The band of Earth between the Tropic of Cancer (23.5°N) and the Tropic of Capricorn (23.5°S) where the sun can be directly overhead. South Asia spans the northern half of the tropics.
climateWhat are the Tropics?
The tropics are the geographic zone between the Tropic of Cancer (23.5° North latitude) and the Tropic of Capricorn (23.5° South latitude). This 47° band of latitude around the equator covers roughly 40% of Earth’s surface and contains about 40% of the global population.
For South Asia, the tropics define the climate of more than half the subcontinent. The Tropic of Cancer crosses Bhuj (Gujarat), Udaipur, Bhopal, Jabalpur, Ranchi, Agartala — so everything south of this line is fully tropical: most of India south of Madhya Pradesh, all of Bangladesh, Sri Lanka, the Maldives, and southern Pakistan (Karachi, Sindh, southern Balochistan).
Tropical regions share several defining characteristics:
- High solar radiation year-round (sun reaches overhead at least once per year)
- Limited seasonal temperature variation (mostly warm year-round)
- Strong monsoon or wet/dry seasons rather than four temperate seasons
- Distinct rainy and dry seasons
- Tropical cyclones, typhoons, hurricanes in coastal regions
- Highest UV Index values on Earth
- Dense vegetation where rainfall is high (rainforests)
- Wide range of cultures, languages, ecosystems
Defining the tropics geographically
The 23.5° latitude marks where the sun is directly overhead at the summer solstice in each hemisphere:
- Tropic of Cancer (23.5°N): sun overhead on June 21 (Northern Hemisphere summer)
- Tropic of Capricorn (23.5°S): sun overhead on December 21
- Equator (0°): sun overhead at the equinoxes (March 21, September 21)
Within this zone, the sun crosses directly overhead at least once per year, while outside it, the sun never quite reaches zenith. This is the fundamental basis of the tropical climate — extreme solar energy input.
Subdivisions of the tropics
Climate scientists subdivide tropical regions by precipitation:
1. Tropical rainforest (Af in Köppen system):
- Year-round rain (>60 mm every month)
- No dry season
- Examples: parts of Western Ghats, Northeast India (Cherrapunji), Sumatra, Java
2. Tropical monsoon (Am):
- High annual rain (>1500 mm)
- One short dry season
- Examples: Kerala, Konkan coast, Mumbai, parts of Bangladesh, Yangon
3. Tropical savanna (Aw):
- Distinct wet and dry seasons
- Less total rainfall (700-1500 mm)
- Examples: Central India, Deccan Plateau, parts of Karnataka
4. Tropical desert (BWh):
- Very low rainfall (<250 mm)
- Hot year-round
- Examples: Sindh, Cholistan, parts of Rajasthan, Thar Desert
Despite shared latitude, these vary hugely. Mumbai gets 2,400 mm rain; Jodhpur gets 360 mm — both technically tropical.
South Asia in the tropics
Fully tropical (south of Tropic of Cancer):
- All of South India (Kerala, TN, Karnataka, AP, Telangana)
- All of Bangladesh
- Sri Lanka, Maldives
- Southern Pakistan (Karachi, Sindh, southern Balochistan)
- Southern India coastal Maharashtra/Gujarat
Partly tropical / mostly tropical:
- Central India (Madhya Pradesh, Chhattisgarh, Maharashtra)
- Bihar, Jharkhand, Odisha
- West Bengal
Sub-tropical (north of Tropic of Cancer):
- Delhi, UP, Punjab, Haryana, Rajasthan
- Pakistan Punjab, KP plains
- Nepal Terai
- Bhutan
- Northern Pakistan (Gilgit-Baltistan)
This is why southern India experiences different weather patterns than the Indo-Gangetic plain — sub-tropical India sees Western Disturbances and four distinct seasons, while tropical India sees two seasons (wet monsoon, dry rest) with a brief cool window.
