Geography: Class 112 — Chapter 9: Atmospheric Circulation and Weather Systems

The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. It is expressed in units of millibar (mb). At sea level, the average atmospheric pressure is 1,013.25 mb.

Air pressure is measured using a mercury barometer or an aneroid barometer. Pressure decreases with height, which is the primary cause of air motion (wind) from high-pressure areas to low-pressure areas.

In the lower atmosphere, pressure decreases rapidly with height at a rate of about 1 mb for every 10 m increase in elevation. However, the vertical pressure gradient force is balanced by the gravitational force, preventing strong upward winds.

Horizontal distribution of pressure is studied using isobars, which are lines connecting places of equal pressure reduced to sea level.

Wind is air in horizontal motion, blowing from high to low pressure. Its velocity and direction are determined by the combined effect of three forces:

• Pressure Gradient Force: The rate of change of pressure with respect to distance. It is strong where isobars are close and weak where they are far apart.
• Frictional Force: Affects wind speed near the surface, extending up to 1–3 km. It is minimal over sea surfaces.
• Coriolis Force: Caused by the Earth's rotation, deflecting winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It is maximum at the poles and zero at the equator.

In the upper atmosphere (2–3 km above the surface), winds are free from friction and controlled by the pressure gradient and Coriolis forces. When these forces balance, the wind blows parallel to the isobars, known as the geostrophic wind.

Wind circulation around a low-pressure system is called cyclonic circulation, while circulation around a high-pressure system is called anticyclonic circulation.

The global pattern of planetary winds is driven by latitudinal heating, pressure belts, the sun's migration, and Earth's rotation. This circulation is structured into three major cells:

• Hadley Cell: Formed by rising warm air at the Inter Tropical Convergence Zone (ITCZ) that sinks at subtropical high-pressure belts (30° N and S) and flows back to the equator as easterlies.
• Ferrel Cell: Characterized by sinking air from the poles rising at subpolar lows, producing prevailing westerlies at mid-latitudes.
• Polar Cell: Formed by cold air subsiding at the poles and flowing towards mid-latitudes as polar easterlies.

The general circulation of the atmosphere drives ocean currents. A key interaction is the El Niño Southern Oscillation (ENSO), where the warming of the central Pacific Ocean (El Niño) alters pressure systems (Southern Oscillation), causing global weather anomalies like droughts in Australia/India and heavy rains in South America.

Diurnal heating and cooling cycles generate localized wind patterns:

• Land and Sea Breezes: During the day, land heats faster, creating low pressure and drawing in a sea breeze. At night, the land cools faster, reversing the gradient to create a land breeze.
• Mountain and Valley Winds: Daytime heating causes air to move upslope (valley breeze). At night, cold dense air flows downslope into the valley (mountain wind or katabatic wind).

An air mass is a large body of air with relatively uniform temperature and humidity. They form over vast homogenous surfaces called source regions, classified into:

• Maritime tropical (mT) - Warm and humid
• Continental tropical (cT) - Warm and dry
• Maritime polar (mP) - Cold and humid
• Continental polar (cP) - Cold and dry
• Continental arctic (cA) - Extremely cold and dry

The boundary zone where two different air masses meet is called a front. The process of front formation is frontogenesis. Fronts are classified into four types:

1. Cold Front: Formed when cold air moves towards warm air, forcing the warm air upward.
2. Warm Front: Formed when warm air moves actively over cold air.
3. Stationary Front: When the boundary between air masses remains immobile.
4. Occluded Front: Formed when an active cold front overtakes a warm front, lifting the warm air completely off the ground.

These systems develop in mid and high latitudes along the polar front. They feature distinct warm and cold fronts, cover large areas, can originate over both land and sea, and generally move from west to east.

Violent, destructive storms that originate over warm tropical oceans (sea surface temperature > 27°C) and dissipate upon reaching land (landfall). They are known by different regional names:

• Cyclones in the Indian Ocean
• Hurricanes in the Atlantic Ocean
• Typhoons in the Western Pacific and South China Sea
• Willy-willies in Western Australia

A mature tropical cyclone features a calm, low-pressure center called the eye, surrounded by a highly turbulent eye wall with torrential rains and winds reaching up to 250 km/h.

Severe, short-duration local storms driven by intense convection:

• Thunderstorms: Characterized by strong updrafts of moist air forming cumulonimbus clouds, producing thunder, lightning, and sometimes hail.
• Tornadoes: Violent, spiralling wind columns descending from severe thunderstorms with extreme low pressure at the center. Over water bodies, they are called water spouts.