Fog, smog, and PM2.5—what exactly are these three?
Release date:
2025-09-01
They first emerged during the First Industrial Revolution in London, England—and since then, fog and smog have become inseparable. What began as a bond of camaraderie among "loser" types has now evolved into something even deeper, transcending mere friendship to embrace a more intense, heartfelt connection: "Better to appear proud than to tremble in humble uncertainty!"
Well~~~
What causes smog and fog?
A lot of the time, is it really hard to tell if it’s fog?
Is it haze, or is it PM2.5?
Fog and haze were never related to begin with.
They first emerged during the First Industrial Revolution in London, England—and since then, fog and smog have become inseparable. What began as a bond of camaraderie among "loser" types has now evolved into something even deeper, transcending mere friendship to embrace a more intense, heartfelt connection: "Better to appear proud than to tremble in humble uncertainty!"
Well~~~
What causes smog and fog?
"Was it supposed to be a fleeting encounter, yet somehow lingering?"
It all starts with where you come from.
Fog is a natural phenomenon—a visible collection of tiny water droplets or ice crystals.
Smog has a complex origin: part of it stems from human activities, such as burning coal and charcoal, vehicle emissions, and construction-related dust; the other part comes from natural sources, like wildfires and volcanic eruptions.
Difference 1: Different relative humidity
Fog is wet, while haze is dry—think of one as "water-based," the other as "dry goods." Fog has a relative humidity greater than 90%, whereas haze features a relative humidity below 80%. When relative humidity falls between 80% and 90%, you’re dealing with a mixture of both fog and haze—but in such cases, haze typically remains the dominant component.
Difference two: Visibility differs.
The horizontal visibility in fog is less than 1 kilometer, while in haze it’s less than 10 kilometers.
Difference three: Different thicknesses.
The fog is only tens to about 200 meters thick, while haze can reach depths of 1 to 3 kilometers.
Difference four: Different colors.
The fog is milky white or bluish-white in color; haze, on the other hand, appears yellow or orange-gray.
Despite these differences, when grayish weather sets in, it remains challenging to strictly distinguish between fog and haze.
Because fog and haze often alternate or mix together, and throughout the day, they can even "switch roles," transforming into one another.
Finally, let’s talk about PM2.5. PM2.5 is just one of the main "raw materials" that make up smog.
PM2.5 particles are small in size, highly reactive, and easily carry toxic or harmful substances. Once inhaled into the human body, they can directly reach the bronchi, disrupting gas exchange in the lungs and potentially triggering conditions such as asthma, bronchitis, and cardiovascular diseases.
The PM2.5 value represents the mass concentration of particulate matter—the higher the number, the more pollutants are present, and the worse the air pollution is.
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