Dense to very dense fog conditions are continuing over Punjab, Haryana, and Chandigarh.

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Formation of Fog and Heat Transfer in the Atmosphere

Fog forms when air at or near the Earth’s surface becomes saturated. This saturation occurs through cooling, addition of moisture, or mixing with another air mass. It usually develops in a stable air mass because limited vertical motion allows moisture to accumulate near the surface. In contrast, unstable air promotes vertical mixing and produces clouds rather than fog.

Meteorologists often describe fog as a stratus cloud at ground level. It develops when the air temperature and dew point move close to the same value, allowing water vapour to condense into tiny droplets.

Types of Fog

1. Advection
Advection fog develops when warm, moist air moves horizontally over a colder surface. The surface cools the air to its dew point, causing condensation. This forms over both land and water, such as when warm air flows over a cold sea surface.

2. Radiation (Ground or Valley Fog)
Radiation fog forms during clear, calm nights. The ground loses heat through long-wave radiation, which cools the air just above it. This cooling creates a temperature inversion and brings moist air to its dew point. Moist soil often adds water vapour, which speeds up fog formation. Valleys experience this type more frequently because cold air settles in low-lying areas.

Heat Transfer Processes in the Atmosphere

Conduction

Conduction transfers heat through direct contact. When the land warms up, it heats the air touching its surface. This process mainly affects the lowest layers of the atmosphere and plays a limited but important role in surface heating.

Convection

Convection involves vertical movement of air. Warm air near the Earth’s surface rises, while cooler air sinks to replace it. These vertical currents transport heat upward and operate mainly within the troposphere.

Advection

Advection transfers heat through the horizontal movement of air. Winds carry warm or cold air from one region to another, causing most day-to-day weather changes, especially in the middle latitudes.

Mist and Visibility

Mist contains fewer water droplets than fog and allows better visibility. It reduces visibility more sharply because it has a higher concentration of suspended droplets.

Temperature Inversion

Normally, air temperature decreases with height. A temperature inversion occurs when warm air overlies cooler air near the surface. This situation commonly develops at night when the ground cools rapidly after sunset. Inversions trap cool, moist air near the surface and favour fog formation.