods spans a factor of fourteen across the measured markets, and it is the number that decides whether a dry season hurts:
Madrid shows why the rate and the annual level must not be conflated. The CIEMAT rooftop campaign fitted a modest 0.16 per cent per day, but after 52 rain-free summer days the loss level reached about 6 per cent, and a 4.5 mm event failed to fully clean low-tilt modules. A low rate with a long dry spell ends in the same dirty glass as a high rate with a short one, which is why the dataset records rate, dry spell and threshold separately.
The same arithmetic reaches the blank rows. At the 0.44 per cent per day rates measured in the blank markets, a dry spell the length of Madrid's 52 rain-free days would put an uncleaned array past a 20 per cent loss level, the bound that defines the desert band. No annual figure is interpolated here, but the measured rate says which band those markets are unlikely to sit below.
The three cleaning bands
The economics fall into three bands. In the temperate 1 to 2 per cent band (most of Europe, the eastern United States), rain does the job and paid cleaning generally returns less than it costs; a Belgian field study concluded annual cleaning would have negligible effect. In the marginal band around 5 per cent (southern California, Australia's coastal cities), the standard practice is a derate and roughly one clean a year: Perth's measured 4 to 6 per cent sits exactly here, and the structural south-west WA rainfall decline lengthens the summer accumulation window that derate assumes. In the desert band cleaning is part of operations: UAE utilities wash plants 40 to 45 times a year to hold soiling under 3 per cent, Atacama operators clean on a 45-day to 4-month cadence, a Moroccan 44 MWp plant cut losses from 5.8 to 1.9 per cent with an optimised schedule worth about US$5.23 per kilowatt per year, and in Riyadh anti-soiling coatings return a measured 3.2 per cent annual yield gain.
40 to 45
How many times a year UAE utilities wash their solar plants to hold soiling under 3%.
The band a market sits in decides whether a cleaning contract returns its cost; in the temperate band, on the Belgian field evidence, it generally does not. In dry-summer markets tilt matters, since rain cleans adequately above about 5 degrees and improves with tilt, so near-flat arrays soil harder and stay dirty longer. And in the desert band the cleaning schedule belongs in the quote from day one, because at a 20 per cent uncleaned loss it is the largest controllable term in this series' yield model.
Behind the forty-market soiling baseline
The figures come from the series' 40-market soiling baseline: annual energy loss with no manual cleaning, dry-period accumulation rates, rain-cleaning thresholds and season metadata, each row named to its source with its observation window and confidence grade. Ranges are recorded at midpoints and bounds are flagged as bounds; sales-blog figures were located and excluded. Deposition is strictly separated from atmospheric attenuation, so haze and smoke dimming belong to part 06 and are not double-counted here. Snow markets (Oslo, Stockholm, Helsinki, Ulaanbaatar) record snow as the dominant mechanism, a different physical process with a different remedy. And nothing was interpolated into the blanks; no neighbour's value was borrowed.