The Invisible Rain: Unraveling the Mystery of Nanjing's Precipitation Fog
When rain falls through dry air, it can create a weather phenomenon that challenges our very perception of fog.
Imagine driving through Nanjing on a winter day when rain begins to fall, yet instead of clearing the air, a thick fog mysteriously forms, reducing visibility to less than 200 meters. This paradoxical weather phenomenon, where precipitation seems to generate rather than dissipate fog, has long puzzled both residents and scientists alike.
Did You Know?
Precipitation fog represents approximately 20% of all fog events in the Nanjing region, presenting unique challenges to transportation and air quality management.
Recent comprehensive research has begun to unravel the mysteries behind its formation, revealing fascinating insights into the complex interactions between falling rain and atmospheric conditions that create this unusual weather event 1 .
When Rain Creates Fog: The Paradox Explained
Precipitation fog, sometimes called "rain fog," forms through a seemingly counterintuitive process. Unlike radiation fog that develops from overnight cooling or advection fog resulting from moist air moving over cold surfaces, precipitation fog occurs when rain droplets fall through dry air beneath cloud level.
As these raindrops descend into the drier layer, they begin to evaporate, increasing the moisture content in the surrounding air. This evaporation process simultaneously cools the air until it reaches its dew point temperature, triggering condensation and fog formation 1 .
The Evaporation-Cooling Mechanism
The essence of precipitation fog lies in this evaporation-cooling mechanism. As water changes from liquid to vapor, it requires energy, which it extracts from the surrounding air in the form of heat. This cooling effect, combined with the added moisture from evaporation, creates the perfect conditions for fog development.
Research from Nanjing indicates that this process typically requires specific atmospheric conditions: weak cold air intrusion, a strong temperature inversion layer that traps moisture, and consistent light precipitation 1 .
Rain Falls Through Dry Air
Rain droplets descend from clouds into a layer of drier air beneath
Evaporation Occurs
Droplets evaporate, adding moisture to the air while cooling it
Fog Forms
Air reaches dew point, triggering condensation and fog formation
The Nanjing Fog Experiment: Decoding the Phenomenon
To better understand precipitation fog, scientists conducted a comprehensive field study in Nanjing during the winters of 2006-2007. This ambitious research project employed multiple observation techniques simultaneously to capture both the macro and micro aspects of fog formation 1 .
Methodology: A Multi-faceted Approach
The research team deployed an array of sophisticated instruments at the Nanjing University of Information Science and Technology observation site:
Boundary Layer Profiling
Using a tethered balloon system to measure temperature, pressure, humidity, and wind parameters at different altitudes
Fog Microphysics Monitoring
With an FM-100 fog droplet spectrometer to record droplet size distribution, number concentration, and liquid water content
Vertical Wind Measurements
Utilizing a planar phased array acoustic radar system to document three-dimensional wind fields up to 1000 meters
Continuous Weather Tracking
Through automated weather stations and visibility sensors recording conditions every second 1
This comprehensive approach allowed scientists to analyze precipitation fog from multiple perspectives simultaneously, from the large-scale weather patterns down to the microscopic droplets that compose the fog itself.
Key Findings: The Structure of Precipitation Fog
The research revealed several critical aspects of precipitation fog's structure and behavior:
- Formation Conditions: Analysis of 14 separate precipitation fog events showed they predominantly occurred under three specific weather patterns: frontal systems (most common), cold high-pressure systems, and low-pressure troughs 1
- Thermal Structure: A defining feature was the presence of a strong temperature inversion layer in the boundary layer, where upper-level air became warmer than surface-level air, creating a "lid" that trapped moisture 1
- Microphysical Properties: Precipitation fog demonstrated notably smaller number density, liquid water content, and average droplet diameter compared to other fog types. The droplet size distribution showed a very narrow spectrum that decreased exponentially with increasing diameter 1
| Weather Pattern Type | Frequency | Key Characteristics |
|---|---|---|
| Frontal Systems | Most common | Weak cold air invasion, temperature inversion |
| Cold High-pressure Systems | Common | Stable atmospheric conditions, light winds |
| Low-pressure Troughs | Less common | Moisture convergence, uplift mechanisms |
Inside the Fog: Microscopic Structure and Characteristics
At the heart of precipitation fog research lies the analysis of its microphysical properties—the characteristics of the individual fog droplets that collectively determine the fog's behavior and impact.
