Hi!
And yet another doubt. I know about the monson typical weather in Hanoi.
I have just checked this website: https://www.weather-atlas.com/en/vietnam/hanoi-climate,
and it seems the weather is rather good, but I think reality is that right now in summer the sensation is of 40 degrees Celsius, and with an unbearable humidity.
Since your question is specifically about Hanoi in the summer, I will strongly reiterate:
It was horrible in Summer 2018 when I stayed for a month.
It was horrible in Summer 2019 when I stayed for another month.
Not just the oppressive heat and humidity--the worst I've ever experienced anywhere in Vietnam, including during my tour of duty here during The American War--but also the air pollution that routinely ranks Hanoi in the top ten WORST cities for air pollution in the world.
It's really sad for me to have to emphasize this to you, because personally, Hanoi is my favorite City in Vietnam.
In fact, I'm currently stranded in Danang during a COVID-19 virus lockdown, waiting to continue my plans to spend August and maybe even September in Hanoi again this year.
I posted in this thread on 1 March 2019 about a routine that allowed me to live somewhat comfortably there, even when the weather and the pollution are terrible in Hanoi:
expat.com/forum/viewtopic.php?id=835023
I was in Hanoi late last August and all of September.
I'm of British Isles and Northern European extraction.
It was pretty miserably hot and humid during most of that time...in "the heat of the day", meaning about 1 PM thru 5 PM, approximately.
These things worked for me:
Rise and walk/exercise for an hour or so by one of the lakes between 4 AM and 7 AM.
It's amazing to see how many people are active during those hours.
Learn to ask restaurant help to turn an electric fan in your direction. In many instances, it's just as effective as air conditioning.
If there isn't a fan in your hotel room, request one.
Try to schedule outdoor activities in the morning before lunch or in the afternoon/evening after 5 PM.
Wear lightweight, loose fitting clothing, plus a hat when you are in the sun.
Carry a travel umbrella if you must be out in the brutal sun and don't let your pride keep you from using it.
Nap and work indoors after lunch, from about 1 till 5 pm.
Don't oversleep in the morning. If you don't go outside until mid morning, the jolt from the heat will be too much for your body to adapt to.
Drink a lot of WARM botted water throughout the day. I didn't always follow that one, but locals kept telling me to do it. Not good to be shocking your overheated system with very cold beverages.
Shower at least twice a day.
Eat what the Vietnamese eat. If you try to maintain a Western diet there, I fear you'll find yourself weighed down too much by your food.
Along the same lines, learn to eat with chopsticks from one of the small individual bowls. Much harder to overeat that way.
So it's possible to live somewhat comfortably in Hanoi during the summer if your employment allows you to arrange your life in a similar manner.
Special Spa Tip:
Most Spas in the city are open by noon (some earlier) but they don't do significant business until the late afternoon/early evening.
They have air conditioning and electric fans, depending on your comfort zone.
Discount prices for massage and other legitimate spa services are usually available between noon and 4 PM.
So you can leave your child and the nanny to continue napping together after lunch and head to your favorite spa for an hour or two of inexpensive therapy.
ALSO, of the inexpensive bottled water brands here, my Vietnamese doctor in Qui Nhơn recommends La Vie mineral water whenever I'm breaking a sweat.
He calls it "as good as Gatorade, without the sugar"