Atmospheric Circulation and Weather Systems

Correct option: (c) 900 mb

Justification: Atmospheric pressure decreases with increasing altitude. As a general rule, pressure decreases by approximately 100 mb for every 1 km rise in altitude. Therefore, at 1 km above the surface where pressure is 1,000 mb, the pressure will be approximately $1,000 - 100 = 900$ mb.

Correct option: (a) near the Equator

Justification: The Inter Tropical Convergence Zone (ITCZ) is a low-pressure belt where the northeast and southeast trade winds converge. It is located near the Equator because solar heating is most intense there, causing warm air to rise, creating a zone of low pressure and convergence.

Correct option: (c) anti-clockwise

Justification: In the Northern Hemisphere, due to the Coriolis force, winds are deflected to the right. Around a low-pressure system (cyclone), winds blow inward and are deflected to the right, resulting in an anti-clockwise (counter-clockwise) circulation. In the Southern Hemisphere, the circulation around a low is clockwise.

Correct option: (c) the Siberian Plain

Justification: Air masses form over large, uniform surface areas (source regions) where air can stagnate long enough to acquire the temperature and humidity characteristics of the underlying surface. The Siberian Plain is a vast, homogeneous landmass that acts as a source region for cold, dry continental air masses. The other options — equatorial forests, the Himalayas, and the Deccan Plateau — are not ideal source regions due to their varied topography or limited horizontal extent.

Given/Concept: Atmospheric pressure is the weight of the air column above a unit area.

Unit of Pressure: Atmospheric pressure is measured in millibar (mb). The SI unit is Pascal (Pa); 1 mb = 100 Pa. Pressure is measured using a barometer.

Reason for Reduction to Sea Level:
Stations are located at different altitudes above sea level. Since pressure decreases with altitude, a station at a higher elevation will always record a lower pressure than a station at sea level, even if the actual atmospheric conditions are the same. To make a meaningful comparison of pressure values across different stations and to draw accurate isobars on weather maps, all pressure readings are reduced (corrected) to sea level. This eliminates the effect of altitude and allows meteorologists to identify actual pressure systems such as cyclones and anticyclones.

Given: Pressure gradient force acts from the subtropical high pressure belt (around 30°N) towards the equatorial low pressure (near 0°), i.e., from north to south in the Northern Hemisphere.

Concept Used: Coriolis Force — The rotation of the Earth deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Explanation:
- The pressure gradient force drives air from the subtropical high (30°N) towards the equator (southward).
- As this air moves southward, the Coriolis force deflects it to the right (i.e., towards the west).
- As a result, the winds that originally blew from north to south get deflected and blow from the northeast to the southwest.
- These winds are therefore called the Northeast Trade Winds or Northeast Easterlies in the tropics of the Northern Hemisphere.

Thus, the combined effect of the pressure gradient force and the Coriolis force results in the northeast trade winds.

Given/Concept: Geostrophic winds are upper-level winds that blow in the free atmosphere (above the friction layer, generally above 600 m from the surface).

Definition and Explanation:
When the pressure gradient force and the Coriolis force acting on moving air become equal and opposite, the wind blows parallel to the isobars. Such a wind is called a geostrophic wind.

- The pressure gradient force pushes air from high pressure to low pressure (perpendicular to isobars).
- As the air starts moving, the Coriolis force deflects it (to the right in the Northern Hemisphere).
- When these two forces balance each other exactly, the wind flows parallel to the isobars without crossing them.
- Geostrophic winds are theoretical winds that occur in the upper atmosphere where friction is negligible.
- They represent a balance between the pressure gradient force and the Coriolis force: $$\text{Pressure Gradient Force} = \text{Coriolis Force}$$

Given/Concept: Land and sea breezes are local winds caused by the differential heating and cooling of land and water surfaces.

Sea Breeze (During the Day):
- During the day, land heats up faster than the sea.
- The air over the land becomes warm, expands, and rises, creating a low-pressure area over the land.
- The sea remains relatively cooler, with higher pressure over it.
- Cool air from the sea blows towards the land to fill the low pressure.
- This cool wind blowing from sea to land during the day is called the sea breeze.
- It brings relief from heat in coastal areas during daytime.

Land Breeze (During the Night):
- During the night, land cools faster than the sea.
- The air over the sea is now warmer, rises, and creates a low-pressure area over the sea.
- The land is cooler with relatively higher pressure.
- Cool air from the land blows towards the sea.
- This wind blowing from land to sea during the night is called the land breeze.

These breezes are most pronounced in tropical coastal regions and reverse their direction between day and night.

Given/Concept: Wind is the horizontal movement of air from high pressure to low pressure. Its speed and direction are controlled by several forces.

Factors Affecting the Speed and Direction of Wind:

1. Pressure Gradient Force:
- Wind is primarily driven by differences in atmospheric pressure.
- The pressure gradient is the rate of change of pressure per unit distance.
- A steeper pressure gradient (isobars closer together) results in stronger, faster winds.
- Wind always blows from high pressure to low pressure, so the pressure gradient determines the initial direction of wind.
- Formula: $$\text{Pressure Gradient} = \frac{\Delta P}{\Delta d}$$

2. Coriolis Force:
- This is an apparent force caused by the rotation of the Earth.
- It deflects winds to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
- The Coriolis force is zero at the equator and maximum at the poles.
- It affects the direction of wind but not its speed.
- It causes winds to blow at an angle to the isobars rather than straight across them.

