A tribute to the northeast monsoon

This past weekend we saw one of the sharpest and most welcome weather changes I can remember here in Hong Kong. This year’s unusually hot and humid autumn finally gave way to the gloriously Mediterranean-style sun and dry air we expected to arrive several weeks earlier.

What was the problem? Why did it stay so uncomfortably steamy so long this year? We can blame it all on the failure of the northeast monsoon to arrive on time.

In point of fact, we had already had a couple of ‘surges of the northeast monsoon’ this autumn, but they were very weak, with winds barely making it around to the northeast, and quickly changing back to the east or southeast thereafter. This is a recipe for stuffy humidity, and it’s just what we got.

But instead of spending the rest of this article moaning about why the northeast monsoon was late this year, let’s instead focus on being grateful we receive it at all. Why?

Simply because Hong Kong is geographically a tropical city. This indisputable fact surprises many people who live here. I’ve had numerous conversations with other expats who assure me that Hong Kong is subtropical, and that’s why we get a nice mild winter instead of year-round heat. I agree, but then point out that it’s only Hong Kong’s climate that’s subtropical: our latitude – 22’ 15” north – is well within the Tropic of Cancer, which lies at 23’ 30”. (This might also be an opportune time to say that I’m very grateful to MrB and the several other extremely tolerant people who are willing to be known as my friends in spite of my tendency to carry on just this sort of conversation.)

In fact, Hong Kong’s latitude is about the same as places like Havana in Cuba, Calcutta in India, and the USA’s Hawaiian islands. Rio de Janeiro is at similar latitude in the southern hemisphere. Although these places also have some seasonal variations, these are not cities you really think of as having ‘winter’.

It’s the northeast monsoon that makes Hong Kong different. It’s this prevailing autumn and winter wind pattern, in which cold air and high pressure build up over the Asian continent and then spill southwards, that finally gives us a break from perpetual summertime down here on China’s southern coast.

So – mild temperatures, sunny skies, dry air: what’s not to like? There are two potential problems with the northeast monsoon.

The first has to do with what lies to our northeast, i.e. the heart of the Guangdong industrial heartland. The northeast monsoon serves as an aerial conveyor belt, shifting clouds of particulates and other pollutants right down over Hong Kong. MrB has explained convincingly how this pollution affects us.

But then why have the past three days been relatively clear? There are both quantitative and qualitative differences in the northeast monsoons that affect Hong Kong. The big, powerful surge in the past few days was strong enough – i.e. the winds were sufficiently brisk – to disperse and blow much of the polluted air right on past us. It’s the ‘weak monsoons’ that are the killers in terms of pollutants, as this article from the Hong Kong Observatory explains:

In winter and spring, reduced visibility usually occurred in association with weak northerly surges of the monsoon or when the northeast monsoon affecting Hong Kong was subsiding.

A weak monsoon gradually shifts southward the clouds of pollutants that would otherwise hang over Guangdong. And then when the monsoon sometimes fails completely for a day or two (watch out for ‘light winds’ in the HKO’s forecasts), those pollutants are left hanging right on top us here in Hong Kong.

The other potential problem with the northeast monsoon is that it gets a little stronger at points than some of us might prefer. That is, it can get really cold in Hong Kong. For example, if you were in town last February, when Hong Kong endured the longest stretch of cold weather we’ve had in the past four decades, you might have wistfully been wondering what the weather was like in Havana or Calcutta.

That stretch of cold was nothing, though, compared to mid-January in 1893. This contemporaneous article from Nature magazine reports on four-day stretch of extreme cold, at the depths of which the Hong Kong Botanical Gardens recorded a temperature of 31 F, or -1 C.

Even worse, throughout much of this cold spell freezing rain fell on Hong Kong. Trees and other vegetation were coated with ice, ships in the harbor had ice on their rigging, and telegraph and telephone wires became ice-laden, snapping many. There was widespread agricultural devastation, as many tropical plants such as bananas and bamboo were destroyed by the cold. Even the sea was affected, with record low water temperatures killing many fish.

This 1893 surge really was historic in its intensity. It seems to be one of the only recorded instances anywhere, anytime, of freezing temperatures at sea level in the tropics. The author of the Nature article mentions that no temperature below 40 F (i.e. 4 C) had previously been recorded in colonial Hong Kong. And the cold air in 1893 rushed on past Hong Kong to cool off other British outposts, too, dropping temperatures in Singapore below 70 F (21 C) and into the high 50s F (13-14 C) in Sri Lanka.

Why was the northeast monsoon so intense that year? The Nature article makes one final useful allusion, to the eruption of a volcano the previous year on Great Sanguir Island (you can find an account here) in what is now Indonesia (it’s now more commonly spelled ‘Sangir’, by the way). The ash and other pollutants really big eruptions pour into the atmosphere commonly result in a year or two of pronounced global cooling, so the 1892 Sanguir eruption may well have been the cause of 1893’s brutal cold.

Given the unusual circumstances in 1892, and the buildup of an enormous urban heat island in today’s Hong Kong, it’s unlikely any of us will witness frost in the harbor. But I do encourage you to enjoy – and appreciate! – the northeast monsoon.



What a great article - thanks!

HK and Kauai

We lived on Kauai for 4 years. Loved the winters. So when we were going to move to HK, I looked on a map and saw the latitude was virtually the same. So I thought, great, warm winters. Then we actually experienced winter in HK and were so cold! I thought, what in the world is going on here? Hadn't thought of the monsoon or the fact that we are connected to a large, cold continent. Now we're more prepared . . . .