1401 - Northeasterlies and ourselves 東北季風與我們

1. Throughout January 2014, nothing has been recalled so clearly on the cold weather and sudden decrease in temperature. In our city, Hong Kong, it is very often to be affected by Northeast Winter Monsoon. At the time it bursts, most of the places in China have suffered from a sharp cooling event (7 - 13 Degrees Celsius). And after a plenty of days, the weather recovers (15 to 18 degrees Celsius) But, at first, why Winter Monsoon called Northeast Monsoon?

2. First of all, it is important to note that the name "NE Monsoon" should always be interpreted as the prevailing winds coming from North-East Direction towards South-West. 

3. And now, the main problem comes, The coldest place on the Northern Hemisphere of the Earth is, theoretically, "North Pole". Why are all Winter monsoons affecting Hong Kong not come directly from the North Pole? Why not the prevailing winds in Winter are coming from the North direction, but NE direction?

4. Well, there are several reasons, of course. 

5. One of the reason is that the mountain ranges in China has blocked most of the Northern prevailing winds. As such, it prevents southern expedition of Polar air masses, protecting us from the temperature below freezing point. To be exact, the most influential mountain range responsible for this protection is QinLing (Locating at 33N 107E) and NanLing (Locating at 25N 112E). These mountain range lies in "West to East" Direction (i.e. parallel to Equator) and directly to the North of Hong Kong. 

6. The second reason, the major reason, is due to Coriolis Force. This is not a easy concept and to understand it in full, physical interpretation must be required. In general, Coriolis Force is not a "force" in physics. To be more precise, it should always be referred as "Coriolis Effect". The Coriolis Effect appears as a result of the Earth's rotation. Earth is tilted at an angle of 23 1/2 degrees and rotate along this axis. Rotation causes that objects in polar region has a greater velocity than equatorial region. Coriolis effect in polar region is, therefore, much significant than the tropical region. 

7. The wind directly come from the North Pole will be affected by Coriolis Effect. Winds will be deflected to the right in the Northern Hemisphere. In a simplified diagram, wind direction from High Pressure in North Pole towards Low Pressure in Tropics will be demonstrated like this:

8. Arrow in aqua colour is the theoretical wind flow without considering Coriolis Effect. Green arrow is the wind flow after taking Coriolis Effect into account.

9. As you may notify, air parcel leaving polar region will be deflected immediately clockwise and towards the West or South-west. Therefore, Hong Kong suffers from winds blowing towards South-west which is originated from North-east. The direction from which the monsoon comes determines its name. So, in Winter, we are suffering from North-east Monsoon.

(1): 東北季侯風的狂嘯

1. 在整個一月, 最深刻莫過於寒冷的天氣和氣溫驟降。我們的城市 - 香港, 一年內十二個月中的八個月 (九月至下年四月) 受東北季侯風影響。反之, 西南季侯風對香港的影響較短暫, 大約只是四個月 (五月至八月)。當東北季候風 (稱為寒潮) 大幅南下時, 幾乎整個中國甚至東亞地區會出現廣泛性的氣溫驟降。但當季候風開始緩和時, 我們可以感受到天氣會逐漸變回暖和 (約攝氏15-20度) 。

2. 或許對於不常接觸天氣報告的人, 會常常混淆東北風是怎麼一回事。東北風是指由東北方向吹來的風, 吹向西南方。以香港作為中心, 東北方是大約是汕頭, 福州一帶, 遠一點就是琉球和日本海。這是一個相當容易犯錯的概念, 運用時要加以注意。

3. 那麼, 為甚麼冬天吹的是東北風而不是正北風呢? 

4. 原因有很多, 最主要的莫過於兩個。

5. 第一個原因是因為中國的山脈遮擋了正北季候風的南侵。但正因為這個原因, 香港本土才免受冰寒的北極氣團影響。亦所以香港市區的氣溫 (在21世紀而言) 是很少低於5度。更準確地說, 中國山脈中的秦嶺 (約北緯33度, 東經107度) 和南嶺 (約北緯25度, 東經112度) 是阻擋北風到達香港的主要角色。而且, 這些山脈是東西向的, 所以北風幾乎被封鎖在向風面。

6. 第二個原因是科氏奧利力。這是拆解這個問題最重要的原因。科氏力是一個較為艱深, 需要物理學的基礎才能完全解析其中心的意思。一般來說,科氏力不是一種力。所以理應稱為科氏奧利效應 (中文好像很少用這個詞來表達)。簡單來說, 這個效應是由於地球的自轉而產生出來的。地球以傾斜23 1/2度的軸來自轉。極地的自轉速度較熱帶地區高, 因此極地所受到的科氏力亦比赤道大。

7. 所以, 盛行風 (即一個地方常常受到其影響的風) 會受到科氏力影響。在北半球, 從高氣壓吹出來的風將會偏向右方 (順時針)。下圖是以最簡化的模式來表達由高壓吹往低壓的風向。

8. 淺藍色的箭咀描述在沒有科氏力影響的風向, 綠色箭咀是描述受到科氏力影響時的風向。

9. 或許你會察覺到當風由極地吹出時, 其風向會立即向順時針的偏離。因此香港便受到吹往西南方向的風所影響, 由於它是從東北方向吹來, (風的方向取決於它是從那個地方吹), 因此我們稱之為東北季候風。在冬天, 香港有大部分時間受到東北風所影響。但有時強烈的寒潮可將極地氣團從正北方帶來, 香港便會大幅降溫。