Physical Geography

The Ionosphere (within the Thermosphere, 80–600 km) contains electrically charged ions that reflect AM radio waves back to Earth's surface — enabling long-distance radio communication beyond line of sight. Critical for IAF communications. The Stratosphere contains ozone (UV protection); Mesosphere burns meteors; Troposphere contains all weather. The IAF specifically relies on ionospheric radio propagation for beyond-horizon communication.

P-waves (Primary/Compressional waves) travel through solids, liquids, and gases — the only seismic waves that pass through all states of matter. S-waves (Secondary/Shear waves) travel through solids only. When an earthquake occurs, S-waves disappear on the far side of Earth because they cannot pass through the liquid outer core — this 'S-wave shadow zone' proved the outer core is liquid.

Marble is a metamorphic rock formed when limestone is subjected to intense heat and pressure underground. Key metamorphic pairs: Limestone → Marble; Shale → Slate; Sandstone → Quartzite; Coal → Graphite → Diamond. Igneous rocks form from cooling magma (Granite = intrusive; Basalt = extrusive). Sedimentary rocks form from compressed sediment layers and contain fossils. This rock-type question appears almost every AFCAT paper.

Loo is a hot, dry, dusty wind blowing during May–June from the west/northwest across the North Indian plains (Rajasthan, Haryana, Punjab, UP). Temperatures can reach 45–48°C, causing severe heat strokes. It blows strongly in the afternoon, usually subsiding by sunset. India's most famous local wind — a direct AFCAT factual question answered in the chapter. Contrast: Mango Showers (Kerala), Blossom Showers (coffee region).

The ozone layer is located in the Stratosphere (15–50 km altitude), specifically between 20–35 km. It absorbs 97–99% of the Sun's harmful ultraviolet (UV-B and UV-C) radiation. Temperature in the stratosphere increases with altitude (unlike the troposphere) because ozone absorbs solar UV energy. CFCs destroy ozone molecules — hence the Montreal Protocol (1987) phased them out. Ozone = O₃ (triatomic oxygen).

The Mohorovičić Discontinuity (Moho) separates the Earth's crust from the mantle — named after Croatian seismologist Andrija Mohorovičić who discovered it in 1909 through seismic wave analysis. The Gutenberg Discontinuity separates the mantle from the outer core. The Lehmann Discontinuity separates the outer core (liquid) from the inner core (solid). These boundaries are detected through sudden changes in seismic wave velocities.

Shield volcanoes have gently sloping sides formed by low-viscosity basaltic lava flowing long distances before solidifying. They are the largest volcanoes — Hawaii's Mauna Loa and Olympus Mons on Mars are shield volcanoes. Stratovolcanoes (composite) have steep sides, explosive eruptions, and thick lava — e.g., Mt Fuji, Mt Vesuvius. Shield volcanoes are non-explosive; stratovolcanoes cause catastrophic eruptions. The IAF monitors volcanic ash clouds for flight safety.

The Coriolis effect (caused by Earth's rotation) deflects winds to the RIGHT in the Northern Hemisphere and to the LEFT in the Southern Hemisphere. This causes anticlockwise rotation in Northern Hemisphere cyclones (low-pressure systems) and clockwise rotation in Southern Hemisphere cyclones. The effect is strongest at the poles and zero at the Equator. It also affects aircraft flight paths — relevant knowledge for AFCAT pilots.

Cold ocean currents along the western coasts of continents (e.g., Benguela Current off southwest Africa, Humboldt Current off South America, Canary Current off West Africa) cool the air above them, reducing evaporation and preventing moisture-laden winds from forming. This creates coastal deserts: Namib Desert (Benguela Current), Atacama Desert (Humboldt Current). Warm currents on eastern coasts do the opposite — bringing rainfall.

Horse Latitudes (25°–35° N and S) are subtropical high-pressure zones characterised by calm, sinking air, low precipitation, and light variable winds. Sinking air warms and dries as it descends — creating desert conditions. The Sahara, Arabian Desert, and Australian Outback all lie under horse latitudes. The name originates from Spanish sailors who supposedly threw horses overboard here when becalmed and running out of water.

