Humid coast or dry interior: where should the heat alert go?
In May, the district health office compares two weather stations before issuing a heat alert. Digha, on the coast, records a maximum of 36.4°C, relative humidity 78%, and a sea breeze of 18 km/h; its minimum is 29.1°C. Bankura, inland, records 43.1°C, relative humidity 26%, almost calm winds, and a minimum of 26.8°C. Both stations are in the same meteorological subdivision and both have large outdoor labouring populations. The office has funds for cooling centres in only one block and must decide where to issue a red alert and what advice to give. Officials disagree: one argues that Bankura is clearly worse because the air temperature is 6.7°C higher, while the other points to Digha's humidity. Newspapers have reported more deaths in the coastal block among elderly residents, but the inland block has more cases of heat exhaustion among construction workers.
1. Explain, with reference to sensible and latent heat exchange, why Digha can produce greater heat stress at a lower air temperature than Bankura.
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Heat stress depends on the body's ability to lose heat, not on air temperature alone. At Digha the air is 36.4°C, close to skin temperature of about 35°C, so sensible heat loss by convection and radiation is small. With 78% relative humidity, vapour pressure is high, so sweat evaporates slowly and latent cooling is limited; the sea breeze gives some ventilation but also brings moist air. The result is a high wet-bulb temperature. At Bankura the air is hotter at 43.1°C, so dry convection adds heat, but 26% relative humidity allows sweat to evaporate rapidly if water is available, giving a lower wet-bulb temperature. A fit, hydrated person can therefore lose heat in Bankura's dry air, whereas in Digha even a resting person may not. The key difference is how the body's heat loss is partitioned between sensible and latent pathways.
2. Which block should receive the red alert and the cooling centres? Justify your choice and explain why a single 'highest temperature' criterion is inadequate.
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Both blocks need alerts, but the interventions should differ. Digha's humid heat produces a high heat index and is especially dangerous for the elderly, infants and people indoors without fans, because evaporation cannot cool them; cooling centres, fans and drinking water should be concentrated there, and advice must stress indoor risk, not only outdoor work. Bankura's dry heat is dangerous for outdoor labourers, who may dehydrate without feeling thirst; shade, water, work-rest cycles and timing of work are the cheap and effective measures. If only one block can have cooling centres, choose Digha, because mechanical cooling is the only reliable relief when evaporation fails, whereas in Bankura shade and water can be distributed widely. A highest-temperature criterion misranks the two places because it ignores humidity, wind and acclimatisation. A better criterion combines heat index or wet-bulb temperature with vulnerability data such as age structure, outdoor employment and housing quality.
3. Account for the difference in night-time temperatures between the two stations and explain why the minimum matters for health planning.
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Digha's minimum of 29.1°C is high because the air is moist and lies over a warm sea surface; water vapour absorbs and re-emits long-wave radiation, so the surface cools slowly, and the sea breeze continues to supply warm, moist air at night. Bankura's dry air allows rapid nocturnal radiative cooling under clear skies, so its minimum falls to 26.8°C despite a much hotter day. The minimum matters because heat stress is cumulative: if nights remain hot, people cannot recover from daytime heat load, sleep is disturbed, and cardiovascular strain continues. Digha's small diurnal range means there is little night-time relief, which helps explain the reported deaths among elderly coastal residents. Health planning should therefore use night-time temperature and humidity as well as daytime maxima, and should time cooling-centre opening through the night in Digha while emphasising daytime water and shade in Bankura.