When the fire is far away but the air is not
In the summers of 2023 and 2025, cities across the northeastern United States and southern Canada experienced some of the worst air quality in recorded history — not because of local fires but because wildfire smoke from thousands of kilometers away had been transported by upper-level winds into dense urban corridors. New York, Washington D.C., and Toronto saw AQI values exceed 300, placing them temporarily among the most polluted cities on the planet.
This long-range transport of wildfire smoke is not rare. It happens every fire season on every continent. Australian bushfire smoke has been tracked crossing the Pacific. Siberian fire smoke reaches Europe. Saharan dust — while not smoke — follows similar atmospheric pathways. Satellites are the primary tool for observing and forecasting these transboundary pollution events.
Aerosol optical depth: measuring smoke from orbit
The key satellite measurement for tracking smoke is aerosol optical depth (AOD) — a dimensionless number describing how much aerosol (smoke, dust, pollution) in the atmosphere attenuates sunlight. An AOD of 0.1 represents clean air. An AOD of 1.0 means the aerosol column absorbs or scatters roughly 63 percent of incoming sunlight. During major smoke events, AOD values above 3.0 have been observed.
MODIS and VIIRS both retrieve AOD globally. The algorithms compare the observed reflectance of the Earth's surface and atmosphere to models of what clean-air reflectance should look like. The difference is attributed to aerosols. Over dark surfaces like oceans and dense vegetation, retrieval is straightforward. Over bright deserts or snow, it is much harder.
Tracking smoke plume transport
Once emitted, wildfire smoke enters the atmosphere and is transported by prevailing winds. Smoke injected into the planetary boundary layer (below about 2 km altitude) stays close to the surface and degrades local air quality quickly but disperses relatively fast. Smoke injected higher — by pyroconvection from intense fires — can reach the free troposphere or even the stratosphere and travel intercontinental distances.
Satellite imagery in the visible and infrared bands can track these plumes day by day. Geostationary satellites provide continuous animation of plume movement. Polar-orbiting satellites provide higher spatial resolution snapshots. Together, they allow forecasters to predict where smoke will travel and when it will reach populated areas.
PM2.5: the health-relevant metric
While AOD measures the total column of aerosol from surface to space, the health-relevant measurement at ground level is PM2.5 — the mass concentration of particulate matter smaller than 2.5 micrometers in diameter. Wildfire smoke is predominantly composed of fine particles in this size range, which penetrate deep into the lungs and enter the bloodstream.
Converting satellite AOD to ground-level PM2.5 requires additional information: how the aerosol is distributed vertically, the relative humidity, the particle composition, and the height of the planetary boundary layer. NASA's MERRA-2 reanalysis and the NOAA HRRR-Smoke model produce PM2.5 estimates that blend satellite AOD, surface monitor data, and atmospheric models.
Smoke forecast models
Several operational models forecast smoke transport and air quality impact. NOAA's HRRR-Smoke model uses satellite fire detections to initialize smoke emissions and then simulates atmospheric transport and dispersion at hourly time steps. The Canadian FireWork system performs similar functions for North America. NASA's GEOS model produces global aerosol forecasts.
These models are imperfect — they depend on accurate fire emission estimates, which can change rapidly as fires grow or are suppressed. But they provide the best available guidance for public health agencies issuing air quality warnings, often 1–3 days in advance of smoke arrival at distant cities.
Satellite imagery layers for smoke visualization
PlanetSentry integrates satellite imagery layers that can make smoke visible on the globe. True-color imagery (RGB visible bands) shows smoke as hazy gray or brown plumes against the landscape. Specialized aerosol products color-code AOD values to highlight smoke concentration. Infrared difference products can distinguish smoke from clouds based on their different thermal properties.
Stacking these layers on a 3D globe alongside active fire markers creates a coherent picture: fires burning in one region, smoke streaming downwind, and air quality degrading hundreds or thousands of kilometers away. This kind of integrated visualization is why multi-source platforms add value beyond single-purpose dashboards.
What individuals can do with smoke data
Access to real-time smoke tracking and forecasts gives individuals actionable information. When models predict smoke arrival within 24–48 hours, sensitive groups (children, elderly, those with respiratory conditions) can prepare by staying indoors, running air purifiers, and adjusting outdoor exercise plans.
Ground-level air quality monitors provide the most accurate local readings, but satellite data fills the vast gaps between monitors. In rural and wildland areas where no ground monitors exist, satellite AOD estimates may be the only source of air quality information. Combining both data types gives the most complete picture of smoke exposure risk.