Dense fog is a common winter phenomenon in northern India, particularly across the Indo-Gangetic Plains. It frequently disrupts rail and air transport, leading to delays, cancellations, and safety risks. The phenomenon is best understood by examining its physical causes and the reasons it affects transport operations more severely during winter.
Physical Causes of Dense Fog in Northern India
- Radiational Cooling in Winter
During winter, long nights and weak solar radiation lead to rapid loss of heat from the ground. This causes surface temperatures to fall close to the dew point, resulting in condensation of water vapour and fog formation. - High Moisture Availability
The Indo-Gangetic Plains have abundant sources of moisture, including rivers, canals, wetlands, and irrigated agricultural fields. Residual soil moisture after the monsoon further increases near-surface humidity. - Calm Wind Conditions
Winter is characterised by weak pressure gradients and light winds. The absence of strong winds prevents horizontal and vertical dispersion of moisture-laden air, allowing fog to persist. - Temperature Inversion
A strong surface inversion often develops in winter, with cold air trapped near the ground beneath warmer air aloft. This inversion suppresses vertical mixing and traps moisture and aerosols close to the surface. - Role of Air Pollution
High concentrations of particulate matter from vehicular emissions, industrial activity, and crop residue burning act as condensation nuclei. This enhances fog density and duration, often converting fog into smog.
Why Fog Disrupts Transport More During Winter
- Severe Reduction in Visibility
Dense fog can reduce visibility to below 50 metres. Such conditions compromise safety and necessitate speed restrictions in both rail and air transport. - Impact on Rail Transport
Railways depend on visual signalling and safe braking distances. Poor visibility increases accident risk, forcing trains to operate at reduced speeds and causing cascading delays across the network. - Impact on Air Transport
Aircraft require minimum visibility for safe take-off and landing. Although airports use Instrument Landing Systems (ILS), extremely dense fog often exceeds operational limits, resulting in diversions and cancellations. - Longer Persistence
Winter fog tends to last from night to late morning or afternoon due to weak sunlight and inversion conditions. This coincides with peak travel hours, intensifying disruption.
Conclusion
Dense fog in northern India arises from a combination of winter cooling, high moisture content, calm winds, temperature inversion, and elevated pollution levels. Its prolonged persistence during winter severely impacts rail and air transport operations. Addressing this issue requires better weather forecasting, pollution mitigation, and technological upgrades in transport infrastructure.
This is sample answer for UPSC Question: Fog-induced disruptions in northern India significantly impact both rail and air transport systems. Discuss the physical causes of dense fog in northern India and explain why this weather phenomenon disproportionately affects transport operations during winter.
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