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Rainfall in the Tropics Part 2 (Lecture 3) (3rd Factor: Orographic Lifting…
Rainfall in the Tropics Part 2 (Lecture 3)
Recap of Lecture Part 1
Stable air
When temperature of the air parcel is the same as the surrounding air,
resists uplift
. Hence, no condensation and no rainfall
When temperature of the air parcel is lower than the surrounding air, air parcel will sink and
resist uplift
. Hence, no condensation, no rainfall
Stable air parcel will only uplift if it is obstructed by a highland or mountain. However, it sinks back to the surface once it crosses the peak of the mountain. Stable air is
dry
, hence no condensation and no rain
Unstable air
When temperature of the air parcel is warmer than the surrounding air, air parcel will be uplifted
Air parcel is uplifted, hence condensation takes place, leading to rainfall
Air is saturated (full of moisture), leading to condensation which forms clouds. This continuous process releases latent heat which heats up the air leading to unstable air again. The cycle repeats and through further cooling and condensation, more clouds are formed, leading to more rainfall
1st Factor influencing rainfall:
Convective Lifting
due to Local Heating
Overhead sun at the Equator (Equatorial Low Pressure Belt) -> Highly Concentrated insolation -> High Terrestrial Radiation and High heating of air -> Higher air temperature
Unstable air (warmer and less dense than the surroundings) -> Convective Uplift -> Condensation -> More latent heat released -> More towering cumulonimbus clouds formed ->Heavy convectional rain (high intensity and short duration)
2nd Factor:
Convergent lifting
(at the Inter-Tropical Convergence Zone)
Temperature difference, Pressure difference, PGF
->Trade winds blow from STH to ELP -> Trade winds converge at the ELP forming ITCZ -> Converging winds are forced to rise, thus convergent lifting (focus is not on the sun)
Similarly, leads to cooling and condensation -> Latent heat released -> Unstable air -> More uplift -> More towering cumulonimbus clouds -> Heavy rain due to convergent lifting at ELP and ITCZ (overhead sun)
3rd Factor:
Orographic Lifting
due to Topography
Topographic barriers (Mountain ranges) block wind movements and forces uplifting of air, possibly producing orographic/relief rain on the windward slope if ascending air is cooled to dew point
Much drier air now sinks as the leeward slope where cooling is replaced by warming and condensation ceases. Hence, little rainfall at leeward slope
Rain shadow is applied to both the leeward slope and the area beyond as far as the drying influence extends
4th Factor: Impact of
ocean currents
on sea-surface evaporation
Warm, moist air over warm ocean currents -> Eastern coasts -> absorb more moisture -> Convective or orographic lifting ->rainfall in the eastern coast (must explain formation of rainfall in detail!)
Cold, dry, stable air over cold ocean currents -> western coasts ->dry conditions(cold) ->no lifting(stable) -> do not form clouds -> no rain in the western coasts (explain in detail how is there no formation of rainfall)
5th Factor: Effect of
Continentality
Further inland -> Summer, moisture from the sea evaporates before reaching inland location -> large range of temperature
Consider stretch of land over which moisture-laden winds blow. Over the long distance covered, winds are no longer holding much moisture (dry wind), hence less rainfall (explain processes!)
Deserts dry in the tropics
Located along STH, stable air, sinking air, no uplifting, no condensation, no rain = DRY
Located along western coast of continents, cold ocean currents, cold wind is dry, no rain = DRY
Located inland, by the time wind covers the long stretch of land, becomes dry, no rain = DRY