What’s Monsoon Wind ; Why Monsoon blow from north east in winter and southwest in summer?

April 5, 2024

Ref:

https://www.hko.gov.hk/en/publica/gen_pub/files/sms_Urdu.pdf

https://www.hko.gov.hk/en/education/weather/meteorology-basics/00010-geostrophic-wind.html

https://www.hko.gov.hk/en/education/weather/monsoons/00068-why-the-winter-monsoon-in-hong-kong-is-generally-called-the-northeast-monsoon.html

 

所謂季候風就係會隨著季節改變既風, 但光是研究呢個季候風既風向都搞左我粒幾鐘, 因為背後講緊既CONCEPT 有好幾個:

  1. 咩叫High pressure system, 咩叫Low pressure system
  2. 咩係風
  3. 咩叫Coriolis effect (科里奧利力/科科氏力)
  4. 咩叫Geostrophic Wind (地轉風)

 

想簡單D咁理解就要背少少野 (明白少少基本CONCEPT)

  1. High, Low pressure system 是相對 (relative) : compare one air mass with another one
    仲要記得通常凍就係有高壓區, 熱就係有低壓區~
  2. 風就是due to two air mass has pressure difference and so the pressure gradient as well as the pressure gradient force (PGF) exist, then due to hydrodynamics  (some physic, boring stuff), the air molecule at high pressure area tend to “flow to ” low pressure area to reach an equilibrium state, this movement (movement of air molecule hitting on object, say human), we can sense it and the it is referring to wind. 再簡單就係因為有pressure difference induce PGF, 所以空氣粒子流動就有風!
  3. Coriolis effect is a finding that in northern hemisphere, when air mass travel long distance, it will deflect to right due to the earth is rotating!
  4. Geostrophic Wind is talking about when the PGF is equal to Coriolis force, then the overall resultant force will parallel to isobar, and the air mass’s movement will same in direction as this resultant force – this is Geostrophic Wind in ideal case!

    So, 實際結果會係

有左呢D基本既知識我地就解釋到點解冬天係香港有東北季候風, 夏天就係西南風~

原因係冬天, 大陸相對凍 (所以HIGH PRESSURE), 而香港位置南海 (相對溫暖就有LOW PRESSURE), 運用HIGH TO LOW CONCEPT , 加埋真實世界有山有OBJ! 同埋地球係有轉! 所以PGF 被FRICTION 同CORILOIS FORCE 搞搞之後就變左所謂既東北風~

Figure 1:  Dot lines are isobars. R is the vector sum of pressure gradient force (P), Coriolis force (C) and friction (F). Prevailing winds over the southern China is generally from the northeasterly in winter monsoon

 

至於夏天，因為水係比較好吸熱，而夏天大陸陸地升溫相對快，所以大陸熱 (LOW), 南海相對冷 (HIGH), 地球都係有轉, 又係有FRICTION, 所以:

希望無理解錯…訓得~

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