Frost risk in a Sonitpur tea garden
The manager of a tea estate in Sonitpur reads a forecast for a clear, calm night in late December. At 6 p.m. the estate office on a ridge (90 m elevation) records 8°C, and the temperature is expected to fall about 0.6°C per hour until 5 a.m. A low-lying section of the garden, 12 m below the office, holds young tea bushes. Last year a similar night damaged 30% of bushes in that section, while bushes on the ridge were unharmed. The manager has eight workers, a limited supply of straw, and can light small smoky fires or use water sprays. She must decide whether to act, where to concentrate effort, and which method to use.
1. Calculate the likely minimum temperatures on the ridge and in the low-lying section. Explain why the low section is colder although it is only 12 m lower.
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From 6 p.m. to 5 a.m. is 11 hours. At 0.6°C per hour, the fall is 6.6°C. Ridge minimum = 8 – 6.6 = 1.4°C. The low section is about 3°C colder on such nights, so it could reach 1.4 – 3 = –1.6°C. The 12 m height difference itself is almost negligible: at a normal lapse rate it would account for less than 0.1°C. The real cause is cold-air drainage. On a clear, calm night the ground loses heat by radiation. Air in contact with the cooled slope becomes dense and flows downhill, collecting in hollows. The ridge loses heat too, but air there is mixed and can drain away. The low section becomes a cold-air pool, so it has an inversion: temperature rises with height instead of falling. That is why frost hits the hollow, not the ridge.
2. Which method should the manager choose: smoky fires, straw, or water spray? Justify using the conditions and resources.
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The immediate aim is to protect young bushes in the coldest hollow. Smoky fires add some heat and smoke, but with only eight workers they cannot be kept going across a large area, and smoke is not reliable on a calm night because it stays low. Straw is best used as a mulch around the base and over low bushes; it reduces heat loss from the soil and protects buds, and the limited supply can be concentrated in the lowest section. Water spray can release latent heat when water freezes, but it must be continuous until after sunrise; if it stops, the wet bushes may freeze worse, and the estate may lack a reliable supply. My choice: concentrate straw in the hollow, prepare small smoky fires only as a backup, and use water spray only on a small block with a guaranteed supply. Do not spread all three thinly everywhere.
3. The forecast changes to a strong wind. How does this change the frost type and the plan?
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A strong wind changes the situation from radiation frost to advection frost. Radiation frost forms in calm, clear conditions when cold air drains into hollows; the wind would mix that air and may reduce the hollow's advantage. But advection frost occurs when a large mass of cold air moves in, and it affects the whole garden, not just low patches. Wind also removes the warm layer that fires and smoke try to create, so smoky fires become even less effective. Water spray can drift and cause evaporative cooling, so it is risky. The best response is to protect the most valuable blocks with windbreaks and mulch, delay pruning or applying nitrogen, and accept that a whole-garden frost cannot be beaten by small fires. The manager should not spend all labour on the hollow.