Technical article

Santiago in Winter: Smog Days, and How Other Chilean Cities Compare

Winter days are mild and nights are cold, and the rain falls in a few months.

By Neil Solomon · 3 min read

Picture a couple named Ruth and Ian. Both are 64, and they love the idea of Santiago: a big city with mountains at the end of the street. Then a friend mentions winter smog. They would like numbers, not stories.

What winter looks like in Santiago

The national weather service, the Dirección Meteorológica de Chile, publishes 30-year averages. For the Quinta Normal station in Santiago, 1991 to 2020:

Yearly rain is 286.3 mm. June to August brings 174.9 mm of that (my arithmetic). Winter days are mild and nights are cold, and the rain falls in a few months.

How the ministry counts bad air days

The Ministerio del Medio Ambiente watches fine particles, called PM2.5, and uses the 24-hour average. Its levels, in micrograms per cubic metre:

• Good: up to 50
• Regular: 51 to 79
• Alert: 80 to 109
• Pre-emergency: 110 to 169
• Emergency: 170 or more

Its 2026 operating plan runs the special management season from 1 May to 31 August. On Alert days the plan includes a ban on all residential heating that burns wood. On Pre-emergency and Emergency days the ban also covers wood pellets. Vehicle restrictions apply as well; the text I read gave the heading but not the rules.

What 2025 looked like

The ministry's September 2025 report covers 1 January to 30 September. The figures are marked as preliminary. For Santiago and the Metropolitan Region:

• 61 days over the 50 limit
• 196 good days and 54 regular days
• 20 Alert episodes, 3 Pre-emergency episodes and no Emergency episodes

How other cities compare

Episodes from the same report, 1 January to 30 September 2025:

I picked these nine from 18 cities in the report. The report also lists Curicó, Linares, Cauquenes, San Carlos, Chillán, Los Ángeles, Curanilahue, La Unión and Aysén. Some cities had Emergency days as well. Los Ángeles had 10 and Linares 5, so their totals are higher than Alert plus Pre-emergency alone.

Santiago, with about 23 episodes, sits in the lower half. Several southern cities, where wood is a common heating fuel, record more. I do not have a page that says why, so I leave the reason open.

What is left open

• Yearly average particle levels. The report I read lists episodes, not averages.
• Coastal cities such as Valparaíso, Viña del Mar and La Serena. They were not in the list of monitored cities I read.
• Official health guidance for people over 60.
• Complete 2026 counts.