Some folks and I were having a chat this morning about the rainfall in Da Nang and Hoi An, and the question of which place gets more rainfall came up. Some of us were sure that Da Nang received more while some of us thought Hoi An received more, so I figured this would be a good one to investigate and find out what was really going on.
For this I decided I needed to come up with two sets of numbers: 1) average yearly rainfall for both places, and 2) average monthly rainfall totals, again for each city. First I consulted with a few different weather sites and their historical data. Next, it was finding total precipitation figures and average rainfall figures inside a host of PDFs I have that about the area. (Most helpful were some documents related to the rivers, including an EIS [Environmental Impact Survey] for a dam in Quang Nam, and another that is a SEA [Strategic Environmental Assessment] for the Vu Gia and Thu Bon River basins.)
Finding historical rainfall averages for Da Nang was easy; thanks to the airport and the studious French colonists we have a whole host of data. Finding these numbers for Hoi An was much more difficult. I found several sources that had "month by month rainfall" for Hoi An but in the end the numbers were taken from either Da Nang, or worse, Hue, and the numbers were identical (DNG) or way off (Hue).. I did finally find a source that was different from both of the larger cities. Next, I stacked the two together and compared the data.
The numbers for yearly rainfall are a little different depending on which site you check but they are all very close to each other. Da Nang comes in between 2050 mm and 2200 mm per year (average), while Hoi An comes in between 2000 mm and 2100 mm. So, almost even total rainfall on a yearly basis. But what about month to month? This data is a little more interesting. (I've made the month that has the higher number in red.)
Finding historical rainfall averages for Da Nang was easy; thanks to the airport and the studious French colonists we have a whole host of data. Finding these numbers for Hoi An was much more difficult. I found several sources that had "month by month rainfall" for Hoi An but in the end the numbers were taken from either Da Nang, or worse, Hue, and the numbers were identical (DNG) or way off (Hue).. I did finally find a source that was different from both of the larger cities. Next, I stacked the two together and compared the data.
The numbers for yearly rainfall are a little different depending on which site you check but they are all very close to each other. Da Nang comes in between 2050 mm and 2200 mm per year (average), while Hoi An comes in between 2000 mm and 2100 mm. So, almost even total rainfall on a yearly basis. But what about month to month? This data is a little more interesting. (I've made the month that has the higher number in red.)
Month Jan. Feb. March April May June July Aug. Sept. Oct. Nov. Dec. | Da Nang 96 33 22 27 63 87 86 103 350 613 366 199 | Hoi An 96 39 80 31 54 91 74 115 326 529 385 212 |
In the end no clear pattern emerges, and we see that although Hoi An has seven months of "more rain" than Da Nang's four, it isn't that obvious of a pattern. So who is correct? All of us, and none of us. Because in the end the yearly rainfalls are pretty close to the same, and the month by month doesn't really matter. What I think is happening are two different perceptions based on the local climate and it's patterns.
For example, Dean mentioned that Hai Van Pass plays an influence on Da Nang's weather and he thinks this means Da Nang gets more rain. I think this is partially true; I think a mountain does play a role in Da Nang's rainfall but I don't think Hai Van plays that big of a role, but instead I think it is Son Tra that plays the much larger role in it. Patrick also thinks Marble Mountain can create an interesting microclimate, which I'm inclined to agree with this based on both personal evidence of working in the area, and page 29 of the Quang Nam Rivers SEA linked above. Here is what I think is happening:
In the months from April to August, many of our rain showers and thunderstorms come from the Southwest. These storms form over the tops of the peaks and mountains to the west and southwest, and then gravity takes hold and these big heavy thunderstorms end up tracking and following the lowest spot (just like water rolling downhill). Since Hoi An is at the river mouth, and thus the lowest spot around for a long ways, this can explain why some days in the summer it will be sunny and nice in Da Nang but raining in Hoi An (this is more so in the afternoons and evenings), and lead to people like me thinking Hoi An gets more.
However, when the pattern changes during rainy season, and lasting through about March, the winds are coming from the Northeast. Since there is a large mountain [Son Tra] directly in the path of those winds (and moisture) it creates a disturbance which dumps rain on the immediate area [Da Nang]. Then it doesn't have quite as much leftover by the time it reaches further south to Hoi An. This can create an effect for people sitting in Hoi An looking north and seeing big rainclouds over Da Nang and thinking Da Nang gets it more.
So in the end, after looking at the data, I think there isn't a whole heck of a lot of difference between the two. Of course there are always going to be storms and weather events where it either looks like it is confirming this or outright going against it, but there are always going to be outliers, and if we look at the consistent data the truth does emerge, and it looks like it is a bit of a wash, since in the end the two numbers that really matter, the yearly rainfall totals, are both within 100 mm of each other. This is just too small of a number to call either way, so I guess we will just have to be happy knowing we all get right about the same! Thanks for reading!
-Kit
For example, Dean mentioned that Hai Van Pass plays an influence on Da Nang's weather and he thinks this means Da Nang gets more rain. I think this is partially true; I think a mountain does play a role in Da Nang's rainfall but I don't think Hai Van plays that big of a role, but instead I think it is Son Tra that plays the much larger role in it. Patrick also thinks Marble Mountain can create an interesting microclimate, which I'm inclined to agree with this based on both personal evidence of working in the area, and page 29 of the Quang Nam Rivers SEA linked above. Here is what I think is happening:
In the months from April to August, many of our rain showers and thunderstorms come from the Southwest. These storms form over the tops of the peaks and mountains to the west and southwest, and then gravity takes hold and these big heavy thunderstorms end up tracking and following the lowest spot (just like water rolling downhill). Since Hoi An is at the river mouth, and thus the lowest spot around for a long ways, this can explain why some days in the summer it will be sunny and nice in Da Nang but raining in Hoi An (this is more so in the afternoons and evenings), and lead to people like me thinking Hoi An gets more.
However, when the pattern changes during rainy season, and lasting through about March, the winds are coming from the Northeast. Since there is a large mountain [Son Tra] directly in the path of those winds (and moisture) it creates a disturbance which dumps rain on the immediate area [Da Nang]. Then it doesn't have quite as much leftover by the time it reaches further south to Hoi An. This can create an effect for people sitting in Hoi An looking north and seeing big rainclouds over Da Nang and thinking Da Nang gets it more.
So in the end, after looking at the data, I think there isn't a whole heck of a lot of difference between the two. Of course there are always going to be storms and weather events where it either looks like it is confirming this or outright going against it, but there are always going to be outliers, and if we look at the consistent data the truth does emerge, and it looks like it is a bit of a wash, since in the end the two numbers that really matter, the yearly rainfall totals, are both within 100 mm of each other. This is just too small of a number to call either way, so I guess we will just have to be happy knowing we all get right about the same! Thanks for reading!
-Kit