Unfortunately you never know as it experiences will vary per location / per day / per gate, etc...
Flew once last year out of Stansted on Ryan Air and they did not weigh carry-ons.

As a general rule, if your bag does not look close to the size limit it won't be weighed. Usually backpacks are given less scrutiny than wheeled bags.
I travel with heavy fragile photography gear so have an overweight carry-on (backpack) on every single flight I fly on and never been weighed.
BUT my advise would be regardless of weather outside is to wear a jacket with pockets. If you witness people ahead of you getting their carry-ons weighed move the heaviest items to your jacket pockets. You can then decide to move them back into bag later if you want.