Which one of the following is a planetary wind ?

1. Atmospheric Pressure and Pressure Gradient (basic)

Welcome to your first step in understanding how the atmosphere moves! To understand winds, we must first understand Atmospheric Pressure. Imagine a column of air reaching from the ground all the way to the top of the atmosphere; the weight of that air pressing down on a unit area of the surface is what we call atmospheric pressure. Because air is thinner the higher you go, pressure decreases rapidly with height—specifically, about 1 millibar (mb) for every 10 meters you climb NCERT Class XI, Atmospheric Circulation and Weather Systems, p.76.

You might wonder: if the pressure at the bottom is so much higher than at the top, why doesn't the air just shoot upward? This is because of a delicate balance. The vertical pressure gradient force (the push from high to low pressure) is indeed very strong, but it is almost perfectly countered by the force of gravity pulling the air down. This state of balance is why we don't experience constant, violent upward winds PMF IAS, Pressure Systems and Wind System, p.306.

When we look at weather maps, we focus on horizontal distribution. To do this fairly, scientists "reduce" all pressure readings to sea level so that altitude doesn't skew the data. We then connect points of equal pressure using lines called Isobars NCERT Class XI, Atmospheric Circulation and Weather Systems, p.77. The change in pressure over a specific horizontal distance is called the Pressure Gradient. This is the "engine" of the wind: the force always acts from high pressure toward low pressure, perpendicular to the isobars.

The spacing of these isobars tells us how intense the wind will be. Think of it like a slope on a mountain:

Isobar Spacing	Pressure Gradient	Wind Velocity
Close together	Steep / Strong	High (Strong winds)
Far apart	Gentle / Weak	Low (Light breeze)

Sources: NCERT Class XI, Atmospheric Circulation and Weather Systems, p.76; NCERT Class XI, Atmospheric Circulation and Weather Systems, p.77; PMF IAS, Pressure Systems and Wind System, p.304; PMF IAS, Pressure Systems and Wind System, p.306

2. Global Pressure Belts of the Earth (basic)

To understand global winds, we must first understand the Global Pressure Belts. Think of these as the 'engine rooms' of the atmosphere. Because the Earth is a sphere, the Sun's heat is not distributed evenly. This temperature difference, combined with the Earth's rotation, creates seven distinct belts of high and low pressure that encircle the globe like a striped sweater. These belts are the primary drivers of our Planetary Winds (Trade winds, Westerlies, and Polar Easterlies). Starting at the center, we have the Equatorial Low Pressure Belt (extending roughly 10°N to 10°S). Here, intense solar heating causes air to expand, become light, and rise through convection currents Certificate Physical and Human Geography, Climate, p.139. This creates a zone of low pressure at the surface. Because the air is mostly moving upward rather than horizontally, surface winds are notoriously calm. Sailors historically called this region the Doldrums. It is also the Intertropical Convergence Zone (ITCZ), where trade winds from both hemispheres meet and rise Physical Geography by PMF IAS, Pressure Systems and Wind System, p.311. As that equatorial air reaches the top of the troposphere, it spreads toward the poles, cools down, and begins to sink around 30°N and 30°S. This sinking (subsiding) air creates the Sub-Tropical High Pressure Belts. Because sinking air is compressed and warmed, it doesn't form clouds, leading to dry, sunny, and calm conditions Physical Geography by PMF IAS, Pressure Systems and Wind System, p.316. These are known as the Horse Latitudes. In the past, sailing ships carrying horses would get stuck in these calm waters; when fodder ran out, they sadly had to throw the horses overboard to lighten the load and conserve water Physical Geography by PMF IAS, Pressure Systems and Wind System, p.312.

Feature	Equatorial Low (Doldrums)	Sub-Tropical High (Horse Latitudes)
Air Movement	Ascending (Rising)	Descending (Sinking)
Pressure Type	Thermal (Heat-induced)	Dynamic (Mechanically induced by sinking air)
Weather	Cloudy, Heavy Rainfall	Clear Skies, Dry (Major Deserts)

Sources: Certificate Physical and Human Geography, Climate, p.139; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.311; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.312; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.316

3. Forces Affecting Wind Direction: Coriolis and Ferrel's Law (intermediate)

Concept: Forces Affecting Wind Direction: Coriolis and Ferrel's Law

4. General Circulation: The Tricellular Model (intermediate)

If the Earth were a stationary, uniform sphere, we might have just one massive circulation cell per hemisphere where hot air rose at the equator and sank at the poles. However, because our planet rotates and features a complex distribution of heat, the atmosphere organizes itself into a much more sophisticated Tricellular Model. This model describes the three distinct loops of air—the Hadley, Ferrel, and Polar cells—that govern our global climate and create the planetary winds we see on maps Environment and Ecology, Majid Hussain, Atmospheric circulation cell, p.100.

