My MacBook sometimes becomes hot when I'm using it, and I want to make sure it is operating within normal temperature. What's the operating temperature for the MacBook that was released in May 2016 normally?
You can find your processor model by entering the following into a terminal:
sysctl -n machdep.cpu.brand_string
which will return a string, for example, like this:
Intel(R) Core(TM) i5-5287U CPU @ 2.90GHz
You can then search for your processor on the intel product specifications page, by pasting for example
i5-5287U into the search box.
On the resulting page you will see something called
This is the maximum temperature recommended for that processor. You can monitor this with htop.
Internally, the processor is rated to run up to 100°C and since this model has no blower or fans, that heat will be conducted out to the frame. The nature of conductive heat transfer is that the case itself won't get that hot unless it's insulated to near perfection.
The system will slow down the processor to keep the internal thermal temperatures in line, but most modern cell phones, tablets and computers will get warmer than most people feel is proper and run just fine for extended periods of time. If a store is close, you might make an appointment and have them run diagnostics - just because they are designed to be within the specs - failures can happen on the chips and on the sensors to make yours painfully hot. I would always have someone trained look over something you feel could be a fire risk or dangerous no matter what I say is normal.
Considering that the entire line of MacBooks have no fans, I wouldn't be surprised if it got to the range of 50°C while idle.
Apple only supplies minimum and maximum ambient temperatures; the specified ambient temperature range for your MacBook 50° to 95° F (10° to 35° C) (source). Unless you're using your MacBook in an abnormally warm environment (i.e. direct sunlight in a tropical country), your MacBook shouldn't get too much hotter than its regular idle temperatures.
My 13" Retina MacBook Pro, mid-2014 version can get rather warm when idle (~50°C-60°C) especially if placed on a non-heat conducive surface like a mattress or my legs. I don't mean to get too technical, but you can think of the entire unibody as a rather large heatsink that conducts heat away from your Mac (another reason why sometimes the entire computer feels warm and not just one region). It's also important to remember that OS X will shut your computer down if it gets too hot (>100°C IIRC).
Unless the MacBook in question is exceeding 80°C while idle with no absurd CPU usage or rogue processes (check with Activity Monitor for process CPU usage), I wouldn't see any cause for concern about the temperature of your MacBook.
To follow up on comments relating to fans, I am unsure why posters are stating that "entire line of MacBooks have no fans," because they do have fans. All of them. It is true that they use their cases as heat sinks, but they also all utilize fans. You can monitor your internal temps with apps such as smc fan control. That particular app will also allow you to set manual fan speeds and create specific profiles that you could select based off of what type of processing you are using you computer for.