Rajasthan Thornthwaite Climate Classification GK
PART - 12: Thornthwaite's Climate Classification#
Detailed Index#
| Section |
Topic |
Exam Importance |
| 12.1 |
Conceptual & Scientific Foundation |
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| 12.2 |
Letter Code Decoding |
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| 12.3 |
Four Zones of Rajasthan |
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| 12.4 |
Detailed Zonal Analysis |
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| 12.5 |
Koppen vs Thornthwaite |
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| 12.6 |
Key Exam Pointers |
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| 12.7 |
Comparative Study |
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π 12.1 Conceptual & Scientific Foundation
American geographer and climatologist C.W. Thornthwaite presented the classification of world climate in 1931, which he revised in 1933 and 1948.
While KΓΆppen's classification was mainly 'Empirical' and focused on vegetation, Thornthwaite's classification is considered more logical as it is based on Water Balance and mathematical indices.
| Basis of Classification |
Scientific Concept |
Variables |
| 1. Precipitation Effectiveness Index (P-E Index) |
Proportional relationship between total rainfall received and total evaporation in an area |
$P\text{-}E\text{ Ratio} = 115 \times \left(\frac{P}{T - 10}\right)^{\frac{10}{9}}$ |
| 2. Thermal Efficiency Index (T-E Index) |
Amount of thermal energy available for plant growth |
$T\text{-}E\text{ Ratio} = \frac{T - 32}{4}$ |
| 3. Potential Evapotranspiration (PET) |
In the 1948 revision, Potential Evapotranspiration and Moisture Index ($I_m$) were made the main basis |
$I_m = \frac{100s - 60d}{\text{PET}}$ |
π 12.2 Decoding of Thornthwaite's Letter Codes
Thornthwaite used a combination of three levels of letter codes to represent the climate of Rajasthan:
| Letter |
Scientific Name |
Meaning & Context in Rajasthan |
| C |
Sub-Humid |
Sub-Humid zone (P-E Index: 32-63); Southern & South-Eastern Rajasthan |
| D |
Semi-Arid |
Semi-Arid zone (P-E Index: 16-31); Central & Northern Rajasthan |
| E |
Arid |
Arid desert zone (P-E Index: <16); Western Thar Desert |
| A' |
Megathermal |
Tropical thermal efficiency (T-E Index β₯ 128); extreme summer heat |
| B' |
Mesothermal |
Mesothermal efficiency (T-E Index: 64-127); Northern Rajasthan |
| d |
Dry in all seasons |
Deficiency of rainfall in all seasons; Jaisalmer region |
| w |
Winter Dry |
Winter dry; monsoon rainfall only in summer/rainy season |
π 12.3 Four Thornthwaite Climatic Zones of Rajasthan
According to Thornthwaite, Rajasthan is divided into 4 main climatic zones:
| S.No. |
Climate Code |
Zone Name |
Representative District |
Avg Rainfall |
Major Districts & Extension |
| 1. |
CA'w |
Tropical Humid Zone |
Dungarpur |
80-100 cm+ |
Banswara, Dungarpur, Pratapgarh, Jhalawar, Baran, Kota, Mount Abu |
| 2. |
DA'w |
Tropical Semi-Arid Zone |
Ajmer |
50-80 cm |
(Largest by area) Sirohi, Pali, Ajmer, Bhilwara, Tonk, Jaipur, Dausa, Sawai Madhopur, Karauli, Bundi |
| 3. |
DB'w |
Mesothermal Semi-Arid Mixed Zone |
Bikaner |
20-40 cm |
Sri Ganganagar, Hanumangarh, Bikaner, Churu, Northern Jhunjhunu |
| 4. |
EA'd |
Tropical Very Arid Desert Zone |
Jaisalmer |
10-20 cm |
Jaisalmer, Barmer, Phalodi, Western Bikaner, Western Jodhpur |
π 12.4 Detailed Zonal Analysis
1οΈβ£ EA'd Zone β Tropical Very Arid Desert Climate#
| Aspect |
Description |
| Code Meaning |
E = Arid, A' = Megathermal, d = Deficiency in all seasons |