The journey begins at the equator with the Hadley Cell. Intense solar heating causes air to expand and rise, creating the Equatorial Low or the Inter-Tropical Convergence Zone (ITCZ) INDIA PHYSICAL ENVIRONMENT, NCERT, Climate, p.30. As this air travels poleward in the upper atmosphere, the Coriolis force deflects it, and it eventually cools and sinks around 30° latitude, forming the Subtropical Highs. On the surface, this air completes the loop by blowing back toward the equator as the Trade Winds. Because this cell is driven directly by solar heating, we call it thermally induced Physical Geography by PMF IAS, Pressure Systems and Wind System, p.317.

At the other extreme, the Polar Cell operates similarly. Cold, dense air sinks at the poles (High Pressure) and flows toward the 60° latitude line as Polar Easterlies. Upon meeting warmer air, it rises at the Subpolar Low. Like the Hadley cell, this is also thermally induced due to the extreme cold. Sandwiched between these two is the Ferrel Cell. Interestingly, the Ferrel cell is dynamically induced; it acts like a mechanical gear driven by the movement of the other two cells and the intense friction of the Westerlies on the surface Physical Geography by PMF IAS, Jet streams, p.385.

Cell Name	Latitudinal Zone	Origin Type	Surface Winds
Hadley Cell	0° to 30° N/S	Thermal	Trade Winds
Ferrel Cell	30° to 60° N/S	Dynamic	Westerlies
Polar Cell	60° to 90° N/S	Thermal	Polar Easterlies

Sources: Environment and Ecology, Majid Hussain (Access publishing 3rd ed.), Major Crops and Cropping Patterns in India, p.100; INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT 2025 ed.), Climate, p.30; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Chapter 23: Pressure Systems and Wind System, p.317; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Jet streams, p.385

5. Connected Concept: Jet Streams and Upper Air Circulation (intermediate)

While we often focus on surface winds like the Trade Winds, the real "engine room" of global weather lies nearly 9 to 15 kilometers above our heads in the upper troposphere. This is the domain of Jet Streams—narrow bands of fast-flowing, geostrophic air currents that circulate the globe from West to East. They are born from two primary ingredients: the sharp temperature gradient between the equator and the poles, and the Coriolis force generated by Earth's rotation Physical Geography by PMF IAS, Jet streams, p.385.

There are two permanent types of jet streams in each hemisphere that you must master for your preparation. The Polar Jet Stream forms at the boundary of cold polar air and warmer temperate air (around 60° latitude). It is the "powerhouse" jet, significantly influencing the path and intensity of temperate cyclones Physical Geography by PMF IAS, Jet streams, p.388. The Subtropical Jet Stream forms at higher altitudes (around 30° latitude) where the Hadley and Ferrel cells meet. While the subtropical jet is more consistent throughout the year, the polar jet is much more variable and aggressive, especially during the winter when the temperature difference between the poles and the equator is most extreme Physical Geography by PMF IAS, Jet streams, p.387.

Feature	Polar Jet Stream	Subtropical Jet Stream
Latitude	Approx. 60° (Mid-latitudes)	Approx. 30° (Subtropics)
Strength	Stronger, especially in winter	Relatively weaker and more stable
Impact	Influences temperate cyclones and fronts	Influences tropical weather and monsoons

Crucially, these jets do not move in a straight line. They meander like a giant river in the sky, creating massive undulating loops known as Rossby Waves Environment and Ecology by Majid Hussain, Major Crops and Cropping Patterns in India, p.120. These waves are responsible for bringing cold polar air down to lower latitudes or pushing warm tropical air toward the poles. When a jet stream "dips" south (a trough), it often brings stormy, cold weather; when it arches north (a ridge), it brings clear, warm conditions. Understanding this upper-air circulation is the key to predicting why weather patterns "stall" or why a particular winter is unusually harsh Physical Geography by PMF IAS, Jet streams, p.386.

