I have always been a person who gets cold easily.
Except during the heat of the summer months, I wear a running jacket whilst running. Even during a hard session I rarely find a need to take it off. I always have the option though.
I currently own two running jackets, one black and one fluorescent yellow. They are both thin and lightweight. They fit so they are not too tight or loose.
I did own a third but the zip broke and would continually open as I ran. So I recycled it.
My running jackets have a number of functions. They keep the cold and wind off my arms. My black jacket has side pockets with zips, which can hold food, gloves and my house key. My fluorescent yellow jacket ensures that my evening runs and coaching are safe.
I love my black jacket in particular, a gift from my partner for my twenty-second birthday. Much of the branding has faded due to its frequent use.
If they are unavailable to use (either unclean or still wet from been washed) I have a selection of substitutes. These include jumpers and long sleeved tops.
I often wear a running jacket during road races, even when most other runners are in vests or t-shirts. I have sometimes rolled up the sleeves but have never regretted wearing this extra layer.