| Representative District |
Jaisalmer |
| Extension |
Entire Jaisalmer, Barmer, Western Phalodi, Western Bikaner, Western Jodhpur |
| Rainfall |
10-20 cm (highly uncertain) |
| Temperature |
Summer: 45-48Β°C; Winter: 0-5Β°C |
| Vegetation |
Xerophytes β Phog, Kair, Lani, Sewan grass |
2οΈβ£ DB'w Zone β Mesothermal Semi-Arid Climate#
| Aspect |
Description |
| Code Meaning |
D = Semi-Arid, B' = Mesothermal, w = Winter Dry |
| Representative District |
Bikaner |
| Extension |
Sri Ganganagar, Hanumangarh, Bikaner, Churu, Northern Jhunjhunu |
| Rainfall |
20-40 cm |
| Temperature |
Reaches freezing point in winter; intense heat in summer |
| Vegetation/Agriculture |
Babool, Khejri; wheat, cotton, kinnow with canal irrigation |
Steppe Type#
| Aspect |
Description |
| Code Meaning |
D = Semi-Arid, A' = Megathermal, w = Winter Dry |
| Representative District |
Ajmer |
| Extension |
Sirohi, Jalore, Pali, Eastern Jodhpur, Nagaur, Ajmer, Tonk, Bhilwara, Jaipur, Dausa, Alwar, Sawai Madhopur, Karauli |
| Rainfall |
50-80 cm |
| Vegetation |
Steppe & Dry Deciduous β Khejri, Rohida, Dhokra, Neem, Ber |
| Special |
Largest zone by area |
Tropical Humid#
| Aspect |
Description |
| Code Meaning |
C = Sub-Humid, A' = Megathermal, w = Winter Dry |
| Representative District |
Dungarpur |
| Extension |
Vagad (Banswara, Dungarpur, Pratapgarh), Hadoti (Jhalawar, Baran, S. Kota), Mount Abu |
| Rainfall |
80-100 cm+ (Mount Abu 150 cm) |
| Vegetation |
Savanna, Bamboo, Mango, Mahua, Dry Teak |
| Special |
Water surplus condition; lowest annual temperature range |
π 12.5 Koppen vs Thornthwaite: Comparative Analysis
| Thornthwaite Code (P-E based) |
Koppen Code (Vegetation based) |
Equivalence |
Main Difference |
| EA'd |
BWhw |
Western Desert (Jaisalmer) |
KΓΆppen emphasized winter dryness (w), while Thornthwaite highlighted year-round dryness (d) |
| DB'w |
BShw |
Northern Rajasthan (Ganganagar, Bikaner) |
KΓΆppen considered it hot (h), while Thornthwaite considered it mesothermal (B') due to lower temperature |
| DA'w |
BShw + Cwg |
Central & Eastern Rajasthan (Ajmer) |
This Thornthwaite zone encompasses large parts of two separate KΓΆppen zones |
| CA'w |
Aw |
Southern & South-Eastern part (Dungarpur) |
Both classifications present almost similar boundaries here |
π 12.6 High-Yield Exam Pointers
| Fact |
Description |
| Representative Districts |
CA'w β Dungarpur, DA'w β Ajmer, DB'w β Bikaner, EA'd β Jaisalmer |
| Largest Zone |
DA'w (Ajmer) β according to Thornthwaite |
| Meaning of 'd' |
Deficiency of rainfall in all seasons |
| Meaning of 'B' |
Mesothermal efficiency |
| Meaning of 'A' |
Megathermal efficiency |
| Main Basis of 1931 |
Precipitation Effectiveness (P-E Ratio) & Thermal Efficiency (T-E Ratio) |
| Main Basis of 1948 |
Potential Evapotranspiration (PET) |
π 12.7 Comparative Study
| Aspect |
EA'd |
DB'w |
DA'w |
CA'w |
| Rainfall |
10-20 cm |
20-40 cm |
50-80 cm |
80-100+ cm |
| Representative District |
Jaisalmer |
Bikaner |
Ajmer |
Dungarpur |
| Vegetation |
Xerophytes |
Steppe |
Steppe/Deciduous |
Savanna/Teak |
| Temperature |
45-48Β°C |
40-45Β°C |
35-40Β°C |
30-35Β°C |
| Area |
Medium |
Medium |
Largest |
Medium |
Exam Revision#
Water balance -> P-E Index and T-E Index -> PET -> Thornthwaite letter codes -> CA'w, DA'w, DB'w, EA'd -> representative districts -> comparison with Koppen
rajasthan-thornthwaite-climate-classification, thornthwaite, thornthwaite-letter-codes, precipitation-effectiveness, thermal-efficiency, pet, water-balance, ca-w, da-w, db-w, ea-d