This MCQ module is based on: Temperature Distribution, Inversion & Exercises
TOPIC 17 OF 29
Temperature Distribution, Inversion & Exercises
🎓 Class 11
Social Science
CBSE
Theory
Ch 8 — Solar Radiation, Heat Balance and Temperature
⏱ ~28 min
🌐 Language: [gtranslate]
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8.8 What Is Temperature?
The interaction of insolation with the atmosphere and the earth's surface creates heat, which is then measured in terms of temperature. The two words are sometimes used loosely, but in physics they are distinct. Heat represents the molecular movement of the particles comprising a substance — the total kinetic energy of all the molecules. Temperature, by contrast, is the measurement in degrees of how hot (or cold) a thing or a place is — it is a measure of the average kinetic energy of the molecules. A swimming pool may contain more total heat than a cup of boiling water, but the cup has a far higher temperature.
📖 Heat vs Temperature
Heat = the energy of molecular motion in a body (a quantity).Temperature = the degree of hotness or coldness of a body (an intensity).
Temperature is what a thermometer reads; heat is what flows from a hotter body to a cooler one when they are in contact.
8.9 Factors Controlling Temperature Distribution
The temperature of the air at any place is influenced by six main factors: (i) the latitude of the place; (ii) the altitude of the place; (iii) distance from the sea; (iv) air-mass and ocean currents; (v) the aspect of the slope; and (vi) cloud cover and rainfall. Each of these works alongside the others, and a place's actual temperature is the combined outcome.
(i) Latitude — The Master Control
The temperature of a place depends on the insolation it receives. As we have seen, insolation varies sharply with latitude — vertical rays at the equator, slant rays at the poles — and so the temperature varies accordingly. This is why mean annual temperatures generally decrease from the equator toward the poles. Latitude is, in this sense, the "master variable" that all the others modify.
(ii) Altitude — Up Is Cooler
The atmosphere is heated indirectly by terrestrial radiation from below. Therefore, places near sea level record higher temperatures than places situated at higher elevations. In other words, temperature generally decreases with increasing height. The rate of decrease of temperature with height is termed the normal lapse rate?. NCERT gives the figure as 6.5°C per 1,000 m (sometimes also stated as approximately 1°C per 165 m).
(iii) Distance From the Sea — Continentality
Another factor that influences temperature is the location of a place with respect to the sea. Compared to land, the sea gets heated slowly and loses heat slowly; land heats up and cools down quickly. Therefore, the variation in temperature over the sea is less than over the land. Places situated near the sea come under the moderating influence of the sea and of the daily land and sea breezes that moderate temperature swings. Places far inland, away from this maritime influence, swing strongly between hot summers and cold winters — a property called continentality?. The Eurasian interior in winter is the textbook case.
(iv) Air-Mass and Ocean Currents
Like the land and sea breezes, the passage of air masses also affects the temperature. Places that come under the influence of warm air-masses experience higher temperatures, and places that come under the influence of cold air-masses experience low temperatures. Similarly, places located on coasts where warm ocean currents flow record higher temperatures than places located on coasts where cold currents flow. The Gulf Stream warms the British Isles; the cold Labrador Current cools the eastern coast of Canada.
(v) Aspect of the Slope
The direction a slope faces — its aspect — determines how much sunlight it intercepts. In the Northern Hemisphere, south-facing slopes receive more direct sunlight and are warmer; north-facing slopes are cooler and often hold snow longer. In Himalayan villages, settlements and apple orchards are clustered on the sunnier south-facing slopes for exactly this reason.
(vi) Cloud Cover and Rainfall
Cloud cover modifies temperature in two opposite ways. By day, clouds reflect incoming solar radiation back to space, lowering surface temperature; by night, they trap outgoing terrestrial radiation, keeping the surface warmer than it would otherwise be. The net effect is to flatten the diurnal temperature range. Rainfall has a similar moderating effect, partly by increasing humidity and partly by cooling through evaporation.
Latitude
Sets the angle of incidence and day length — the dominant control of mean annual temperature.
Altitude
Temperature falls 6.5°C per 1,000 m — the normal lapse rate of the troposphere.
Distance from Sea
Coastal places mild; continental interiors swing from hot summer to cold winter (continentality).
