Which one of the following does not control climate ?

1. Earth's Heat Budget and Insolation (basic)

Welcome to the start of our journey into world climates! To understand why the Sahara is hot or why the Tundra is frozen, we must first understand the Earth's energy currency: Insolation. Short for Incoming Solar Radiation, insolation is the solar energy that reaches the Earth's surface. Interestingly, the amount of energy we receive isn't uniform. As noted in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Solar Radiation, Heat Balance and Temperature, p.67, this variation is driven by factors like the angle of inclination of the sun's rays, the rotation of the Earth, and the transparency of the atmosphere.

Think of a flashlight: if you shine it directly down, the circle of light is small and intense. If you tilt it, the same energy spreads over a larger area and becomes weaker. This is exactly what happens at the poles compared to the equator. Because of the Earth's curvature and its axial tilt of 66½° with the orbital plane, the equator receives vertical rays (concentrated heat), while the poles receive oblique rays (spread out heat). Furthermore, oblique rays must travel through a thicker layer of the atmosphere, losing more energy to scattering and absorption before they ever touch the ground.

Now, why doesn't the Earth just keep getting hotter every day? This is due to the Heat Budget. The Earth maintains a delicate equilibrium where the amount of heat received equals the amount lost through terrestrial radiation. If we imagine 100 units of energy entering the top of the atmosphere, the breakdown looks like this:

Process	Units	Description
Albedo	35 Units	Reflected back to space by clouds, ice, and the atmosphere before reaching the surface.
Atmospheric Absorption	14 Units	Absorbed directly by gases and water vapor in the air.
Surface Absorption	51 Units	Actually reaches and warms the Earth's land and oceans.

As explained in FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Solar Radiation, Heat Balance and Temperature, p.69, these 51 units absorbed by the Earth, plus the 14 units in the atmosphere, are eventually radiated back into space as long-wave radiation. This perfect balance ensures our planet remains habitable rather than turning into a furnace or an icebox.

Sources: FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Solar Radiation, Heat Balance and Temperature, p.67; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI, Solar Radiation, Heat Balance and Temperature, p.69

2. Atmospheric Circulation and Pressure Belts (intermediate)

To understand global climate, we must first understand why air moves at all. At its simplest, atmospheric circulation is nature’s attempt to redistribute heat from the sun-drenched equator to the colder poles. This movement is governed by two primary forces: the Pressure Gradient Force (PGF), which pushes air from high to low pressure, and the Coriolis Force. Due to the Earth's rotation, the Coriolis force deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere Certificate Physical and Human Geography, GC Leong (Oxford University press 3rd ed.), Chapter 14, p.139. At high altitudes, where friction is absent, this deflection is so strong that winds eventually blow parallel to the isobars, a phenomenon known as Geostrophic Wind Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Jet streams, p.384.

The Earth doesn't just have one giant loop of air; it is divided into three distinct 'cells' in each hemisphere—the Hadley, Ferrel, and Polar cells. These cells create seven alternating pressure belts across the globe. Some are thermally induced (like the Equatorial Low and Polar High), while others are dynamically induced by the sinking and rising of air due to Earth's rotation (like the Sub-Tropical Highs and Sub-Polar Lows) FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 9, p.79.

Pressure Belt	Nature	Associated Winds
Equatorial Low (Doldrums)	Thermal (Rising air)	Trade Winds converge here
Sub-Tropical High (Horse Latitudes)	Dynamic (Sinking air)	Westerlies and Trades originate here
Sub-Polar Low	Dynamic (Rising air)	Westerlies and Polar Easterlies meet
Polar High	Thermal (Sinking air)	Polar Easterlies originate here

Crucially, these belts are not static. Because they are driven by solar heating, they migrate north and south following the apparent path of the sun throughout the year Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Pressure Systems and Wind System, p.316. This migration is the secret ingredient behind seasonal rainfall patterns, such as the Mediterranean climate or the Indian Monsoon, which we will explore in later hops.

Sources: Certificate Physical and Human Geography, GC Leong (Oxford University press 3rd ed.), Chapter 14: Climate, p.139; Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.), Pressure Systems and Wind System, p.316, 384; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Geography Class XI (NCERT 2025 ed.), Chapter 9: Atmospheric Circulation and Weather Systems, p.79

3. Ocean Currents and Maritime Influence (intermediate)

To understand world climate, we must look beyond just how much sun a place gets. We must look at the maritime influence—the way the massive oceans act as a global thermostat. Oceans have a high specific heat capacity, meaning they heat up and cool down much slower than land. This creates a distinct difference between maritime climates (coastal areas with moderate temperature ranges) and continental climates (inland areas with extreme summers and winters) Certificate Physical and Human Geography, GC Leong, Chapter 14, p.134.

