Inside a smog dome

Popular Science - - CHARTED -

IN SALT LAKE CITY, UTAH, RES­I­DENTS CALL WIN­TER “INVERSION SEA­SON” for its ten­dency to bring on a strange weather con­di­tion: Snow­storms in­vert the usual trend of air get­ting colder as the al­ti­tude gets higher by cool­ing the ground, which leaves be­hind a layer of warm air high above the me­trop­o­lis. An inversion cre­ates a sort of at­mo­spheric roof while the moun­tains sur­round­ing the city act like walls, trap­ping pol­lu­tion in place un­til an­other storm blows it away. The re­sult­ing “smog dome” lasts four days on av­er­age, but some have lin­gered al­most three weeks. These pe­ri­ods aren’t just dreary; they’re dan­ger­ous. Here’s a peek into the dome.

A Dome Is Born

Nor­mally, higher al­ti­tudes have colder air, but a cou­ple of sce­nar­ios can set a warm-air “lid” over an area. Salt Lake City‘s in­ver­sions usu­ally fol­low snow­storms. Other lo­cales more of­ten get night­time ra­di­a­tion in­ver­sions, when Earth gives off more heat than it re­ceives from the sun.

Peak Prob­lems

Smog domes can hap­pen any­where but oc­cur more of­ten in ci­ties—such as Boise, Idaho, and Los An­ge­les—that lie near moun­tain ranges, which trap cold air in val­leys. Domes also tend to hit ar­eas that get heavy snow­fall, which makes air near the ground cooler than what’s above.

Some­thing in the Air

In­ver­sions trap fine par­ti­cles known as PM2.5, pro­duced by car en­gines, bon­fires, and in­dus­trial emis­sions. The buildup of these and other pollutants, such as ozone, can re­duce vis­i­bil­ity, in­crease the acid­ity of wa­ter, and de­plete soil nu­tri­ents.

Bated Breath

In­hal­ing PM2.5 can ex­ac­er­bate health is­sues such as asthma and con­ges­tive heart fail­ure: When­ever Salt Lake City has an inversion, emer­gen­cy­room vis­its spike. And the longer it lasts, the sicker peo­ple get. If con­cen­tra­tions rise too high, stu­dents stay in­doors and adults telecom­mute.


Hu­mans can’t stop in­ver­sions. But we can cut down on dome­fill­ing emis­sions. Salt Lake aims to in­crease car-ef­fi­ciency stan­dards and re­duce traf­fic with bet­ter public-trans­porta­tion op­tions. Of­fi­cials hope that small mea­sures like these, taken to­gether, will lighten the dome by 2040.

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