Air-Mass & Currents
Warm/cold air masses and ocean currents redistribute heat across continents and seas.
Aspect of Slope
South-facing slopes (in N. Hemisphere) catch more sun — apple orchards thrive there.
Cloud Cover
Cools by day (reflection), warms by night (trapping IR) — flattens daily range.
8.10 Distribution of Temperature
The global distribution of temperature can be well understood by studying the temperature distribution in January and July — the typical northern winter and northern summer months. Temperature distribution is generally shown on the map with the help of isotherms? — lines joining places that have equal temperature.
📖 Definition — Isotherm
An isotherm is a line on a map joining places that have the same temperature at a given time. Just as contour lines connect points of equal height and isobars connect points of equal pressure, isotherms connect points of equal temperature. They are the working tool for visualising temperature distribution at the global, regional or local scale.
Horizontal Distribution — Reading Isotherm Maps
In general, the effect of latitude on temperature is well pronounced on the map: isotherms are generally parallel to the latitudes. The deviation from this general trend is more pronounced in January than in July, especially in the northern hemisphere. This is because in the northern hemisphere the land surface area is much larger than in the southern hemisphere — so the effects of land mass and ocean currents are felt more strongly.
January isotherms
In January the isotherms deviate to the north over the ocean and to the south over the continent. This pattern is clearly seen over the North Atlantic Ocean. The presence of warm ocean currents — the Gulf Stream and the North Atlantic Drift — make the northern Atlantic Ocean warmer, so the isotherms bend northward over the water. Over the European and Asian land mass, by contrast, the temperature drops sharply and the isotherms bend southward.
This effect is most pronounced in the Siberian plain. The mean January temperature along 60° E longitude is −20°C at both 80° N and 50° N latitudes — the isotherm of −20°C runs almost vertically across thirty degrees of latitude. The mean monthly temperature for January is over 27°C in the equatorial oceans, over 24°C in the tropics, between 2°C and 0°C in the middle latitudes, and from −18°C to −48°C in the Eurasian continental interior. The effect of the ocean is much more pronounced in the southern hemisphere — there the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual. The 20°C, 10°C and 0°C isotherms run parallel to the 35° S, 45° S and 60° S latitudes respectively.
July isotherms
In July the isotherms generally run parallel to the latitude. The equatorial oceans record warmer temperatures, more than 27°C. Over the land, more than 30°C is noticed in the subtropical continental region of Asia, along the 30° N latitude. Along the 40° N runs the isotherm of 10°C, and along 40° S the temperature is also about 10°C. The annual range of temperature — the difference between January and July averages — is highest (more than 60°C) over the north-eastern part of the Eurasian continent (the Verkhoyansk region), due to extreme continentality, and lowest (about 3°C) between 20° S and 15° N, in the equatorial belt where seasonal change in insolation is small.
Schematic Isotherm Map — January & July
Vertical Distribution — The Normal Lapse Rate
Temperature normally decreases with elevation in the troposphere. The rate of decrease is the normal lapse rate of 6.5°C per 1,000 m (i.e. about 1°C per 165 m). This is why mountain peaks are snow-covered while their bases lie in tropical heat — the summit of Mt Everest at 8,848 m is on average about 57°C colder than sea level at the same latitude.
Temperature vs Altitude — Normal Lapse Rate
Each 1,000 m of ascent reduces temperature by about 6.5°C in the troposphere, until the tropopause is reached.
8.11 Inversion of Temperature
Normally, temperature decreases with increase in elevation. It is called the normal lapse rate. At times, the situation is reversed, and the normal lapse rate is inverted. This is called inversion of temperature?. Inversion is usually of short duration but quite common nonetheless.
Causes of Surface Inversion
A long winter night with clear skies and still air is the ideal situation for inversion. The heat of the day is radiated off during the night, and by early morning hours the earth is cooler than the air above. The thin layer of air closest to the ground cools fastest — colder air below, warmer air above — and the lapse rate is reversed. Over polar areas, temperature inversion is normal throughout the year because the surface remains chronically cold.
