National Weather Service Wilmington hypothesis
Historic Heat Waves in the Carolinas
The NWS chronology places July 23 inside the prolonged July 20-31, 1999 Carolinas heat wave and records extreme temperatures across the affected Carolinas corridor.
Stagnant pollution mixture Possible
1999-07-23 Charlotte-Concord-Gastonia, NC-SC stagnant mixed-pollution episode
Daily PM2.5 peaked at 41.2 µg/m³ near Charlotte across 1 covered metro home to 2.9 million people.
Counting the extra PM2.5 above each metro's typical day, this event handed the average affected resident ≈ 1.4 cigarettes. Across everyone living in the affected metros, that is the collective equivalent of ≈ 4.0 million cigarettes. An awareness illustration of outdoor exposure at 22 µg/m³ per cigarette (Berkeley Earth), counting only the event's ranked days; not a medical dose.
Why we think this happened
The basis for “Stagnant pollution mixture”
The NWS chronology places July 23 inside the prolonged July 20-31, 1999 Carolinas heat wave and records extreme temperatures across the affected Carolinas corridor. The retained scope and limitations support possible confidence for this local episode.
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
Inference, not direct attribution
documented_heatwave_overlap_possible.v1: This inference is recorded in the catalog methodology.
Evidence path
From source to city
A cause is more convincing when the record connects three things: something happened, the air mass moved toward the region, and local instruments responded. Here is how much of that chain is documented for this event.
- 1Inferred
Source or mechanism
Pattern-based explanation
documented_heatwave_overlap_possible.v1. This inference is recorded in the catalog methodology.
- 2Research gap
Air-mass movement
The transport path has not been reconstructed
The retained record does not yet show how air traveled from the proposed source to these metros. A trajectory replay or dated plume analysis belongs here.
- 3Documented
Local response
41.2 µg/m³ near Charlotte
EPA monitors recorded the event across 1 metro. The peak daily concentration is roughly 1.9 cigarette-equivalents at the outdoor daily rate.
Inspect the pollutant and weather signals ↓
Event autopsy
What the instruments saw
Before, during, and after the episode: nearby pollutant monitors and surface-weather stations show what changed. This context tests the “Stagnant pollution mixture” explanation; it does not assign the cause by itself.
Charlotte-Concord-Gastonia, NC-SC
Monitor comparison window 1999-07-20 to 1999-07-26
35% above nearby days
20% lower than surrounding days
No adjacent-day comparison
Pollution fingerprint
Metro-wide median of reporting monitors PM2.5 39.3 µg/m³
35% higher than surrounding days
Before29.1 µg/m³ Event39.3 µg/m³ After23.6 µg/m³
PM10 54.5 µg/m³
No adjacent-day comparison
Before— Event54.5 µg/m³ After—
Carbon monoxide 0.730 ppm
32% higher than surrounding days
Before0.570 ppm Event0.730 ppm After0.543 ppm
Nitrogen dioxide 25.3 ppb
16% higher than surrounding days
Before21.9 ppb Event25.3 ppb After18.8 ppb
Sulfur dioxide 5.21 ppb
13% higher than surrounding days
Before3.58 ppb Event5.21 ppb After4.63 ppb
Ozone 0.054 ppm
11% higher than surrounding days
Before0.049 ppm Event0.054 ppm After0.048 ppm
Atmospheric conditions
Nearby EPA weather observationsAbout the same as surrounding days
About the same as surrounding days
About the same as surrounding days
20% lower than surrounding days
Source: U.S. EPA AirData daily observations and, where available, nearby ASOS/AWOS surface observations. Values are descriptive medians over the retained monitor set; missing measurements remain missing. Transport and source evidence remain separate layers because this instrument pattern does not assign a cause by itself.
Measured daily PM2.5 at EPA monitors, as cigarette-equivalents
EPA monitors sit in metro areas, so the colour clusters there. Land between and beyond
them is unmeasured, not clean: this map shows where IAQng has readings, not the extent
of the smoke.
—
Episode peak
-- cigarettes
The smoke map's scale, replayed over this event: the surface interpolates between covered metros' daily records, framed to the affected region. Hover for a reading, click a metro to pin it to the card. Days a monitor did not report draw nothing.
Views from the selected metro
Camera views near
Select a metro on the map to see nearby cameras. Where an operator archive reaches the event, drag the divider to compare two moments, adjust either date and time, or play the available event-day frames. Camera imagery is visual context, not an air-quality measurement or proof of smoke.
No useful public camera views are currently available within 80 km of this metro.
Affected metros
| Metro | Peak PM2.5 | Cigarettes, whole event | Peak day | All-time standing |
|---|---|---|---|---|
| Charlotte-Concord-Gastonia, NC-SC | 41.2 µg/m³ | 1.9 | Jul 23, 1999 | #20 of its top days |
"Cigarettes, whole event" totals a metro's ranked event days at 22 µg/m³ per cigarette: what a person outdoors there breathed across the event, as the illustration. "All-time standing" is where this episode's days sit in each metro's ranked record (worst-area measure, since 1999). Full local context is on each metro's Air Records page.
Public evidence library
What supports this account
Each card states what a source supports—and what it does not. Official records, research, reporting, community observations, and transparent inferences remain visibly different kinds of evidence.
Historic Heat Waves in the Carolinas
National Weather Service Wilmington
The NWS chronology places July 23 inside the prolonged July 20-31, 1999 Carolinas heat wave and records extreme temperatures across the affected Carolinas corridor.
What it supports
This source supports a possible mechanism or occurrence; it does not establish the cause by itself.
Limit
A heat wave can promote pollution accumulation but does not by itself identify the particles or prove stagnation at every monitor.
What remains uncertain
- A heat wave can promote pollution accumulation but does not by itself identify the particles or prove stagnation at every monitor.
Status and provenance
Possible This is a transparent hypothesis supported by limited evidence or an explicit inference, not a definitive attribution.
This episode was grouped from the monitor record by the catalog pipeline and given its cause from the evidence listed above. It has not been individually reviewed and signed off, so its boundaries, cause, and name may change as evidence accumulates. Grouping policy common_sense_root_cause_grouping.v2. The cause label reflects the assembled evidence above, not an IAQng detection formula. If it is wrong, tell us.