Calm

In geography and meteorology, a calm refers to a condition or region of the atmosphere or ocean where wind speeds are very low or nearly absent. It represents a state of atmospheric stability, often associated with areas of high pressure, descending air, or equilibrium between opposing wind systems. Calms are characteristic of certain latitudinal zones and local weather phenomena, and they play a significant role in understanding global wind circulation and climatic patterns.

Definition and Concept

The term calm denotes an atmospheric state in which air movement is minimal, usually defined as wind speeds below 1 knot (approximately 1.85 km/h). In meteorological observations, a calm is recorded when the wind direction cannot be determined due to its weakness.
Calms occur both locally, as temporary weather conditions, and regionally, as part of the planetary wind system. On a global scale, calms correspond to zones where air converges or subsides, resulting in weak or variable winds.

Causes of Calms

Calms arise due to several physical processes affecting the movement and pressure of air masses:

1. Atmospheric Pressure Systems:
   • High-pressure systems (anticyclones) often produce calm conditions as air descends and diverges outward, suppressing strong horizontal movement.
   • The descending air warms adiabatically, creating stable atmospheric conditions that reduce vertical motion and turbulence.
2. Equatorial Convergence:
   • Near the equator, calms occur where trade winds from both hemispheres converge and neutralise one another, creating a zone of weak winds and frequent convection.
3. Temperature and Pressure Gradients:
   • When temperature differences between adjacent air masses are small, the resulting pressure gradients are weak, producing minimal wind.
4. Topographical and Local Influences:
   • Valleys, basins, and sheltered areas often experience calm conditions due to the obstruction of prevailing winds or nocturnal cooling that stabilises air near the ground.

Major Calm Zones of the Earth

On a global scale, two principal calm zones are recognised as part of the Earth’s general circulation of the atmosphere:

1. Equatorial Calm or Doldrums:
   • Found near the Equator (around 5° N to 5° S latitude).
   • Known scientifically as the Intertropical Convergence Zone (ITCZ).
   • Characterised by light and variable winds, intense heat, and frequent thunderstorms due to rising moist air.
   • Historically dreaded by sailors, as sailing ships could be trapped for days without wind.
2. Subtropical Calms or Horse Latitudes:
   • Occur around 30° N and 30° S latitudes, forming part of the subtropical high-pressure belts.
   • In the Northern Hemisphere, the region is known as the Calms of Cancer; in the Southern Hemisphere, it is called the Calms of Capricorn.
   • Characterised by descending dry air, light or variable winds, and clear skies, resulting in arid conditions and the formation of subtropical deserts.

Characteristics of Calms

• Low Wind Velocity: Wind speeds are extremely weak or absent, with little horizontal air movement.
• Atmospheric Stability: Air masses are stable, with limited convection or vertical motion.
• High Pressure: In subtropical regions, calms coincide with zones of high barometric pressure.
• Temperature Effects: Depending on location, calms may lead to either extreme heat (as in the equatorial doldrums) or clear, dry conditions (as in subtropical regions).
• Humidity and Precipitation: Varies with latitude—high humidity and heavy rainfall near the equator; low humidity and aridity near 30° latitudes.

Geographic Distribution

Calm zones occur in distinct latitudinal belts associated with global wind circulation:

Seasonal shifts of the Sun and global circulation cause these calm belts to migrate slightly northward or southward, following the solar zenith.

Climatic and Environmental Effects

1. Desert Formation:
   • Subtropical calm zones promote aridity due to descending dry air, leading to the formation of major deserts such as the Sahara, Arabian, Kalahari, and Atacama.
2. Equatorial Rain Belts:
   • In the equatorial calms, ascending moist air leads to heavy rainfall, supporting the tropical rainforest biome.
3. Weather Predictability:
   • Calm zones are regions of stable weather, often characterised by clear skies and limited atmospheric disturbance.
4. Impact on Navigation:
   • Historically, calm belts posed challenges for maritime navigation, as sailing vessels could remain stationary for long periods without wind propulsion.

Examples of Local and Temporary Calms

Apart from global calm zones, localised or temporary calm conditions may occur due to diurnal and topographical effects:

• Night-time Radiational Calms: During clear nights, surface cooling reduces vertical air mixing, creating calm conditions in valleys or lowlands.
• Sheltered Basin Calms: Mountain basins or enclosed valleys experience calm weather when surrounded by barriers that block prevailing winds.
• Sea and Land Breezes: Transitional periods between sea and land breeze cycles may create short-lived calms when opposing airflows balance each other.

Significance in Geography and Meteorology

The study of calm zones is vital for understanding:

• Global circulation and energy balance: Calms mark areas where atmospheric convection either originates or terminates.
• Climate classification: Calm belts define the equatorial (Af) and subtropical (BW, BS) climate zones in Köppen’s classification.
• Weather forecasting: Knowledge of calm regions aids in predicting wind systems and pressure movements across latitudes.
• Marine and aviation meteorology: Awareness of calm regions is essential for route planning, as they influence air and sea travel efficiency.