The four classic conditions favouring surface inversion are: (i) long winter night — extended cooling time; (ii) clear sky — terrestrial radiation escapes freely with no cloud blanket; (iii) dry air — little water vapour to absorb the radiation; (iv) snow surface — high albedo plus rapid radiative cooling.
Effects on Air Quality
Surface inversion promotes stability in the lower layers of the atmosphere. Smoke and dust particles get trapped beneath the inversion layer and spread horizontally to fill the lower strata of the atmosphere. Dense fogs in mornings are common occurrences, especially during the winter season — and the smog of Delhi, Beijing and London is a textbook example of pollutants pinned down by inversion. This inversion commonly lasts for a few hours until the Sun rises and begins to warm the earth, breaking the inversion.
Inversion in Hills and Valleys — The Frost Hollow
Inversion takes a striking form in hills and mountains, due to air drainage. Cold air, produced at the higher slopes during the night, flows under the influence of gravity. Being heavy and dense, the cold air acts almost like water and moves down the slope to pile up deeply in pockets and valley bottoms, with warmer air above. This is called air drainage. The chilled lowest layer of a valley, where frost is most likely to form, is called a frost hollow?.
Air drainage actually protects plants from frost damage — but only the plants on the upper slopes. Plants on the valley floor are exposed to repeated frost. This is why apple orchards in the Himalayan region (Himachal Pradesh, Kashmir) and tea plantations in the hills of South India are deliberately planted on the upper and middle slopes, not in the valley bottoms. In the famous Mahabaleshwar hills of Maharashtra, Kashmir Valley in winter, and the Himalayan apple orchards, this principle is observed every cold night of the season.
Frost Hollow — Cold Air Drainage in a Valley
Upper-air Inversion
Inversion is not confined to surface valleys. Upper-air inversion occurs at the boundaries between two contrasting air masses — for example, where a warm air mass overrides a cold one along a frontal surface. Such inversions can persist for long distances horizontally and act as caps on convective rising air.
Normal Lapse Rate vs Inverted Profile
Normal: temperature falls with height. Inversion: temperature rises with height in the lowest layer, trapping pollutants.
LET'S EXPLORE — Why Are Apple Orchards on the Slopes, Not in the Valley?
L4 Analyse
In the apple-growing belts of Kashmir and Himachal Pradesh, the orchards are deliberately planted on the upper and middle parts of the hill slopes, not on the valley floor — even though the valley floor is more level and easier to cultivate. Use the concept of air drainage to explain.
On clear winter nights, the air at the higher slopes cools rapidly through terrestrial radiation. Cold air is heavy and dense, so it slides down the slopes under gravity — the process of air drainage — and accumulates in the valley bottom, forming a frost hollow with sub-zero temperatures. Apple blossoms and young fruit are extremely vulnerable to frost; a single hard frost in spring can destroy an entire crop. By planting orchards on the upper and middle slopes — the warmer "thermal belt" above the inversion — farmers keep their trees out of the frost zone. The valley floor is reserved for hardier crops or seasonal grazing. This is direct application of physical geography to agricultural planning.
8.12 Plank's Law and Specific Heat — Two Quick Definitions
The NCERT box on the right of the inversion section introduces two short physics ideas relevant to atmospheric heating.
📐 Two Useful Physics Ideas
- Plank's Law: The hotter a body, the more energy it will radiate, and the shorter the wavelength of that radiation. The hot Sun therefore radiates intense, short-wavelength energy (visible/UV), while the cooler earth radiates weaker, long-wavelength energy (infrared).
- Specific Heat: The energy needed to raise the temperature of one gram of a substance by one Celsius. Water has a much higher specific heat than land — that is why oceans heat and cool more slowly than continents, and why coastal places have moderate climates.
THINK ABOUT IT — Why Does Mahabaleshwar Get Cold Despite Being in the Tropics?
L3 Apply
Mahabaleshwar (Maharashtra, ~17.9° N, altitude 1,353 m) is well within the Indian tropics, yet its winter mornings record temperatures close to 5°C, sometimes with frost in low-lying valleys. Use altitude and temperature inversion to explain.