The real "movers and shakers" of maritime influence are ocean currents. Think of them as massive conveyor belts of energy. Warm currents flow from the equator toward the poles, carrying heat to higher latitudes. Conversely, cold currents move from the poles toward the equator, bringing a cooling effect to the tropics. This is why two cities at the exact same latitude can have vastly different climates. For instance, the Gulf Stream (warm) keeps the ports of Western Europe ice-free in winter, while at the same latitude in North-East Canada, the Labrador Current (cold) causes ports to freeze for several months FUNDAMENTALS OF PHYSICAL GEOGRAPHY, NCERT Class XI, Chapter 13, p.103.

The interaction between winds and currents is crucial. For an ocean current to significantly change the climate of a coastal region, the prevailing winds must be onshore (blowing from sea to land). If the wind is offshore, the moderating effect of the current is largely lost to the sea. In certain regions, like North-Eastern North America, we see an intermediate climate called the Laurentian type. It sits between the extreme cold of the Siberian interior and the mild maritime climate of the British type, showing features of both maritime and continental influences Certificate Physical and Human Geography, GC Leong, Chapter 24, p.224.

Feature	Warm Currents	Cold Currents
Origin	Equatorial regions (moving poleward)	Polar regions (moving equatorward)
Climate Impact	Raise temperature; increase rainfall	Lower temperature; creates aridity/fog
Typical Location	East coasts (low/mid lats); West coasts (high lats)	West coasts (low/mid lats); East coasts (high lats)

Sources: Certificate Physical and Human Geography, GC Leong, Chapter 14: Climate, p.134; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, NCERT Class XI, Chapter 13: Water (Oceans), p.103; Certificate Physical and Human Geography, GC Leong, Chapter 24: The Cool Temperate Eastern Margin (Laurentian) Climate, p.224; Physical Geography by PMF IAS, Ocean Movements Ocean Currents And Tides, p.488

4. Longitude, Time Zones, and Earth's Rotation (basic)

While latitude determines how much solar energy a place receives, longitude is primarily a tool for measuring time and position. Longitude consists of imaginary vertical lines, known as meridians, that run from the North Pole to the South Pole. Unlike latitudes, which vary in length, all meridians of longitude are of equal length Physical Geography by PMF IAS, Latitudes and Longitudes, p.243. The starting point is the Prime Meridian (0°) passing through Greenwich, near London. Everything east or west of this line is measured up to 180°.

The core relationship between longitude and time is driven by the Earth's rotation. Since the Earth completes one full rotation of 360° in 24 hours, we can calculate the rate of time change as follows:

Rotation Angle	Time Duration
360°	24 Hours
15°	1 Hour
1°	4 Minutes

Because the Earth rotates from West to East, places located to the East see the sun earlier and are "ahead" in time, while places to the West see the sun later and are "behind" GC Leong, Chapter 1, p.11. This is why when it is noon in London (0°), a place at 15°E will already be at 1:00 PM. At the opposite side of the world lies the International Date Line (180°). Crossing this line is a major event because it doesn't just change the hour; it changes the calendar date by one full day NCERT Class VI, Locating Places on the Earth, p.24.

In the context of Climate Classification, it is vital to remember that longitude is not a physical control of climate. While a change in latitude or altitude will drastically change the temperature and vegetation of a region, moving along the same latitude to a different longitude primarily changes the local time, not the fundamental climatic zone GC Leong, Chapter 14, p.131.

Sources: Physical Geography by PMF IAS, Latitudes and Longitudes, p.243; Certificate Physical and Human Geography, GC Leong, The Earth's Crust, p.11; Exploring Society: India and Beyond. Social Science-Class VI. NCERT, Locating Places on the Earth, p.24; Certificate Physical and Human Geography, GC Leong, Climate, p.131

5. Vegetation, Albedo, and Local Climate (intermediate)

While global climate is largely dictated by latitude and altitude, the local climate is profoundly shaped by the surface characteristics of the Earth—specifically vegetation cover and albedo. Think of these as the 'active' surface layer that determines how much solar energy is retained and how moisture is cycled. Albedo refers to the reflectivity of a surface; it is the proportion of solar radiation reflected back into space without being absorbed. Fresh snow has a very high albedo (reflecting 70-90% of sunlight), whereas dark forests or deep oceans have low albedo, meaning they absorb more heat Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p. 283. This absorption and radiation of heat are central to the Earth's heat budget, where roughly 31 units of incoming solar radiation are reflected back immediately—27 from clouds, 2 from snow/ice, and 2 from the atmosphere—leaving the rest to drive our weather systems Fundamentals of Physical Geography, Solar Radiation, Heat Balance and Temperature, p. 69.

Natural vegetation acts as a biological thermostat. Forests regulate the local climate through evapotranspiration—a process where trees 'breathe' moisture from the soil into the atmosphere. This moisture then cools the air and eventually falls back as precipitation, creating a self-sustaining cycle. There is a direct correlation: higher forest cover generally leads to higher rainfall Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p. 68. For instance, evergreen forests thrive in humid zones with over 200 cm of rain, while desert vegetation is restricted to very dry zones with less than 50 cm Geography of India, Majid Husain, Natural Vegetation and National Parks, p. 13. When we remove this cover, we don't just lose trees; we break the local water cycle.

