How Nanjing's Precipitation Fog Forms
Imagine trillions of tiny water droplets, each one a complex chemical factory, floating in the air and reducing visibility to less than 1 kilometer. This is the fascinating reality of fog, a common meteorological phenomenon in the Yangtze River Delta region, including Nanjing, that impacts everything from transportation to air quality.
Fog is officially defined as water droplets or ice crystals suspended in the atmosphere that reduce horizontal visibility to less than 1,000 meters. While often mistaken for low-lying clouds, fog represents a unique atmospheric environment where complex physical and chemical processes interact in ways scientists are still working to fully understand.
Forms when falling rain evaporates into drier air below the cloud, increasing humidity until saturation occurs.
Forms on clear, calm nights when the ground cools, cooling the air above to its dew point.
Occurs when warm, moist air moves over a cooler surface, cooling the air to its dew point.
At the heart of fog formation are tiny atmospheric particles called aerosols, which serve as Fog Condensation Nuclei (FCN). These microscopic particles, ranging from sea salt to pollution particles, provide the essential surfaces upon which water vapor can condense.
The transformation of a dry aerosol particle into a fog droplet is elegantly described by Köhler theory, which explains how particles take up water vapor as humidity increases 3 .
Particles absorb water and grow steadily through deliquescence
Particles reach a stable size but aren't yet activated
Particles pass a critical supersaturation point and activate into growing fog droplets
This activation process is crucial—it creates the larger droplets that substantially reduce visibility and define true fog conditions 3 . The chemical composition of the original aerosol particle significantly influences how easily it activates, with soluble components like salts making more efficient condensation nuclei.
A landmark international research initiative called SOFOG3D (SOuth west FOGs 3D experiment for processes study) has provided unprecedented insights into the complex structure of fog layers through innovative measurement techniques .
During the winter of 2019-2020, researchers employed an integrated observation strategy:
The SOFOG3D data revealed surprising patterns in how liquid water content (LWC) distributes through fog layers:
| Fog Type | LWC Profile Pattern | Atmospheric Conditions | Frequency of Observation |
|---|---|---|---|
| Thin Fog | Decreasing with height | Stable temperature profile | Common in early formation |
| Thick Fog | Increasing with height | Slightly unstable, well-mixed | Mature development phase |
| Transitional | Highly variable | Shifting stability | ~50% of cases |
The transition from thin to thick fog proved more complex than models predicted, with only about 50% of observed events making this transition .
Beyond its physical structure, fog represents a remarkable chemical processing environment where fascinating transformations occur.
Fog droplets act as micro-reactors where inorganic gases like SO₂ and NOx dissolve and oxidize to form sulfuric and nitric acids 2 . These acidic solutions can then be neutralized by ammonia, forming ammonium sulfate and nitrate salts. The chemical concentrations in fog water can be up to 100 times higher than in rain, owing to the smaller droplet size and longer atmospheric residence time 2 .
| Chemical Species | Typical Sources | Transformation Process | Environmental Impact |
|---|---|---|---|
| Sulfate (SO₄²⁻) | SO₂ from fossil fuel combustion | Aqueous oxidation in droplets | Acid deposition, aerosol formation |
| Nitrate (NO₃⁻) | NOx from vehicles and industry | Dissolution and oxidation | Ecosystem fertilization |
| Ammonium (NH₄⁺) | Agricultural emissions | Neutralization of acidity | Alters nutrient cycles |
| Organic Compounds | VOCs from various sources | Aqueous phase reactions | Secondary organic aerosol formation |
Recent studies using single-particle analysis techniques have revealed that different types of aerosol particles are preferentially activated into fog droplets based on their size and chemical composition 5 . This selective processing means that fog doesn't just reflect local air quality—it actively transforms it by scavenging particular pollutants and potentially generating new aerosol particles when droplets evaporate.
Contemporary fog researchers employ sophisticated tools to unravel fog's mysteries:
| Instrument | Primary Function | Key Measurements | Deployment Method |
|---|---|---|---|
| Counter-Flow Virtual Impactor (CVI) | Separates fog droplets from interstitial particles | Cloud droplet residual composition | Aircraft or ground-based |
| Tethered Balloon Profiling System | Vertical measurements through fog layer | LWC, temperature, humidity profiles | Balloon-borne in-situ sensors |
| Single Particle Mass Spectrometer | Analyzes individual particle composition | Size and chemical fingerprints of residuals | Online or offline analysis |
| Microwave Radiometer | Remote sensing of liquid water | Liquid water path (LWP), humidity | Ground-based continuous monitoring |
| Cloud Radar/Lidar Systems | 3D mapping of fog structure | Fog top height, droplet distribution | Remote sensing vertical profiles |
Understanding fog formation and structure has never been more critical. As climate patterns shift and urban areas like Nanjing continue to grow, fog frequency and characteristics are changing. Research shows that both climate change and improving air quality are reducing fog frequency in many regions 3 .
Improved fog forecasting could prevent accidents in aviation, marine, and land transportation.
Fog harvesting technologies are being deployed in water-scarce regions worldwide.
Understanding fog's role in processing pollutants helps develop better emission control strategies.
More accurate fog representation improves regional climate projections.
For residents of Nanjing, understanding precipitation fog formation provides insights into those days when drizzly weather transforms into thick fog, impacting daily life but also quietly cleansing the air of pollutants through scavenging and deposition processes.
Fog represents one of nature's most fascinating intersection points—where physics meets chemistry, where aerosols transform into droplets, and where humble water vapor becomes both a visibility hazard and an atmospheric purifier. Research initiatives like SOFOG3D have revealed that fog's internal structure is far more complex than previously imagined, with distinct vertical profiles that evolve throughout the fog life cycle.
The next time you find yourself in a thick fog in Nanjing, remember that you're witnessing not just a meteorological phenomenon but a dynamic chemical processing system—one that scientists are only beginning to fully understand. Each droplet contains a story of atmospheric interactions that connects human activities with natural processes in the air we breathe.