Fifteen degrees is the most misjudged temperature of the year. It is warm enough that a coat feels absurd by noon and cool enough that a shirt alone feels thin at eight in the morning. Deciding what jacket to wear in 15 degree weather is really a question about three things the forecast does not tell you: how much wind there is, how long you will be outside, and what you are wearing underneath. Settle those and the jacket almost picks itself.
Do you need a jacket in 15 degree weather?
In still, dry air, and if you are walking rather than standing, most people are comfortable at 15°C in a long-sleeve layer alone. Add wind and that changes fast: a 20km/h breeze makes 15°C feel closer to 11°C against bare skin, and any moving air pulls warmth off you far quicker than still air of the same temperature. So the honest answer is a rule rather than a yes or no — if you will be standing still, waiting for anything, or out after sunset, take a jacket. If you are walking briskly between indoor places in daylight, a knit is enough.
Is 15 degrees cold enough for a coat?
No. A wool coat at 15°C will have you carrying it by midday, and carrying a coat is worse than being slightly cool. Fifteen degrees is light-jacket territory: an unlined or lightly lined shell that blocks wind and weighs almost nothing. The job of the layer at this temperature is not insulation, it is stopping air moving across you. That is why a thin nylon shell often feels warmer at 15°C than a much heavier knit with nothing over it.
A jacket cut to the hip, like The Bomber Jacket, is well suited to this because the ribbed hem closes off the gap at your waist where warm air otherwise escapes. A lightweight high-collar shell such as The Solis Jacket does the same job at the neck, which is the other place heat leaves quickest.
What to wear under a jacket at 15 degrees
One mid layer, not two. At this temperature the mistake is stacking, and stacked layers mostly fight each other at the armhole. A fine-gauge knit is the right weight: The Alpine Crewneck for men or The Vesper Sweater for women will sit under a shell without bulk and can be worn on their own the moment the sun comes out.
If you would rather have the adjustment built into the knit than carry a jacket at all, a half-zip does it in one piece — The Willow Zip Knit and The Quarter-Zip Knit both let you open the neck when the temperature drifts up two degrees, which at 15°C is the difference between comfortable and warm.
Is 15 degrees hoodie weather?
Temperature-wise, yes — a hoodie is roughly the right weight for 15°C. The problem is that a hoodie is doing the job of the mid layer and the outer layer at once, badly: cotton jersey soaks up drizzle, holds it, and then cools you as it dries, and it does nothing about wind. A knit plus a light shell weighs about the same, handles both, and looks considerably better doing it. If you want one garment, make it a structured piece that reads finished on its own, such as The Monroe Blazer.
Fifteen degrees at eight in the morning is not fifteen degrees at four
This is the part the forecast hides. An autumn day that peaks at 15°C usually starts around 8 or 9°C and drops back within an hour of sunset. If your day spans both ends of that curve, dress for the cold end and make the warm end removable — which in practice means a jacket you are happy to carry, not a coat you resent carrying. A light shell folds into a bag. A wool coat does not.
Two practical checks before you leave. Look at the wind speed, not just the temperature; above about 15km/h, add the shell regardless. And look at when you will come home; if it is after dark, the temperature you dressed for has already gone.
Once you have chosen the jacket, keeping it looking right is mostly about how you wash it — which is covered in how to wash a nylon jacket without ruining it. More short, specific answers on fit, fabric and layering are gathered on Style Answers.