Decide whether to wear a jacket from the forecast for your route, the time outdoors and what you will be doing. Temperature alone is not enough. Wind, rain, shade and long periods without movement can change comfort substantially.
Check the relevant conditions
Use a current local forecast for the hours you will be outside, not a general climate description. Look at precipitation and wind as well as temperature. A short walk between buildings and a long wait outdoors require different planning.
Consider how easily you can return indoors or add a layer. If conditions may change, carrying a suitable light layer can be more practical than choosing one heavy garment for the entire day.
Match the jacket to the problem
For rain, read the jacket's actual weather protection. For cold, consider insulation and coverage. A fleece, leather fashion jacket and rain shell solve different needs. The word jacket does not identify which one belongs in your plan.
Try the layer over your normal clothing and check movement. If you will carry a bag, assess the fit and pocket access with it. Plan somewhere to store the jacket if you remove it indoors.
Can one temperature provide a universal answer?
No. Personal comfort, activity and exposure vary. Use a temperature as context rather than a guarantee. If you are asking about today in a specific place, consult live weather and your itinerary before deciding; a general article cannot supply a local forecast without that information.
Related reading
Jacket fabric weight and GSM guide. For another useful reference, see Waterproof jacket fabric guide.