Two effects combine. First, the normal lapse rate of 6.5°C per 1,000 m means Mahabaleshwar is roughly 9°C cooler than the Konkan coastal plain at sea level — even at the same latitude. Second, on clear winter nights with calm air, the nocturnal radiative cooling sets up a temperature inversion. Cold air drains down the slopes into low-lying pockets, forming local frost hollows. So a place that tropical-zone latitude predicts should be warm becomes a winter frost-prone zone — exactly because of altitude and inversion working together.
8.13 NCERT EXERCISES — Solutions & Model Answers
1. Multiple Choice Questions
(i) The Sun is directly overhead at noon on 21st June at:
(a) The equator (b) 23.5° S (c) 23.5° N (d) 66.5° N
Answer: (c) 23.5° N — the Tropic of Cancer. On the summer solstice (21 June) the Sun is directly overhead at noon along 23.5° N. This is the northernmost latitude where the Sun is ever vertical at noon, due to the earth's 23.5° axial tilt.
(ii) In which one of the following cities are the days the longest?
(a) Tiruvanantpuram (b) Chandigarh (c) Hyderabad (d) Nagpur
Answer: (b) Chandigarh. In summer, the Northern Hemisphere is tilted toward the Sun, so day length increases with latitude. Chandigarh (~30.7° N) lies at the highest latitude among the four cities, so it has the longest summer days.
(iii) The atmosphere is mainly heated by the:
(a) Short wave solar radiation (b) Reflected solar radiation (c) Long wave terrestrial radiation (d) Scattered solar radiation
Answer: (c) Long wave terrestrial radiation. The atmosphere is largely transparent to incoming short-wave solar radiation. The earth's surface, once warmed, emits long-wave (infrared) terrestrial radiation, which is absorbed by atmospheric gases like CO₂ and water vapour. So the atmosphere is heated indirectly, from below.
(iv) Match the correct pairs:
(i) Insolation — (a) The difference between the mean temperature of the warmest and the coldest months
(ii) Albedo — (b) The lines joining the places of equal temperature
(iii) Isotherm — (c) The incoming solar radiation
(iv) Annual range — (d) The percentage of visible light reflected by an object
Correct pairs:
(i) Insolation → (c) The incoming solar radiation
(ii) Albedo → (d) The percentage of visible light reflected by an object
(iii) Isotherm → (b) The lines joining the places of equal temperature
(iv) Annual range → (a) The difference between the mean temperature of the warmest and the coldest months
(i) Insolation → (c) The incoming solar radiation
(ii) Albedo → (d) The percentage of visible light reflected by an object
(iii) Isotherm → (b) The lines joining the places of equal temperature
(iv) Annual range → (a) The difference between the mean temperature of the warmest and the coldest months
(v) The main reason that the earth experiences highest temperatures in the subtropics in the northern hemisphere rather than at the equator is:
(a) Subtropical areas have less cloud cover than equatorial areas.
(b) Subtropical areas have longer day hours in summer than equatorial.
(c) Subtropical areas have an enhanced "greenhouse effect" compared to equatorial areas.
(d) Subtropical areas are nearer to the oceanic areas than equatorial locations.
Answer: (a) Subtropical areas have less cloud cover than equatorial areas. Equatorial regions are dominated by ascending air, deep convective clouds and abundant rain, all of which reduce the insolation reaching the surface. Subtropical regions sit beneath the descending limb of the Hadley cell — clear, dry skies, minimal cloud cover, full insolation, and therefore the world's hottest air temperatures (Sahara, Thar, Death Valley).
2. Answer in About 30 Words
(i) How does the unequal distribution of heat over the planet earth in space and time cause variations in weather and climate?
Unequal heating creates pressure differences, which drive winds, ocean currents and air-mass exchange. These transport heat and moisture between regions, producing seasonal monsoons, cyclones, droughts and the long-term climates that distinguish equator, mid-latitudes and poles.
(ii) What are the factors that control temperature distribution on the surface of the earth?
Six factors: (i) latitude — sets sun-angle and day length; (ii) altitude — temperature falls 6.5°C per 1,000 m; (iii) distance from the sea — continentality vs maritime moderation; (iv) air-mass and ocean currents; (v) aspect of slope; (vi) cloud cover and rainfall.
(iii) In India, why is the day temperature maximum in May and not after the summer solstice?