3. Friction Force:
- Friction acts near the Earth's surface (up to about 1–3 km altitude, the friction layer).
- It opposes the motion of wind and reduces its speed.
- Friction also changes the direction of wind by reducing the Coriolis effect, causing winds to cross isobars at an angle towards the low pressure.
- Over rough surfaces (forests, mountains), friction is greater; over smooth surfaces (oceans), it is less.
- Above the friction layer, winds are faster and blow nearly parallel to isobars (geostrophic winds).

4. Centripetal Acceleration (for curved flow):
- When wind flows along curved isobars (around cyclones or anticyclones), an additional centripetal force acts towards the centre of curvature.
- This modifies the balance between pressure gradient and Coriolis force, affecting wind speed.

Conclusion: The speed of wind is mainly determined by the pressure gradient force and friction, while the direction is determined by the pressure gradient force, Coriolis force, and friction acting together.

Given/Concept: The general circulation of the atmosphere refers to the large-scale pattern of winds and pressure belts that distribute heat and moisture around the globe.

Simplified Diagram of General Atmospheric Circulation:
(Note: Draw the following schematic in your answer book)

The diagram should show:
- Equatorial Low Pressure Belt (0°) — ITCZ, rising air
- Northeast Trade Winds (0°–30°N) blowing from subtropical high to equator
- Southeast Trade Winds (0°–30°S)
- Subtropical High Pressure Belt (30°N and 30°S) — descending air
- Westerlies (30°–60°N and S) blowing from subtropical high to sub-polar low
- Sub-polar Low Pressure Belt (60°N and S) — rising air
- Polar Easterlies (60°–90°N and S)
- Polar High Pressure Belt (90°N and S) — descending cold air

Reasons for Formation of Subtropical High Pressure at 30° N and S:

1. Convergence and Subsidence of Air:
- At the equator, intense solar heating causes air to rise strongly (ITCZ).
- This rising air moves poleward at high altitudes in the upper troposphere.
- As it moves towards 30° latitude, it cools, becomes denser, and converges.
- The converging air has nowhere to go but descend.
- This descending (subsiding) air creates high pressure at the surface around 30° N and S.

2. Rotation of the Earth (Coriolis Effect):
- The poleward-moving upper air is deflected by the Coriolis force.
- By the time it reaches about 30° latitude, it is deflected so much that it starts blowing parallel to the latitude (westerly direction).
- This prevents further poleward movement, causing air to pile up and sink, enhancing the high pressure.

3. Adiabatic Warming:
- As the air descends, it is compressed and warms adiabatically.
- This warming further stabilises the atmosphere and inhibits cloud formation, resulting in clear skies and dry conditions (deserts are found at these latitudes).

Conclusion: The subtropical high pressure belts at 30° N and S are the result of the convergence and subsidence of upper-level air driven by the Hadley Cell circulation and the deflecting effect of the Earth's rotation.

Given/Concept: A tropical cyclone is an intense low-pressure system that develops over warm tropical oceans. It is characterised by strong inward-spiralling winds, heavy rainfall, and a calm central eye.

Why Tropical Cyclones Originate Over Seas:

1. Warm Ocean Water (Energy Source):
- Tropical cyclones require a sea surface temperature of at least 27°C to form and sustain themselves.
- Warm ocean water provides the heat and moisture necessary for the development of the cyclone.
- Evaporation from the warm sea surface supplies large amounts of water vapour to the atmosphere.

2. Latent Heat Release:
- As the warm, moist air rises, water vapour condenses to form clouds and rain.
- This condensation releases enormous amounts of latent heat, which warms the surrounding air further, causing it to rise more rapidly.
- This positive feedback mechanism intensifies the cyclone.

3. Low Coriolis Force Requirement:
- Cyclones need the Coriolis force to initiate rotation. They form between 5°–20° latitude where the Coriolis force is sufficient but not too strong.
- Over land, the supply of moisture is cut off and friction increases, causing the cyclone to weaken.

4. Absence of Wind Shear:
- Calm upper-level conditions (low vertical wind shear) over oceans allow the cyclone to develop vertically without being disrupted.

Part of the Cyclone with Torrential Rains and High Velocity Winds:

- The Wall Cloud (Eye Wall) region, which immediately surrounds the calm central eye, experiences the most torrential rains and the highest wind velocities.

Reasons:
1. Strongest Pressure Gradient: The pressure gradient is steepest near the eye, so winds are fastest here.
2. Maximum Convergence and Uplift: Air spirals inward most rapidly near the eye wall, causing intense uplift of warm, moist air.
3. Maximum Condensation: The rapid uplift causes intense condensation, releasing maximum latent heat and producing torrential rainfall.
4. Positive Feedback: The heat released further intensifies the updraft, sustaining the violent winds and heavy rain in this zone.

The Eye (Centre):
- Paradoxically, the centre (eye) of the cyclone is calm, with clear skies, light winds, and no rain.
- This is because the eye is a region of descending air (subsidence), which suppresses cloud formation.

Conclusion: Tropical cyclones originate over warm seas because of the abundant heat and moisture available. The eye wall is the most destructive part of the cyclone due to the steepest pressure gradient, maximum convergence, and intense latent heat release.