SmogRank / Episodes / United States
California and Oregon, United States, December 2023
For 6 days, from 13 to 18 December 2023, 20 cities in the United States had PM2.5 at least twice their usual for the time of year. The worst reading was Bakersfield, California, United States: 58.6 µg/m³ on 17 December, about 3.2× its usual 18.4.
No cause named We have no fire or smoke records for these days, so we only say what the measurements show.
Cities reached 20 in 1 country
Days 6 13 to 18 December 2023
Worst day 16 December 13 cities counted
Furthest above usual 6.2× Redding
Day by day
Play 16 December 13 cities at twice their usual or more
Folsom Lincoln Roseville Galt Woodland Davis Truckee South Lake Tahoe Paradise Vacaville Tracy Carson City Reno Concord Livermore Spanish Springs Pleasanton San Leandro Richmond Oakland San Rafael Santa Rosa San Francisco Redwood City Gilroy Hollister Santa Cruz Ukiah Salinas Atascadero Altamont San Luis Obispo Arroyo Grande Ashland Nipomo Orcutt Grants Pass Madera Clovis Visalia Porterville Sacramento Corcoran Fresno San Jose Merced Chico Arden-Arcade Medford Turlock Manteca Stockton Bakersfield Modesto Hanford Yuba City Redding 10 Dec 11 12 13 14 15 16 17 18 19 20 21 Every measured city in the area, coloured by its PM2.5 that day; a dark ring marks a city at twice its usual or more. Bars: cities counted each day, coloured by their median level. Choose a day or press play; without JavaScript, the days link to that day's world ranking.
How it spread
Hanford Hanford, 10 December: 22.6 µg/m³ Hanford, 11 December: 27.5 µg/m³ Hanford, 12 December: 30.6 µg/m³ Hanford, 13 December: 43.1 µg/m³, 2.7× its usual 16.1 Hanford, 14 December: 47.6 µg/m³, 3.0× its usual 16.1 Hanford, 15 December: 53.5 µg/m³, 3.3× its usual 16.0 Hanford, 16 December: 51.9 µg/m³, 3.2× its usual 16.1 Hanford, 17 December: 53.8 µg/m³, 3.3× its usual 16.2 Hanford, 18 December: 45.3 µg/m³, 2.8× its usual 16.2 Hanford, 19 December: 7.7 µg/m³ Hanford, 20 December: 7.0 µg/m³ Hanford, 21 December: 8.7 µg/m³ Fresno Fresno, 10 December: 29.7 µg/m³ Fresno, 11 December: 30.6 µg/m³ Fresno, 12 December: 30.3 µg/m³ Fresno, 13 December: 42.6 µg/m³, 2.2× its usual 19.1 Fresno, 14 December: 39.0 µg/m³, 2.1× its usual 19.0 Fresno, 15 December: 41.7 µg/m³, 2.2× its usual 19.0 Fresno, 16 December: 39.7 µg/m³, 2.1× its usual 19.1 Fresno, 17 December: 46.6 µg/m³, 2.5× its usual 19.0 Fresno, 18 December: 39.9 µg/m³, 2.1× its usual 19.0 Fresno, 19 December: 6.8 µg/m³ Fresno, 20 December: 5.2 µg/m³ Fresno, 21 December: 5.9 µg/m³ Corcoran Corcoran, 10 December: 20.4 µg/m³ Corcoran, 11 December: 24.4 µg/m³ Corcoran, 12 December: 25.7 µg/m³ Corcoran, 13 December: 36.8 µg/m³, 2.5× its usual 14.8 Corcoran, 14 December: no reading Corcoran, 15 December: no reading Corcoran, 16 December: no reading Corcoran, 17 December: no reading Corcoran, 18 December: no reading Corcoran, 19 December: no reading Corcoran, 20 December: 7.5 µg/m³ Corcoran, 21 December: 5.9 µg/m³ Visalia Visalia, 10 December: 29.9 µg/m³ Visalia, 11 December: no reading Visalia, 12 December: no reading Visalia, 13 December: 36.8 µg/m³, 2.1× its usual 17.4 Visalia, 14 December: 36.4 µg/m³, 2.1× its usual 17.2 Visalia, 15 December: 38.5 µg/m³, 2.3× its usual 16.8 Visalia, 16 December: 34.5 µg/m³, 2.1× its usual 16.6 Visalia, 17 December: 36.4 µg/m³, 2.2× its usual 16.6 Visalia, 18 December: 34.7 µg/m³, 2.1× its usual 16.3 Visalia, 19 December: 4.3 µg/m³ Visalia, 20 December: 4.0 µg/m³ Visalia, 21 December: 7.8 µg/m³ Stockton Stockton, 10 December: 33.4 µg/m³ Stockton, 11 December: 29.3 µg/m³ Stockton, 12 December: 22.8 µg/m³ Stockton, 13 December: 34.7 µg/m³, 2.5× its usual 13.8 Stockton, 14 December: 29.0 µg/m³, 2.1× its usual 13.5 Stockton, 15 December: 36.8 µg/m³, 2.7× its usual 13.4 Stockton, 16 December: 35.4 µg/m³, 2.7× its usual 13.2 Stockton, 17 December: 39.6 µg/m³, 3.1× its usual 12.9 Stockton, 18 December: 21.1 µg/m³ Stockton, 19 December: 8.2 µg/m³ Stockton, 20 December: 7.4 µg/m³ Stockton, 21 December: 10.1 µg/m³ Modesto Modesto, 10 December: 20.9 µg/m³ Modesto, 11 December: 18.9 µg/m³ Modesto, 12 December: 23.0 µg/m³ Modesto, 13 December: 32.4 µg/m³, 2.2× its usual 14.6 Modesto, 14 December: 35.2 µg/m³, 2.4× its usual 14.4 Modesto, 15 December: 37.3 µg/m³, 2.7× its usual 13.9 Modesto, 16 December: 41.6 µg/m³, 3.0× its usual 13.9 Modesto, 17 December: 43.3 µg/m³, 3.3× its usual 13.3 Modesto, 18 December: 23.5 µg/m³ Modesto, 19 December: 7.4 µg/m³ Modesto, 20 December: 6.2 µg/m³ Modesto, 21 December: 8.9 µg/m³ Clovis Clovis, 10 December: 20.4 µg/m³ Clovis, 11 December: 18.9 µg/m³ Clovis, 12 December: 23.5 µg/m³ Clovis, 13 December: 29.8 µg/m³, 2.2× its usual 13.5 Clovis, 14 December: 30.0 µg/m³, 2.3× its usual 13.3 Clovis, 15 December: 22.7 µg/m³ Clovis, 16 December: 21.4 µg/m³ Clovis, 17 December: 21.2 µg/m³ Clovis, 18 December: 30.9 µg/m³, 2.3× its usual 13.2 Clovis, 19 December: 2.4 µg/m³ Clovis, 20 December: 2.7 µg/m³ Clovis, 21 December: 7.3 µg/m³ Turlock Turlock, 10 December: 21.5 µg/m³ Turlock, 11 December: 19.3 µg/m³ Turlock, 12 December: 16.3 µg/m³ Turlock, 13 December: 28.2 µg/m³, 2.0× its usual 14.0 Turlock, 14 December: 32.9 µg/m³, 2.4× its usual 13.7 Turlock, 15 December: 34.8 µg/m³, 2.6× its usual 13.6 Turlock, 16 December: 41.0 µg/m³, 2.9× its usual 14.0 Turlock, 17 December: 37.0 µg/m³, 2.7× its usual 13.6 Turlock, 18 December: 24.2 µg/m³ Turlock, 19 December: 4.2 µg/m³ Turlock, 20 December: 4.7 µg/m³ Turlock, 21 December: 11.1 µg/m³ Arden-Arcade Arden-Arcade, 10 December: 27.4 µg/m³ Arden-Arcade, 11 December: 22.7 µg/m³ Arden-Arcade, 12 December: 20.9 µg/m³ Arden-Arcade, 13 December: 25.8 µg/m³, 2.3× its usual 11.1 Arden-Arcade, 14 December: 27.2 µg/m³, 2.5× its usual 11.0 Arden-Arcade, 15 December: 23.1 µg/m³ Arden-Arcade, 16 December: 25.8 µg/m³, 2.4× its usual 10.8 Arden-Arcade, 17 December: 29.1 µg/m³, 2.8× its usual 10.5 