After the summer solstice (21 June), the south-west monsoon arrives over India. Cloud cover, rainfall and high humidity cool the surface and absorb solar radiation, so day temperatures drop. May, just before the monsoon, has clear cloudless skies and intense direct insolation, producing peak heat.
(iv) Why is the annual range of temperature high in the Siberian plains?
The Siberian plains lie deep inside the Eurasian land mass, far from any moderating ocean. Land heats and cools rapidly, so summers can exceed 20°C while winters fall below −40°C. This pure continentality gives Siberia (and especially Verkhoyansk) the world's largest annual range — over 60°C.
3. Answer in About 150 Words
(i) How do the latitude and the tilt in the axis of rotation of the earth affect the amount of radiation received at the earth's surface?
Latitude and axial tilt jointly determine insolation. The earth's axis is inclined at 66½° to the plane of its orbit (i.e. 23½° from the perpendicular). This tilt controls which hemisphere "leans toward" the Sun across the year. At the equator, sun rays are nearly vertical year-round, concentrating energy on a small surface and passing through a thin column of atmosphere. At the poles, rays are slant, spread over a much larger surface, and pass through a thick atmospheric column that absorbs and scatters more energy. Because of the tilt, day length also varies with latitude and season — equatorial days are nearly 12 hours all year, while polar days range from 24-hour darkness to 24-hour sunlight. Combined, these effects produce the surplus radiation at low latitudes and deficit at high latitudes that drives all global atmospheric and oceanic circulation. Without the tilt, there would be no seasons; without latitudinal differences, no winds.
(ii) Discuss the processes through which the earth-atmosphere system maintains heat balance.
The earth-atmosphere system is in radiative balance. Of every 100 units of solar radiation reaching the top of the atmosphere, 35 are reflected back to space — 27 by cloud tops, 6 by atmospheric scattering and 2 by snow/ice — together making up the planet's albedo. Of the remaining 65 units, 14 are absorbed within the atmosphere (by water vapour, ozone and dust) and 51 are absorbed by the earth's surface. The warmed surface emits long-wave terrestrial radiation; 17 units pass directly back to space, while 34 units are absorbed by atmospheric gases — 6 by direct absorption, 9 by convection and turbulence, and 19 by latent heat of condensation when water vapour turns to cloud. The atmosphere thus has 14 + 34 = 48 units to return; combined with the 17 directly radiated from the surface, total outgoing equals 65 — precisely matching the 65 units absorbed. The system thereby balances perfectly.
(iii) Compare the global distribution of temperature in January over the northern and southern hemisphere of the earth.
The two hemispheres show very different January patterns. The northern hemisphere is in winter and is dominated by a vast land mass — Eurasia and North America — which loses heat rapidly. Isotherms therefore deviate sharply: they bend southward over the cold continents (notably the Siberian plain, where the −20°C isotherm runs from 50° N to 80° N along 60° E) and northward over warm oceans, particularly the North Atlantic warmed by the Gulf Stream and North Atlantic Drift. Mean January temperatures range from over 27°C in equatorial oceans to as low as −18°C to −48°C in the Eurasian interior. The southern hemisphere is in summer, but more importantly it is overwhelmingly oceanic — and the moderating effect of water keeps the isotherms much more parallel to latitudes. The 20°C, 10°C and 0°C isotherms run almost regularly along 35° S, 45° S and 60° S. So January isotherms zig-zag in the north and run smooth in the south.
Project Work — Climatological Observations
PW Select a meteorological observatory located in your city or near your town. Tabulate temperature data: (i) note altitude, latitude and the period of the mean; (ii) define temperature-related terms in the table; (iii) calculate the daily mean monthly temperature; (iv) draw a graph showing daily mean maximum, minimum and mean temperature; (v) calculate the annual range of temperature; (vi) find which months have the highest and lowest daily ranges; (vii) list the factors that determine the temperature of the place and explain causes of variation in January, May, July and October.
Worked Example — New Delhi (Safdarjung), 28°35' N, altitude 216 m, observations 1951–1980.
January: mean of daily maximum = 21.1°C; mean of daily minimum = 7.3°C. Daily mean monthly temperature = (21.1 + 7.3) / 2 = 14.2°C.
