If you've stepped outside on a smoky morning and thought the air felt worse than it did the day before, you're not imagining it. Wildfire smoke follows a predictable daily rhythm, and morning is usually its worst phase.

Right now, that pattern is playing out across central and northeastern Washington, where smoke from the Little Giant and Sinlahekin Fires has triggered an air quality alert covering 33 cities across Chelan, Douglas, Ferry, Grant and Okanogan counties.

(MORE: Bad air quality? Here’s what you can do)

Wenatchee has logged multiple mornings with hazardous readings — at an Air Quality Index (AQI) of 301 or higher — only to see conditions ease later in the day.

The mechanism: a nightly lid on the air

The reason comes down to a process called a temperature inversion. 

On clear, calm nights, the ground loses heat quickly while the air above it stays relatively warm. Because warm air is less dense, it settles on top like a lid, and the cooler air underneath — along with any smoke mixed into it — has nowhere to go. 

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(MORE: When you can open your windows when there is wildfire smoke or poor air quality outside)

Meteorologists call the space where air can move freely and disperse pollution the "mixing height." Above the mixing height is the inversion or lid. Overnight, that lid sinks, sometimes to just a few hundred feet, trapping smoke close to the ground.

In Washington, smoke often settles into valleys after dark as mixing heights decrease.

Once the sun comes up and starts heating the ground, the inversion weakens from the bottom. As the ground-level air warms, it becomes buoyant enough to mix out that lid, mixing heights climb, and the trapped smoke finally has room to disperse. 

Why this matters for your day

This is why an AQI forecast can look alarming at 7 a.m. and far more manageable by early afternoon — the actual amount of smoke in the region may not have changed much, but how well it's mixing through the atmosphere has. 

For anyone timing a run, a commute or school drop-off around wildfire smoke, mornings — especially in valleys and low-lying areas — are consistently the riskiest window, and conditions tend to improve as the day goes on.

(MORE: How air pollution affects you)

There's a health reason to care about that timing beyond just visibility and irritation. One study of a Montana community trapped for 35 straight days at "very unhealthy" or worse levels — largely due to this same valley-inversion pattern — found lung function effects in residents that persisted well after the smoke cleared.

Beyond this event

The Pacific Northwest is this week's example, but the mechanism isn't specific to Washington, or to wildfire smoke season in general. 

The same inversion-and-mixing-height cycle governs smoky mornings in California, Colorado and anywhere else smoke settles overnight. 

(MORE: Wildfire smoke myths: What actually protects you — and what doesn't)

Which means the guidance holds any time a wildfire is producing regional haze: check the forecast, expect the worst air in the early hours and expect it to ease as the day warms up.

Content writer Ada Wood enjoys exploring the stories that science and climate teach us about our natural world and how it influences the way we live in it.