I like living in Vietnam but many times I think that I feel that way because of the fact that it is extremely inexpensive to live here and not so much because it is a lovely or wonderful place. Vietnam is extremely hot and humid, especially where I live at sea level. Most of the year it is so damn hot and humid that just a simple walk for a couple of blocks has me already sweating and getting involved in physical activities outdoors can be something that might actually be dangerous for you based on how hot and sweaty most foreigners get.
There are some out there that say that you will acclimate to the temps, but after nearly 6 years my reaction to the weather and general feeling of being outdoors is about the same as it was when I first got here. You just get used to being sweaty.
That is, except for the next couple of months that the locals seem to despise but for me, it is the best time of the year.
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People that have proper winters will probably get a laugh out of this because our "winter" gets us down to the low 70's F and the C is shown above. I wouldn't say that it EVER gets cold here, but there are times at night in December and January where it is actually comfortable to wear long sleeves and a jacket, which is oh so nice after yet another extremely harsh "summer" where even if you were wearing nothing but a bathing suit you will likely be covered in sweat just by being outdoors.
In our "winter" there are many reasons why the temperature changes and most of them are not related to our position on the planet. We are so close to the equator and have a hot current in the China Sea nearby so that we don't really have any sort of temperature drops that are massively related to the tilting of the earth's axis. For reasons that I don't really understand, December to February or so is when we tend to get a lot more clouds and therefore a lot more rain.
Rain comes with winds and the lack of exposure to sun will make it much cooler here. There are many who own motorbikes or have to walk around a lot that are probably not real keen on the rain being a constant threat, but for someone like me who works in his living room and doesn't really NEED to go anywhere, this is the best time of year. It is a good idea to remember to bring an umbrella with you wherever you go all the time because even if the forecast isn't calling for it, I have seen the weather patterns change so dramatically in just an hour to the point where forecasts are something that the average Vietnamese person doesn't even pay any attention to.
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With rain of course comes increased humidity but because of the drop in temperature people tend to notice it less. During the truly painful times of year in what would be considered the northern hemisphere's summer, things are truly nasty here because the heat never dissipates even at night.
Tourism drops dramatically during the rainy winter season and I honestly don't understand why. During the rest of the year it is absurdly hot here and to me and many of my friends, the months of December to maybe early March are the only times of year that the temperature is what I would consider even so much as bearable.
So if you have been holding off on a vacation visit to Da Nang know that if you do decide to come now through February, the prices on all accommodation plummets to damn near free and the beach and other nature places that are normally packed are relatively free of people. For most westerners the temperature during this time of year is ideal, you just have to be prepared for rain on a daily basis. It doesn't necessarily rain every day, but it could, so you need to be prepared.