May: mean of daily maximum = 39.6°C; mean of daily minimum = 25.9°C. Daily mean monthly temperature = (39.6 + 25.9) / 2 = 32.75°C.
Annual range of temperature = Mean Max in May − Mean Temp in January = 32.75°C − 14.2°C = 18.55°C.
Procedure for your station: (1) Visit the IMD website or the local met office; (2) tabulate monthly mean max and min for at least 30 years; (3) plot a line graph with month on x-axis and three lines (max, min, mean) on y-axis; (4) compute the daily range each month and find the largest (typically pre-monsoon May) and smallest (monsoon July); (5) explain January variation by latitude + clear-sky cooling, May by clear-sky pre-monsoon insolation, July by monsoon cloud-cover, October by retreating monsoon and post-monsoon clarity.
January: mean of daily maximum = 21.1°C; mean of daily minimum = 7.3°C. Daily mean monthly temperature = (21.1 + 7.3) / 2 = 14.2°C.
May: mean of daily maximum = 39.6°C; mean of daily minimum = 25.9°C. Daily mean monthly temperature = (39.6 + 25.9) / 2 = 32.75°C.
Annual range of temperature = Mean Max in May − Mean Temp in January = 32.75°C − 14.2°C = 18.55°C.
Procedure for your station: (1) Visit the IMD website or the local met office; (2) tabulate monthly mean max and min for at least 30 years; (3) plot a line graph with month on x-axis and three lines (max, min, mean) on y-axis; (4) compute the daily range each month and find the largest (typically pre-monsoon May) and smallest (monsoon July); (5) explain January variation by latitude + clear-sky cooling, May by clear-sky pre-monsoon insolation, July by monsoon cloud-cover, October by retreating monsoon and post-monsoon clarity.
8.14 Chapter Summary & Key Terms
📚 Chapter Summary
- The earth receives almost all its energy from the Sun as insolation — short-wave radiation averaging 1.94 cal/sq cm/min ≈ 1.367 kW/m² at the top of the atmosphere.
- Earth-Sun distance varies: perihelion (3 January, 147 mn km) and aphelion (4 July, 152 mn km); the effect on weather is small, masked by the axial tilt.
- Five factors govern surface insolation: rotation, angle of incidence, day length, atmospheric transparency, and land configuration.
- The atmosphere is heated by four processes: radiation, conduction, convection, advection, plus terrestrial long-wave radiation from the warmed surface.
- The earth's heat budget balances perfectly: 35 units reflected (albedo), 14 absorbed by atmosphere, 51 by surface; 17 + 48 = 65 units returned to space — net change zero.
- Temperature distribution depends on latitude, altitude, distance from sea, air-mass & currents, slope aspect and cloud cover. Isotherms visualise it on maps.
- The normal lapse rate is 6.5°C per 1,000 m. When inverted, an inversion of temperature occurs — common on clear, calm winter nights and over polar regions.
- In valleys, cold air drainage produces a frost hollow; this is why apple orchards in Kashmir and Himachal sit on slopes, not valley floors.
| Term | Definition |
|---|---|
| Insolation | Incoming solar radiation received by earth, in short wavelengths. |
| Solar constant | ≈ 1.94 cal/sq cm/min ≈ 1.367 kW/m² at top of atmosphere. |
| Perihelion | 3 January position, earth nearest the Sun (147 mn km). |
| Aphelion | 4 July position, earth farthest from the Sun (152 mn km). |
| Angle of incidence | Angle at which Sun's rays strike a surface; high near equator, low near poles. |
| Albedo | Percentage of incoming radiation reflected by a surface; earth as a whole ≈ 35%. |
| Conduction | Heat transfer between bodies in direct contact. |
| Convection | Vertical heat transfer by rising warm air; confined to troposphere. |
| Advection | Horizontal heat transfer by moving air; dominant in middle latitudes. |
| Terrestrial radiation | Long-wave radiation emitted by the warmed earth's surface. |
| Heat budget | Annual balance of incoming and outgoing radiation; 65 in = 65 out. |
| Isotherm | Line on a map joining places of equal temperature. |
| Lapse rate (normal) | Rate of temperature decrease with height; 6.5°C per 1,000 m. |
| Continentality | Strong seasonal temperature swing inland, away from sea's moderating effect. |
| Temperature inversion | Reversal of normal lapse rate; air warmer above, cooler below. |
| Frost hollow | Valley-bottom pocket of pooled cold air that forms frost on calm clear nights. |
🎯 Competency-Based Questions — Temperature, Inversion & Distribution
Case Stem. An apple farmer in Shimla (31° N, 2,205 m) records the following on a clear December night: temperature at the orchard (mid-slope, 2,100 m) at midnight is 4°C; in the valley floor 600 m below at the same hour the temperature is −2°C. By 5 a.m., a thick fog blankets the valley. The next morning, smoke from a roadside fire 200 m below in the valley spreads horizontally rather than rising. Use this scenario to answer Q1–Q4.
