10-20 Celcius is light jacket weather.
20 Celsius is not even jacket weather - 20 Celsius is sleep naked with no covers weather. 20 Celsius is the stuff dreams are made of.
When it gets to be around 10 Celsius - which around here might be a handful of days in January, if you're lucky - I break out the long pants. A few degrees cooler and I'll get my light jacket; for I do not own a coat.
Buncha w**sies.