Tropical weather phenomena
Monsoon:
- The Asian monsoon is the world’s largest tropical weather system
- Driven by land-sea temperature contrast and ITCZ migration
- Delivers 75% of South Asia’s annual rainfall
Tropical cyclones:
- Form over warm tropical oceans (SST ≥ 26.5°C)
- Bay of Bengal, Arabian Sea, Western Pacific, Atlantic, etc.
- Bay of Bengal is the world’s deadliest cyclone basin per capita
Thunderstorms:
- Most frequent in tropics — equatorial regions can see daily storms
- Pre-monsoon Nor’westers over Bangladesh and Bengal are particularly violent
Heatwaves:
- Pre-monsoon (April-June) sees record temperatures
- South Asian wet-bulb temperatures approaching survivability limit
Tropical air pollution:
- High solar radiation accelerates ozone formation in summer
- Monsoon cleans air; dry season concentrates it
Climate change in the tropics
Tropical regions face disproportionate climate risks:
- Heat extremes: tropical land areas already at human survivability margins
- Tropical cyclones: intensifying as ocean warms
- Monsoon disruption: more intense rain in shorter bursts, longer dry spells
- Sea-level rise: coastal cities Mumbai, Karachi, Dhaka, Chennai threatened
- Glacier loss: Himalayan headwaters of tropical Asian rivers retreating
- Agricultural stress: heat + irregular rain + crop pests
- Health impacts: heat-related mortality, vector-borne disease expansion
The tropics are home to the majority of climate-vulnerable populations — densely populated, often poor, dependent on stable monsoons and coastal stability.
UV Index in the tropics
Tropical sunlight is dramatically more intense than at higher latitudes:
| Location | Latitude | Peak UV Index |
|---|---|---|
| Maldives (Male) | 4°N | 14+ |
| Sri Lanka (Colombo) | 7°N | 14+ |
| Cochin (Kerala) | 10°N | 13 |
| Chennai | 13°N | 12-13 |
| Mumbai | 19°N | 11-12 |
| Delhi | 28°N | 9-10 |
| London | 51°N | 6-7 |
| Moscow | 56°N | 5-6 |
This is why sun protection is essential year-round in tropical South Asia. Sunscreen, hats, sunglasses are not optional.
Health considerations for tropical living
The tropics present specific health concerns:
- Heat-related illness (heat stroke, dehydration)
- Vector-borne diseases (dengue, malaria, chikungunya, Japanese encephalitis)
- Waterborne diseases during monsoon (cholera, typhoid)
- Skin cancer from cumulative UV exposure
- Respiratory infections during dry season + winter haze
- Foodborne illness in hot/humid conditions
Mitigation: hydration, cool indoor spaces, mosquito protection, UV sunscreen, food hygiene.
Frequently asked questions
Where does the Tropic of Cancer cross India? The Tropic of Cancer crosses approximately through Gandhinagar (Gujarat) → Udaipur → Bhopal → Jabalpur → Ranchi → Agartala. Eight Indian states straddle it: Gujarat, Rajasthan, MP, Chhattisgarh, Jharkhand, West Bengal, Tripura, Mizoram.
Are all warm climates tropical? No — “tropical” is defined geographically by latitude (between 23.5°N and 23.5°S), not by temperature. Phoenix, Arizona is hot but not tropical (33°N). Hong Kong is warm but barely sub-tropical (22°N). The defining feature is solar geometry.
Why is rainfall so variable across the tropics? Because tropical climates depend on the ITCZ (Inter-Tropical Convergence Zone), monsoon circulations, and ocean conditions — all of which vary by location and season. Western Ghats receive 4,000+ mm/year, while Thar Desert just 200 km away gets less than 250 mm.
Will climate change affect tropical regions more? Yes in some ways (heat extremes already at margins of human tolerance, agricultural disruption from rainfall changes) and less in others (no extreme temperature swings like the Arctic). Net impact is severe — particularly given population density and limited adaptation resources.
Where can I check weather for tropical cities? Mausam Online covers all major South Asian and Southeast Asian cities. See Mumbai, Chennai, Dhaka, Karachi, Colombo, Bangkok, Singapore, Jakarta.