Sources: Physical Geography by PMF IAS, Jet streams, p.385; Physical Geography by PMF IAS, Jet streams, p.386; Physical Geography by PMF IAS, Jet streams, p.387; Physical Geography by PMF IAS, Jet streams, p.388; Environment and Ecology by Majid Hussain, Major Crops and Cropping Patterns in India, p.120

6. Classification of Winds: Permanent, Periodic, and Local (exam-level)

To understand the movement of the atmosphere, we categorize winds based on their scale, duration, and regularity. Think of this as a hierarchy: from global systems that never stop, to seasonal shifts, down to local breezes that might only last a few hours. This classification helps us predict weather patterns and understand the climatic character of different regions.

The first and most significant category is Primary or Planetary Winds (also called Permanent or Prevailing winds). These are the "giant engines" of the atmosphere that blow consistently in the same direction throughout the year. They are driven by the Earth's rotation and the permanent latitudinal pressure belts. The three main members of this group are the Trade Winds, the Westerlies, and the Polar Easterlies Physical Geography by PMF IAS, Pressure Systems and Wind System, p.318. For example, the Polar Easterlies are cold, dry winds blowing from the high-pressure polar areas toward the sub-polar lows Physical Geography by PMF IAS, Pressure Systems and Wind System, p.320.

Next, we have Secondary or Periodic Winds. Unlike planetary winds, these systems change their direction at regular intervals—either seasonally or daily. The most famous example is the Monsoon, which involves a massive seasonal reversal of wind direction due to the differential heating of land and sea. On a smaller scale, Land and Sea breezes or Mountain and Valley breezes also fall into this category because they depend on the time of day Certificate Physical and Human Geography, Climate, p.139.

Finally, we look at Tertiary or Local Winds. These are localized phenomena caused by specific geographical features like mountains or local temperature differences. They blow over small areas and for short periods. Examples include the Loo (hot and dry wind in Northern India), the Chinook (a warm wind in the Rockies known as the 'snow-eater'), or the Mistral (a cold wind blowing from the Alps). Understanding these helps us grasp why two cities just 100km apart can have vastly different daily weather.

Classification	Characteristics	Examples
Primary (Planetary)	Global scale, constant direction year-round.	Trade Winds, Westerlies, Polar Easterlies
Secondary (Periodic)	Reverse direction periodically (seasonal/daily).	Monsoons, Land/Sea Breeze
Tertiary (Local)	Specific to a local area and topography.	Loo, Mistral, Chinook, Foehn

Sources: Physical Geography by PMF IAS, Pressure Systems and Wind System, p.318; Physical Geography by PMF IAS, Pressure Systems and Wind System, p.320; Certificate Physical and Human Geography, GC Leong, Climate, p.139

7. Solving the Original PYQ (exam-level)

Now that you have mastered the building blocks of atmospheric circulation—specifically how Earth's rotation and latitudinal heating create permanent pressure belts—this question asks you to identify the planetary wind among various atmospheric movements. As you learned in Physical Geography by PMF IAS, planetary winds (or primary winds) are the large-scale systems that form the 'permanent skeleton' of global air circulation. To answer this correctly, you must distinguish between winds that blow consistently across the globe and those that are seasonal or localized.

To arrive at the correct answer, ask yourself: Which of these winds is a permanent fixture of the global pressure belts? The Trade Winds, which blow from the Subtropical High-Pressure belt toward the Equatorial Low-Pressure belt, operate year-round on a global scale. This consistency makes (B) Trade the correct choice. As highlighted in Certificate Physical and Human Geography by GC Leong, these winds, along with the Westerlies and Polar Easterlies, are the only ones categorized as planetary because they are driven by the fundamental general circulation of the atmosphere.

UPSC often includes "traps" by mixing different scales of wind systems. For instance, the Monsoon (A) is a periodic or secondary wind; while it covers a vast area, its defining characteristic is its seasonal reversal, not its permanence. Similarly, Chinook (C) and Mistral (D) are local or tertiary winds. They are influenced by specific regional topography—like the Rockies or the Alps—rather than global pressure belts. By eliminating these seasonal and regional systems, you are left with the Trade winds as the only true planetary system.