Q1. The fact that the valley floor is colder than the mid-slope orchard at midnight is best described as —
L3 ApplyAnswer: (c). Cold air, being heavy, has drained from the higher slopes into the valley bottom, forming a frost hollow. The orchard sits in the warmer "thermal belt" above the inversion. Normal lapse rate would predict the opposite — temperature falling with height.
Q2. Why does the smoke spread horizontally instead of rising?
L4 AnalyseBecause the inversion layer acts as a stable lid. Warm air sits above the cold pooled valley air; rising smoke meets denser, warmer air above and stops climbing — instead it spreads sideways under the inversion. This is exactly why surface inversions trap dust and smog over winter cities.
Q3. Calculate the apparent vertical temperature gradient between the orchard (2,100 m, 4°C) and the valley floor (1,500 m, −2°C). Compare it with the normal lapse rate.
L5 EvaluateApparent gradient: 4°C − (−2°C) = +6°C per 600 m of descent = temperature rises 6°C as you go down. The normal lapse rate would predict a fall of (6.5 × 0.6) = 3.9°C per 600 m descent — i.e. valley should be warmer by about 4°C, not colder. The observed reversal of nearly 6°C demonstrates a strong surface inversion.
HOT Q. Imagine you are advising a horticulture department on where to plant a new apple orchard in a Himalayan valley. Write a 4-point note explaining where, why and how you would site the orchard, using the concepts from this chapter.
L6 CreateHint: (1) Site orchards on the middle slopes (the thermal belt) — above the frost hollow on the valley floor, but not so high that altitude lapse cools them excessively. (2) Choose south-facing aspect in the Northern Hemisphere for maximum direct insolation and longer effective daylight. (3) Avoid steep slopes that would erode after irrigation — moderate slope (15–25°) is ideal. (4) Provide windbreaks on the northern side to block cold advection from higher peaks. The note should mention air drainage, inversion, aspect, lapse rate and continentality — all chapter terms applied to a real planning problem.
⚖️ Assertion–Reason Questions — Class 11
Options:
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
(A) Both A and R are true, and R is the correct explanation of A.
(B) Both A and R are true, but R is NOT the correct explanation of A.
(C) A is true, but R is false.
(D) A is false, but R is true.
Assertion (A): In January, the isotherms over the Northern Hemisphere bend southward over land but northward over the ocean.
Reason (R): Land cools faster than water, and the warm Gulf Stream and North Atlantic Drift keep the northern Atlantic Ocean warmer than the surrounding continents.
Answer: (A) — Both true and R precisely explains A. The much larger northern landmass and the ocean currents combine to push isotherms into a strongly zig-zag pattern in winter.
Assertion (A): Apple orchards in Kashmir and Himachal Pradesh are planted on the middle slopes, not on the valley floors.
Reason (R): Cold air drains downward by gravity at night, forming frost hollows at valley bottoms; the middle slopes form a warmer thermal belt safe from frost damage.
Answer: (A) — Both true and R is the correct explanation. This is a classic application of the air-drainage and inversion concept to agricultural land use.
Assertion (A): The annual range of temperature is highest over the north-eastern part of the Eurasian continent.
Reason (R): The region lies far from any moderating ocean and experiences extreme continentality, with very hot summers and very cold winters.
Answer: (A) — Both true and R is the correct explanation. The continentality of the Siberian/Verkhoyansk region produces an annual range of more than 60°C — the largest on earth.
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Class 11 Geography — Fundamentals of Physical Geography
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