This might seem fairly straightforward but in all my traveling I rarely look ahead at the forecast because, you know, why bother. You're there to travel, who cares what the temperature is. But SOMETIMES you should absolutely check whether it be snow, rain, or frogs.
Some things to keep in mind:
North and South are the most important directions when it comes to temperature! Think of the world like a straw you can either go really far up or down but moving horizontally just takes you in a quick little circle. So, the temperature difference between Argentina and Mexico is (normally) more significant than say France to New York.
When you cross the equator(going N/S) the seasons flip. So a warm New York summer can be the middle of Winter in say Brazil, so when you step off the plane— dress appropriately. The closer you get to the equator the less it matters(you'll be comfortable either way)
Layers! When you're backpacking or traveling super light layers are your best friend. A hoodie + longsleeve tshirt + coat and I'm good to go in 0° weather. Unless absolutely necessary, leave the parkas at home.
And that's that.
Sincerely,
A freezing cold traveler that packed last minute
p.s to the Americans— learn Celsius, it's far more practical
yas