Granite is an intrusive igneous rock — formed when magma cools slowly deep underground, allowing large crystals to form. It is coarse-grained and very hard. Basalt is extrusive igneous — formed when lava cools rapidly at the surface, resulting in fine-grained texture. Granite forms the core of continents (continental crust). The Deccan Traps are basaltic lava flows. Granite is used extensively in construction and monuments.

The midnight sun is experienced in regions above the Arctic Circle (66.5°N) and Antarctic Circle (66.5°S) during their respective summers. Due to Earth's 23.5° axial tilt, areas above these circles experience 24-hour daylight during summer solstice and 24-hour darkness during winter solstice. Norway, Finland, Iceland, northern Canada, and Russia experience midnight sun. Antarctica experiences it during Southern Hemisphere summer.

The Richter Scale measures the magnitude (amplitude of seismic waves, related to energy released) at the earthquake's focus (hypocentre). It is a logarithmic scale — each whole number increase represents 10× more ground motion and approximately 31.6× more energy. The Mercalli Scale measures intensity (damage/felt effects at specific locations). The epicentre is the point on Earth's surface directly above the focus.

The Indian Monsoon (June–September) is driven by Southwest Monsoon winds — moist air masses from the Indian Ocean and Arabian Sea that blow toward the Indian subcontinent due to differential heating between land and sea. The land heats faster, creating low pressure; the cooler ocean creates high pressure. Winds blow from high to low pressure, carrying moisture that falls as rainfall across India. The onset in Kerala is usually around June 1.

The Troposphere is the lowest atmospheric layer (0–12 km altitude), containing 75% of atmospheric mass and almost all water vapour. All weather phenomena occur here — clouds, rain, storms, hurricanes, fog. Temperature decreases with altitude at a rate of ~6.5°C per km (environmental lapse rate). The tropopause marks its upper boundary. Aircraft fly in the lower stratosphere to avoid turbulence from tropospheric weather.

Coal is a sedimentary rock — formed from compressed plant material (peat → lignite → bituminous → anthracite) over millions of years under heat and pressure. It is classified as an organic sedimentary rock. It contains fossils of ancient plants. When subjected to extreme metamorphism, coal transforms to graphite and then diamond. India's major coal fields: Jharia (Jharkhand), Raniganj (WB), Singrauli (MP/UP). Coal = energy sedimentary rock.

Fold mountains form at convergent plate boundaries where two tectonic plates collide — compressing and folding rock layers upward like a crumpled tablecloth. Examples: Himalayas (India-Eurasian plate collision), Alps (African-Eurasian), Andes (Nazca-South American), Rockies. They are the world's highest and most extensive mountain ranges. Fault-block mountains (e.g., Vindhya, Satpura) form by faulting — different mechanism.

Earth's interior from surface to centre: Crust (0–35 km) → Mantle (35–2,900 km) → Outer Core (2,900–5,100 km, liquid iron-nickel) → Inner Core (5,100–6,371 km, solid iron-nickel). The liquid outer core generates Earth's magnetic field through convection currents (dynamo effect). The Moho separates crust from mantle; the Gutenberg Discontinuity separates mantle from outer core; the Lehmann separates outer from inner core.

Convectional rainfall occurs when the ground is heated strongly (tropics/summers), causing warm moist air to rise rapidly, cool, condense, and form towering cumulonimbus clouds producing heavy, localised afternoon showers with thunder. Orographic rainfall occurs when air is forced to rise over mountains. Frontal rainfall occurs when warm and cold air masses meet. Convectional rainfall is the dominant type in the equatorial tropics and Indian afternoons in summer.

The Sahara Desert (North Africa; ~9.2 million km²) is the world's largest hot desert. Important distinction: the Antarctic Desert (~14 million km²) is larger overall but is a cold desert. The Gobi Desert (Asia) is the largest cold desert outside Antarctica. The Atacama (Chile/Peru) is the driest non-polar desert. Arabian Desert is the 2nd largest hot desert. AFCAT always specifies 'hot desert' — look for that qualifier.