The unit describes mass in air
Micrograms per cubic meter expresses how much particle mass is present in a volume of air. HRRR-Smoke’s near-surface MASSDEN field estimates primary fine particles emitted by fires near the lowest model level. Smoke Window samples that value at a chosen grid cell and groups it into a readable display band.
A modeled concentration is not collected by a sensor. It reflects the model’s fire emissions, atmospheric state, transport, and removal processes. Treat decimal precision as computational detail, not evidence of exact local accuracy.
The display scale is a planning device
Smoke Window uses five bands: Low below 4, Light from 4 to under 12, Smoky from 12 to under 40, Heavy from 40 to under 100, and Very heavy at 100 or more micrograms per cubic meter. These labels and breakpoints are defined by this product to make forecast changes easier to scan.
They are not EPA AQI categories, health standards, or official NOAA thresholds. The colors may look familiar, so the legend always includes the numeric range and the phrase “Modeled-smoke scale · not AQI.”
Interpret patterns before exact values
A stable band over several forecast samples can be more meaningful than a small numerical difference between two cells. Near sharp plume boundaries, a slight wind shift can move a high concentration into or out of the selected point. Complex terrain adds further uncertainty.
Compare the model with the latest nearby preliminary AirNow NowCast AQI, but do not expect a one-to-one match. AirNow measures ambient pollutants at a reporting area and may reflect multiple pollutants and sources.
Why mass concentration is not visibility
Visibility depends on particle size, humidity, lighting, cloud, and the vertical distribution of haze, not only on near-surface particle mass. Two scenes with the same modeled micrograms per cubic meter can look different. Conversely, a dramatic elevated plume can color the sky while the lowest model level remains lower. Smoke Window therefore describes concentration bands rather than promising a visual condition such as blue sky or clear mountains.
Why model decimals are rounded into bands
A decoder can return many decimal places, but the atmosphere and fire inputs do not support street-level numerical precision. Bands make the time pattern readable and discourage false certainty. They also keep the map faithful to source cells without interpolating new values. The selected-point chip retains a concentration range so users can distinguish a low cell from a much higher one while remembering that the range is still a model estimate.
Concentration changes at plume edges
A plume edge can cross several bands over one or two grid cells. When wind direction shifts slightly in a new run, the location sample may jump even though the regional forecast remains broadly similar. Inspect the neighboring cells and animation before interpreting the jump as a sudden physical wall of smoke. Terrain and unresolved local flows can further alter the real boundary. Trends across multiple cells and times are more robust than one edge value.
The threshold in plain language
Smoke Window calls a sample Low when modeled primary smoke mass is under 4 micrograms per cubic meter. A planning window requires at least two consecutive three-hour samples under that value. Conditions between the discrete samples may differ. The breakpoint was selected to provide a consistent comparative interface; it is not an EPA standard, AQI breakpoint, clean-air promise, or recommendation for any person’s health or activity level.
Compare values only within their scope
Compare HRRR near-surface smoke values with other HRRR near-surface smoke values from compatible runs and locations. Do not compare the number directly with an AirNow AQI, because AQI is an index derived from ambient pollutant observations. Even observed PM2.5 includes sources beyond primary fire particles. Use the model concentration for relative smoke timing and use official air-quality products for observed categories and health-oriented information.
Bottom line: modeled smoke concentration
A concentration number becomes useful only after its source, height, grid, time, and units are known. Here it is modeled primary fire-particle mass near the lowest HRRR level, sampled at one cell and shown in micrograms per cubic meter. It is not a sensor reading or AQI. Use the band to compare the model’s own frames, and resist translating it into health meaning that the field was not designed to carry.
A careful reading of modeled smoke concentration
Modeled smoke concentration estimates smoke-specific particle mass in a volume of air. It is a model quantity with uncertainty, not a monitor reading or health category. Keep that immediate answer attached to the valid time, selected geography, model run, and product limits described above. Those details are part of the result, not fine print, and they should travel with any shared interpretation of this guide.