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Winter Inversion Guide

Temperature inversions trap cold air and pollution near the ground in valleys and basins during winter months. Cities in mountain valleys can experience some of the worst air quality in the nation when inversions persist for days or weeks.

Dec-Feb

Peak Inversion Season

3-14 Days

Typical Duration

Valley Cities

Most Affected

PM2.5

Primary Pollutant

What is a Temperature Inversion?

Normally, air temperature decreases with altitude - warm air at ground level rises and disperses pollution. During a temperature inversion, a layer of warm air sits above cold air near the ground, creating a lid that traps pollution in place. This can cause air quality to deteriorate rapidly in affected areas.

Normal Conditions

• Ground level: Warm air

• Higher altitude: Cold air

• Warm air rises naturally

• Pollution disperses vertically

• Good air quality maintained

Inversion Conditions

• Ground level: Cold air trapped

• Higher altitude: Warm air lid

• Cold air cannot rise

• Pollution accumulates daily

• Air quality deteriorates rapidly

How Inversions Form

1. Cold Clear Nights

Ground radiates heat into space. Air near ground cools rapidly. Creates initial temperature inversion.

2. High Pressure System

Descending air warms and compresses. Creates warm layer aloft. Prevents mixing with ground-level air.

3. Stagnant Conditions

Low wind speeds. No weather systems. Inversion persists for days. Pollution builds continuously.

Cities Most Affected by Inversions

Severe Inversion Areas

Salt Lake City, UT

Worst inversions in the nation. Valley location surrounded by mountains. Can last 2+ weeks. PM2.5 can reach 100+ AQI during severe events.

Peak: December-January

Provo, UT

Similar to Salt Lake with even worse geography. Narrow valley traps pollution effectively. Regular Code Red days during winter.

Peak: December-February

Fairbanks, AK

Extreme cold creates strong inversions. Wood smoke from heating dominates. Ice fog reduces visibility. AQI often Unhealthy for weeks.

Peak: December-January

Moderate Inversion Areas

Fresno, CA

San Joaquin Valley location creates frequent inversions. Agricultural activities add to pollution load. Worst during December-January.

Peak: December-January

Phoenix, AZ

Valley of the Sun experiences occasional winter inversions. Less severe than northern valleys but still causes elevated PM2.5.

Peak: January

Denver, CO

Front Range location sees periodic inversions. Brown cloud visible during events. Usually breaks within a few days.

Peak: December-January

Typical Inversion Event Timeline

Day 1

Inversion Formation

Clear, calm night allows ground to cool. Morning AQI: Good to Moderate (30-60). Light haze may be visible. Weather forecast shows high pressure settling in.

Day 2-3

Pollution Builds

Inversion persists, pollution accumulates. AQI rises to Moderate to Unhealthy for Sensitive Groups (60-120). Visible haze layer. Morning and evening traffic creates spikes. Sensitive individuals notice symptoms.

Day 4-7

Unhealthy Conditions

AQI reaches Unhealthy to Very Unhealthy (120-200+). Thick visible smog. Strong odor of pollution. Health advisories issued. Schools may cancel outdoor activities. General population affected.

Day 8+

Severe Pollution Event

Extended inversions create dangerous conditions. AQI may exceed 200 (Very Unhealthy to Hazardous). Emergency advisories. Stay indoors recommendations. Hospitals see increased respiratory cases.

Break

Weather System Arrives

Wind or precipitation breaks inversion. Cold front or storm system moves through. Air mixes and pollution disperses. AQI drops to Good within hours. Dramatic visible improvement as smog clears.

Health Impacts & Protection

Health Effects by AQI Level

Moderate (50-100)

Sensitive groups may experience minor irritation. Slight cough possible for those with asthma.

Unhealthy for Sensitive (100-150)

Increased symptoms for people with respiratory conditions. Children and elderly should limit prolonged outdoor exertion.

Unhealthy (150-200)

Everyone may experience health effects. Sensitive groups experience serious effects. Avoid outdoor activities.

Very Unhealthy (200+)

Health alert. Everyone should avoid outdoor exertion. Serious respiratory effects expected for sensitive groups.

Survival Strategies

✓Stay Indoors: Keep windows and doors closed. Run HVAC on recirculate mode.
✓Use Air Purifiers: HEPA filters remove PM2.5. Place in bedrooms and main living areas.
✓Avoid Exercise: Move workouts indoors. Gym or home exercise until inversion breaks.
✓Reduce Emissions: Limit driving. No wood burning. Avoid idling vehicles. Every bit helps.
✓Wear N95 Masks: If must go outside, properly fitted N95 protects against PM2.5.
✓Monitor Conditions: Check AQI multiple times daily. Plan activities for breaks in pollution.
✓Have Rescue Meds: Keep asthma inhalers filled. Consider staying with friends in cleaner areas.
✓Travel if Severe: If inversions last 10+ days, consider temporary relocation to nearby areas above inversion layer.