The Nanjing study revealed that precipitation fog droplets are exceptionally small, with a narrow size range. The average droplet spectrum follows what scientists call a Deirmendjian distribution, where the number concentration drops exponentially as droplet size increases . This means the fog consists predominantly of tiny droplets, with progressively fewer larger droplets.
| Fog Type | Number Concentration (cm³) | Liquid Water Content (g/m³) | Average Diameter (μm) |
|---|---|---|---|
| Precipitation Fog | Lower than other types | Smaller than other types | Smaller than other types |
| Radiation Fog | Higher values | Larger values | Larger values |
| Advection Fog | Moderate values | Moderate values | Moderate values |
Visibility and Light-Scattering Properties
The small droplet size and concentration in precipitation fog have direct implications for its light-scattering properties and effect on visibility. According to recent research, the reduction in visibility during fog events is primarily determined by the number concentration of hydrated aerosol particles and their size distribution 6 . The relationship between droplet characteristics and visibility follows the formula:
Visibility Formula
Visibility = 3.912 / (∑(Droplet Cross-sectional Area × Number Concentration × Scattering Efficiency))
This mathematical relationship explains why even a modest concentration of appropriately-sized droplets can dramatically reduce visibility, creating hazardous transportation conditions 6 .
The Scientist's Toolkit: Instruments for Fog Research
Modern fog research relies on specialized equipment designed to capture the delicate and transient nature of fog droplets. The instruments used in the Nanjing study represent the cutting edge of atmospheric monitoring technology:
| Instrument | Primary Function | Key Features |
|---|---|---|
| FM-100 Fog Droplet Spectrometer | Measures size distribution of fog droplets | Detection range of 2-50 μm; provides data on number concentration, liquid water content |
| Tethered Balloon System | Profiles atmospheric conditions at different altitudes | Measures temperature, pressure, humidity, wind parameters through boundary layer |
| Planar Phased Array Acoustic Radar | Records 3D wind fields | Monitors wind patterns up to 1000 meters altitude |
| CASCC-1 Cloud Water Collector | Collects size-fractionated fog water samples | Separates droplets into size categories (4-16 μm, 16-22 μm, >22 μm) for chemical analysis |
These tools enable researchers to move beyond simple observation to comprehensive analysis of fog formation, duration, and dissipation processes. The integration of data from these various instruments provides a holistic understanding of precipitation fog that would be impossible with any single measurement approach 1 7 .
Field Measurement Setup
Researchers deploy specialized instruments to capture detailed data on fog formation and characteristics.
Data Analysis
Scientists analyze complex datasets to understand the microphysical properties of precipitation fog.
Fog in a Changing Climate: Implications and Future Research
The study of precipitation fog extends beyond academic curiosity, with significant implications for transportation safety, air quality, and understanding regional climate patterns. The persistent fogs that blanketed Nanjing in December 2006—including a super-dense fog with visibility below 50 meters that lasted over 40 hours—demonstrate the potential severity of these events .
Transportation Impact
Reduced visibility during precipitation fog events creates hazardous conditions for road, air, and maritime transportation, leading to delays, accidents, and economic impacts.
Future Research Directions
As climate patterns shift and air quality changes, understanding the behavior of precipitation fog becomes increasingly crucial. Current research focuses on improving fog prediction models, with recent studies incorporating both adiabatic and radiative cooling processes to better simulate droplet formation 3 . These enhanced models promise more accurate forecasting of fog events, potentially mitigating their disruptive impacts on transportation and economic activity.
The investigation into Nanjing's precipitation fog continues, with scientists building upon these findings to develop increasingly sophisticated understanding of this captivating meteorological phenomenon. Each research campaign brings new insights, gradually transforming what was once a mysterious occurrence into a predictable, understandable element of our atmospheric system.
Looking to the Future
The study of precipitation fog stands as a testament to how seemingly simple natural phenomena can reveal astonishing complexity when examined with scientific rigor and curiosity.