Arden-Arcade, 18 December: 8.9 µg/m³ Arden-Arcade, 19 December: 1.9 µg/m³ Arden-Arcade, 20 December: 7.2 µg/m³ Arden-Arcade, 21 December: 8.9 µg/m³ Bakersfield Bakersfield, 10 December: 30.6 µg/m³ Bakersfield, 11 December: 29.2 µg/m³ Bakersfield, 12 December: 34.4 µg/m³ Bakersfield, 13 December: 33.1 µg/m³ Bakersfield, 14 December: 40.3 µg/m³, 2.1× its usual 19.2 Bakersfield, 15 December: 38.7 µg/m³, 2.1× its usual 18.6 Bakersfield, 16 December: 51.9 µg/m³, 2.9× its usual 18.1 Bakersfield, 17 December: 58.6 µg/m³, 3.2× its usual 18.4 Bakersfield, 18 December: 31.0 µg/m³ Bakersfield, 19 December: 20.7 µg/m³ Bakersfield, 20 December: 13.6 µg/m³ Bakersfield, 21 December: 5.7 µg/m³ Manteca Manteca, 10 December: 16.3 µg/m³ Manteca, 11 December: 11.3 µg/m³ Manteca, 12 December: 13.6 µg/m³ Manteca, 13 December: 23.8 µg/m³ Manteca, 14 December: 30.4 µg/m³, 2.4× its usual 12.5 Manteca, 15 December: 31.6 µg/m³, 2.5× its usual 12.4 Manteca, 16 December: 36.0 µg/m³, 2.9× its usual 12.5 Manteca, 17 December: 38.1 µg/m³, 3.1× its usual 12.4 Manteca, 18 December: 21.0 µg/m³ Manteca, 19 December: 7.7 µg/m³ Manteca, 20 December: 5.8 µg/m³ Manteca, 21 December: 8.1 µg/m³ Merced Merced, 10 December: 17.6 µg/m³ Merced, 11 December: 16.1 µg/m³ Merced, 12 December: 16.3 µg/m³ Merced, 13 December: 24.0 µg/m³ Merced, 14 December: 30.3 µg/m³, 2.1× its usual 14.1 Merced, 15 December: 36.5 µg/m³, 2.6× its usual 14.1 Merced, 16 December: 27.4 µg/m³ Merced, 17 December: 27.5 µg/m³, 2.0× its usual 13.6 Merced, 18 December: 28.6 µg/m³, 2.1× its usual 13.6 Merced, 19 December: 2.7 µg/m³ Merced, 20 December: 3.5 µg/m³ Merced, 21 December: 10.8 µg/m³ Sacramento Sacramento, 10 December: 15.6 µg/m³ Sacramento, 11 December: 13.1 µg/m³ Sacramento, 12 December: 17.8 µg/m³ Sacramento, 13 December: 22.7 µg/m³ Sacramento, 14 December: 25.2 µg/m³, 2.5× its usual 10.1 Sacramento, 15 December: 17.4 µg/m³ Sacramento, 16 December: 20.7 µg/m³ Sacramento, 17 December: 23.3 µg/m³ Sacramento, 18 December: 10.7 µg/m³ Sacramento, 19 December: 3.3 µg/m³ Sacramento, 20 December: 4.8 µg/m³ Sacramento, 21 December: 7.8 µg/m³ Madera Madera, 10 December: 16.8 µg/m³ Madera, 11 December: 13.1 µg/m³ Madera, 12 December: 15.8 µg/m³ Madera, 13 December: 25.0 µg/m³ Madera, 14 December: 27.8 µg/m³ Madera, 15 December: 28.6 µg/m³, 2.1× its usual 13.9 Madera, 16 December: 22.2 µg/m³ Madera, 17 December: 21.3 µg/m³ Madera, 18 December: 30.7 µg/m³, 2.3× its usual 13.6 Madera, 19 December: 2.4 µg/m³ Madera, 20 December: 2.7 µg/m³ Madera, 21 December: 5.8 µg/m³ San Jose San Jose, 10 December: 16.8 µg/m³ San Jose, 11 December: 14.9 µg/m³ San Jose, 12 December: 11.4 µg/m³ San Jose, 13 December: 16.9 µg/m³ San Jose, 14 December: 18.9 µg/m³ San Jose, 15 December: 25.2 µg/m³, 2.4× its usual 10.5 San Jose, 16 December: 26.8 µg/m³, 2.6× its usual 10.5 San Jose, 17 December: 23.2 µg/m³ San Jose, 18 December: 8.1 µg/m³ San Jose, 19 December: 2.1 µg/m³ San Jose, 20 December: 3.6 µg/m³ San Jose, 21 December: 10.9 µg/m³ Medford Medford, 10 December: 18.2 µg/m³ Medford, 11 December: 14.7 µg/m³ Medford, 12 December: 9.6 µg/m³ Medford, 13 December: 12.3 µg/m³ Medford, 14 December: 18.1 µg/m³ Medford, 15 December: 22.1 µg/m³ Medford, 16 December: 28.0 µg/m³, 2.6× its usual 10.6 Medford, 17 December: 29.9 µg/m³, 2.8× its usual 10.6 Medford, 18 December: 11.7 µg/m³ Medford, 19 December: 4.9 µg/m³ Medford, 20 December: 8.5 µg/m³ Medford, 21 December: 10.2 µg/m³ Yuba City Yuba City, 10 December: 24.7 µg/m³ Yuba City, 11 December: 22.5 µg/m³ Yuba City, 12 December: 14.7 µg/m³ Yuba City, 13 December: 16.2 µg/m³ Yuba City, 14 December: 17.0 µg/m³ Yuba City, 15 December: 17.5 µg/m³ Yuba City, 16 December: 27.7 µg/m³, 3.6× its usual 7.7 Yuba City, 17 December: 18.2 µg/m³ Yuba City, 18 December: 8.7 µg/m³ Yuba City, 19 December: 3.6 µg/m³ Yuba City, 20 December: 4.4 µg/m³ Yuba City, 21 December: 10.2 µg/m³ Chico Chico, 10 December: 21.7 µg/m³ Chico, 11 December: 19.0 µg/m³ Chico, 12 December: 18.3 µg/m³ Chico, 13 December: 18.6 µg/m³ Chico, 14 December: 20.2 µg/m³ Chico, 15 December: 23.4 µg/m³ Chico, 16 December: 25.0 µg/m³, 2.7× its usual 9.1 Chico, 17 December: 17.3 µg/m³ Chico, 18 December: 7.5 µg/m³ Chico, 19 December: 2.6 µg/m³ Chico, 20 December: 4.2 µg/m³ Chico, 21 December: 9.3 µg/m³ Redding Redding, 10 December: 13.8 µg/m³ Redding, 11 December: 18.5 µg/m³ Redding, 12 December: 8.3 µg/m³ Redding, 13 December: 5.2 µg/m³ Redding, 14 December: 12.1 µg/m³ Redding, 15 December: 10.5 µg/m³ Redding, 16 December: 13.4 µg/m³ Redding, 17 December: 45.9 µg/m³, 6.2× its usual 7.4 Redding, 18 December: 8.1 µg/m³ Redding, 19 December: 2.8 µg/m³ Redding, 20 December: 3.9 µg/m³ Redding, 21 December: 9.4 µg/m³ Porterville Porterville, 10 December: 16.7 µg/m³ Porterville, 11 December: 19.0 µg/m³ Porterville, 12 December: 23.7 µg/m³ Porterville, 13 December: 24.7 µg/m³ Porterville, 14 December: 18.9 µg/m³ Porterville, 15 December: 17.7 µg/m³ Porterville, 16 December: 18.9 µg/m³ Porterville, 17 December: 26.4 µg/m³, 2.4× its usual 11.2 Porterville, 18 December: 21.1 µg/m³ Porterville, 19 December: 3.7 µg/m³ Porterville, 20 December: 5.1 µg/m³ Porterville, 21 December: 7.1 µg/m³ 10 Dec 12 14 16 18 20 Every counted city, in the order the episode reached them, 3 days before to 3 after. Each square is a day, coloured by that day's PM2.5; outlined: at twice the city's usual or more. Good 0–50
Moderate 51–100
Sensitive groups 101–150
Unhealthy 151–200
Very unhealthy 201–300
Hazardous 301+
Every city it reached
City Days Highest Usual Above usual