Deforestation triggers a double-whammy for the local climate. First, it reduces transpiration, leading to lower groundwater levels and reduced local rainfall over time. Second, it often changes the surface albedo and breaks the 'natural reuse cycle' of water, causing rapid runoff instead of atmospheric recycling Environment, Shankar IAS Academy, Terrestrial Ecosystems, p. 30. This transformation can turn a once-humid microclimate into a semi-arid or dry zone. The following table illustrates how different vegetation types reflect the moisture availability of a region:

Vegetation Type	Average Annual Rainfall	Climatic Zone
Evergreen Forests	Above 200 cm	Humid
Monsoon Forests	100–200 cm	Semi-Humid
Dry Forests	50–100 cm	Dry
Desert Forests	Below 50 cm	Very Dry

Sources: Physical Geography by PMF IAS, Horizontal Distribution of Temperature, p.283; Fundamentals of Physical Geography, Solar Radiation, Heat Balance and Temperature, p.69; Environment and Ecology, Majid Hussain, Natural Hazards and Disaster Management, p.68; Geography of India, Majid Husain, Natural Vegetation and National Parks, p.13; Environment, Shankar IAS Academy, Terrestrial Ecosystems, p.30

6. The Six Major Controls of Climate (exam-level)

Climate is not a random occurrence; it is the masterpiece of six massive geographical architects working in tandem. These are known as the Climatic Controls. The most fundamental control is Latitude. Due to the Earth's curvature, solar energy (insolation) is most intense at the Equator and decreases toward the poles. This creates the primary temperature gradient of our planet. FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Chapter 8, p.67. Close behind is Altitude; as we move from the Earth's surface to higher elevations, the atmosphere becomes less dense and temperature decreases—this is why even in tropical India, hill stations remain cool while the plains swelter. INDIA PHYSICAL ENVIRONMENT, Chapter 4, p.29.

The movement of air and water further refines these patterns. The Pressure and Wind System of any area depends on its latitude and altitude, dictating the distribution of rainfall. Then there is the Distance from the Sea (often called continentality). As the distance from the sea increases, its moderating influence decreases, leading to extreme weather conditions—scorching summers and freezing winters. CONTEMPORARY INDIA-I, Chapter 4, p.27. This is complemented by Ocean Currents; coastal areas with warm currents will be warmer than those with cold currents, provided the winds are blowing onshore.

Finally, Relief Features play a major role as physical barriers. High mountains can stop cold or hot winds and force moisture-bearing winds to rise and shed rain on their windward slopes, leaving the other side (the leeward side) dry. While factors like Natural Vegetation also influence local climates by acting as carbon sinks and regulating evaporation, it is vital to remember that Longitude is not a climatic control. Longitude is a tool for time and positioning; it does not physically dictate temperature or pressure systems. CONTEMPORARY INDIA-I, Chapter 4, p.27.

Control	Primary Impact
Latitude	Determines the angle of sun rays and total insolation.
Altitude	Causes temperature to drop as air density decreases.
Relief	Acts as a barrier for winds and creates rain-shadow zones.

Sources: CONTEMPORARY INDIA-I, Chapter 4: Climate, p.27; INDIA PHYSICAL ENVIRONMENT, Chapter 4: Climate, p.29; FUNDAMENTALS OF PHYSICAL GEOGRAPHY, Chapter 8: Solar Radiation, Heat Balance and Temperature, p.67

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental principles of solar insolation and the vertical structure of the atmosphere, this question tests your ability to synthesize those building blocks. To determine what controls climate, you must look for factors that physically alter the energy budget or the moisture cycle of a region. As you learned in CONTEMPORARY INDIA-I, Geography, Class IX (NCERT), there are six major climatic controls. This question asks you to identify the non-physical coordinate among physical drivers.

To arrive at the correct answer, (C) Longitude, we must evaluate how each option affects heat and pressure. Latitude is a primary control because the Earth's curvature causes solar energy to decrease from the equator toward the poles. Altitude directly influences temperature via the normal lapse rate, where the atmosphere becomes less dense and cooler at higher elevations, as detailed in INDIA PHYSICAL ENVIRONMENT, Geography Class XI (NCERT). Even Forest cover serves as a vital control by regulating evapotranspiration, surface albedo, and carbon sequestration. In contrast, Longitude is a human-defined coordinate used to determine time and horizontal position; it has no inherent physical impact on how much heat a location receives or how much rain falls.

UPSC often uses Forest cover as a distractor because students may focus exclusively on geological factors and overlook biological influences on the micro-climate. Another common trap is confusing "longitude" with "continentality" (distance from the sea). While your longitudinal position might coincide with being deep inland, the Longitude itself is not the causal mechanism for the climate—the distribution of land and water is. As Certificate Physical and Human Geography, GC Leong clarifies, while longitude helps us organize data, it is not a physical determinant of the climate system.