NCERT Solutions for Class 9th Science Chapter 13 .2.1 Local winds / 13.2.3 Ocean currents — Pause and Ponder
Book page 26113 Updated on2026-09-08
Q2.
How does the cool mountain breeze benefit agriculture activity, particularly the crops and soil?
Answer
The night-time mountain breeze lowers the temperature in the valley, keeps the air moist and stirring, and slows the loss of water from soil and leaves — all of which help crops and protect soil.
By day the sunlit slope heats, air rises and cool valley air flows up — the valley breeze. After sunset the slope cools faster, and dense cold air slides down into the valley — the mountain breeze.
Why it helps, point by point:
It cuts heat stress. Plants close their stomata and stop photosynthesising efficiently when they get too hot. A cool downslope wind at night brings the field temperature down, so the plant recovers overnight.
It reduces water loss. Cooler air holds less water vapour before saturating, so evaporation from the soil and transpiration from leaves both slow down. The soil keeps its moisture for the next day. The chapter says these winds "help regulate temperature, moisture conditions and support soil and crop health".
It gives a wide day–night temperature difference. Warm days and cool nights are exactly what apples, plums, tea and many spices need for good flavour, colour and sugar content. This is why hill agriculture around Shimla and Dehradun grows crops the plains cannot.
It keeps the air moving and drains cold air away. Still, damp air over a crop invites fungal disease; a gentle breeze dries the leaf surface. The downslope flow also carries cold air off the slopes, reducing frost damage on the slope itself.
It protects the soil. Because the mountain breeze is gentle and does not blow in gusts across bare ground, and because the soil stays moist, wind erosion and drying-out of the topsoil are both reduced.
Tip: the mountain breeze forms because the slope loses heat faster than the valley floor after sunset. The air on the slope cools, becomes denser, and gravity pulls it down. It is dense cold air sinking — nothing pushes it.
Q3.
What happens to the warm surface of water from the equator as it travels toward the poles? What impact does this movement have on the area?
Answer
The warm surface water gives up its heat to the air as it moves poleward, gradually cools, becomes denser, sinks at high latitudes and returns to the equator through the deep ocean. The heat it delivers on the way makes the coasts it passes far warmer than their latitude would suggest.
Warm, less dense equatorial water flows poleward at the surface
Along the way it loses heat to the colder air above → water cools
Cooling (and evaporation, which raises salinity) → density increases
Denser water sinks at high latitudes
Cold, dense water returns towards the equator at deeper levels
→ a continuous conveyor of heat from equator to poles
The effect on the regions it reaches:
It reduces the temperature difference across the planet. The chapter says ocean currents, "by transporting heat from the equator towards the poles … reduce temperature differences across the planet". The equator would be hotter and the poles colder without them.
It keeps high-latitude ports usable. The book's example is the North Atlantic Drift, an extension of the Gulf Stream, which "flows toward the northwestern coast of Europe and keeps many ports ice-free during winter, even at high latitudes."
It supports trade and settlement. Ice-free ports all year mean shipping, trade and commerce can continue — the chapter names this directly.
It feeds marine life. The same movement transports nutrients, so ocean currents "support a massive ecosystem". Where cold, nutrient-rich deep water rises again, fisheries are rich.
Note: the return flow at depth is driven by density, and density depends on both temperature and salinity. "Water with lower salinity, being less dense, tends to remain near the surface, while higher salinity, denser water sinks." Cooling and evaporation together are what start the sinking.