Redding California, United States 1 17 December 7.4 6.2× Yuba City California, United States 1 16 December 7.7 3.6× Hanford California, United States 6 17 December 16.2 3.3× Modesto California, United States 5 17 December 13.3 3.3× Bakersfield California, United States 4 17 December 18.4 3.2× Stockton California, United States 5 17 December 12.9 3.1× Manteca California, United States 4 17 December 12.4 3.1× Turlock California, United States 5 16 December 14.0 2.9× Medford Oregon, United States 2 17 December 10.6 2.8× Arden-Arcade California, United States 4 17 December 10.5 2.8× Chico California, United States 1 16 December 9.1 2.7× Merced California, United States 4 15 December 14.1 2.6× San Jose California, United States 2 16 December 10.5 2.6× Fresno California, United States 6 17 December 19.0 2.5× Corcoran California, United States 1 13 December 14.8 2.5× Sacramento California, United States 1 14 December 10.1 2.5× Porterville California, United States 1 17 December 11.2 2.4× Visalia California, United States 6 15 December 16.8 2.3× Clovis California, United States 3 18 December 13.2 2.3× Madera California, United States 2 18 December 13.6 2.3×
Daily means in µg/m³, over the days each city counted in this episode. Highest: its highest of those days. Usual: the city's median for the time of year of that day (its days within 21 days of the date, every year, leaving out the week around it). Above usual: its highest day against that, with a usual under 5 µg/m³ taken as 5.
Against other episodes in the United States
Number 54 of 130 episodes in the United States since our records began, by cities reached.
Eastern United States and Canada, 25 June to 2 July 2023 280 North-eastern United States and Canada, July 2026 264 Eastern United States and Canada, May and June 2023 261 North-eastern United States and Canada, 30 July to 9 August 2025 241 Eastern United States and Canada, July 2023 240 North-eastern United States and Canada, 3 to 9 June 2025 196 Eastern United States and Canada, 14 to 19 June 2023 161 North-eastern United States and Canada, 12 to 16 July 2025 119 North-western United States and Canada, August 2023 110 North-eastern United States and Canada, 10 to 14 June 2025 104 North-western United States and Canada, 4 to 11 August 2026 99 North-eastern United States and Canada, 14 to 19 February 2026 96 California and Oregon, United States, December 2023 20 Bars: cities reached; colour: the level of the episode's highest daily reading. All 130 .
Use this page SmogRank, California and Oregon, United States, December 2023: 20 cities with PM2.5 at twice their usual or more, 13 to 18 December 2023, smogrank.com/episodes/2023-12-california-and-oregon-united-states (accessed 4 October 2026).
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You're welcome to reuse a single chart, image or figure with this citation and a link. Terms .
An episode is a run of days when many cities close to each other (four or more within 350 km on a day, 8 or more in all) each had a daily PM2.5 mean of at least 25 µg/m³ and at least 2 times their usual for that time of year. Found in our own measurements, not picked by hand. How we find episodes . Values are each city's daily mean of its own monitors; causes are the usual kinds for a pattern like this, not a confirmed source.
