We obtain data on nighttime radiance of cities from NOAA VIIRS – comparison for Bratislava district
For Bratislava Region, NASA Black Marble / VIIRS gives us a compact way to follow changes in nighttime-light activity and to place them in the context of Slovakia as a whole. The current comparison uses the complete monthly composites for 2025 and 2024, while the separate long-term table uses the official annual composites for 2015–2024. This distinction keeps the most recent comparison methodologically consistent and still preserves a longer historical view. At the Department of Optics and Thermophysics of USTARCH SAV, the data are relevant to research on light pollution, skyglow, diffuse night-sky light and the propagation of artificial light through the atmosphere. The share of Slovakia shown in the article also helps indicate how large a part of the national nighttime-light indicator is associated with the selected region.
The first table focuses on the latest complete period and compares 2025 with 2024. For both years we calculate the same indicator as the mean of the monthly VIIRS spatial sums, so the year-on-year change of +3.8% is methodologically comparable. The same indicator is also shown for Slovakia as a whole, where the change is +14.4%, allowing the local development to be separated from the broader national trend. The selected region accounts for 20.14% of the current Slovak indicator. For our light-pollution research, combining local change with national context is more useful than presenting an isolated absolute value.
Latest comparison
| Current year | 2025 |
|---|---|
| Months used in current year | 12 |
| Mean monthly VIIRS spatial sum – selected area | 60 860 |
| Mean monthly VIIRS spatial sum – Slovakia | 302 142 |
| Previous year | 2024 |
| Selected area – previous year | 58 625 |
| Slovakia – previous year | 264 110 |
| Selected-area change | +3.8% |
| Slovakia change | +14.4% |
| Share of Slovakia | 20.14% |
The second view is longer-term and uses the official NASA Black Marble annual composites, so it is deliberately not merged numerically with the current monthly-derived indicator into one series. Over 2015–2024, the spatial sum in Bratislava Region changed by +1.2%, with an average annual rate of +0.1%. The three-year-smoothed change of +2.0% helps reduce the influence of one unusual year. The highest and lowest years provide additional context for whether the observed area has been becoming brighter or darker over the long term. This time series is a useful complement to studies of skyglow, propagation of artificial light through the atmosphere and the optical properties of the night sky.
Long-term summary of annual composites
| Annual-composite period | 2015–2024 |
|---|---|
| Latest annual composite | 2024 |
| Change over displayed period | +1.2% |
| Average annual rate (CAGR) | +0.1% |
| Three-year smoothed change | +2.0% |
| Highest-value year | 2022 |
| Lowest-value year | 2020 |
The third table shows the individual years of the long-term series and normalizes the first year to 100. This makes it possible to follow the relative change of the same locality without letting its physical size dominate the interpretation. For both regions and districts, the Slovakia index and the locality share of the national spatial sum are included for every available year. Annual changes highlight stronger shifts, but they are interpreted together with the complete trend. When different districts or regions are published more frequently, the methodology therefore remains consistent while the spatial perspective changes.
Annual trend of the selected area and Slovakia
| Year | VIIRS spatial radiance sum | Annual change | Index (first year = 100) | Slovakia index | Share of Slovakia |
|---|---|---|---|---|---|
| 2015 | 55 317 | – | 100.0 | 100.0 | 25.73% |
| 2016 | 55 958 | +1.2% | 101.2 | 102.3 | 25.45% |
| 2017 | 56 056 | +0.2% | 101.3 | 101.8 | 25.61% |
| 2018 | 55 786 | -0.5% | 100.8 | 104.4 | 24.86% |
| 2019 | 54 688 | -2.0% | 98.9 | 102.8 | 24.74% |
| 2020 | 53 649 | -1.9% | 97.0 | 102.5 | 24.35% |
| 2021 | 55 326 | +3.1% | 100.0 | 109.5 | 23.49% |
| 2022 | 59 450 | +7.5% | 107.5 | 118.7 | 23.29% |
| 2023 | 55 280 | -7.0% | 99.9 | 112.9 | 22.77% |
| 2024 | 56 007 | +1.3% | 101.2 | 115.8 | 22.50% |
The current 2025/2024 comparison is derived from the same mean of monthly ADM2 composites for both years. The long-term series uses the separate official annual Black Marble product. Absolute values from these two products are therefore not compared directly; relative changes are interpreted within each method. The spatial sum is an indicator of nighttime-light activity, not a direct measurement of sky brightness or illuminance.
Method note
The World Bank aggregates approximately 500 m NASA Black Marble pixels directly to ADM2 administrative units. A district uses its ADM2 row; a region and Slovakia are calculated by summing their ADM2 units. All localities therefore use the same source, and after the first load the Slovakia subset is cached so publishing additional regions and districts does not require repeatedly downloading the global files.
Source: NASA Black Marble / World Bank Space2Stats Monthly & Annual Black Marble Nighttime Lights.
