In a car seat, the coat comes off. A puffy jacket or snowsuit under the harness leaves the straps too loose to hold in a crash, so the app keeps the warm thin layers under the straps and puts a blanket over them where the jacket would be.
What the app suggests in a car seat, from the skin out, for a baby of 6–12 months:
20°C: short-sleeve bodysuit, pants and socks. Warm — light single layers. 19°C: long-sleeve bodysuit, pants and socks. Mild — add a light mid layer. 15–18°C: long-sleeve bodysuit, pants, socks and light sweater. Mild — add a light mid layer. 14°C: long-sleeve bodysuit, pants, socks, light sweater and warm hat. Mild — add a light mid layer. 13°C: long-sleeve bodysuit, pants, socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 12°C: long-sleeve bodysuit, warm pants, socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 10–11°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater and warm hat. Cool — a mid layer plus a hat. 6–9°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps and warm hat. Cold — warm layers under the straps, a blanket over them, and a hat. 1–5°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps, warm hat and mittens. Cold — warm layers under the straps, a blanket over them, and a hat. −15 – 0°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps, warm hat and warm mittens. Freezing — warm layers under the straps, a blanket over them, hands covered.
The American Academy of Pediatrics advises against bulky coats and snowsuits under car seat straps. Buckle the harness snug over thin layers, then lay the coat or a blanket over the top, never between your baby and the straps.
Below 10°C, the app swaps the jacket for a blanket over the buckled straps. Above it there is no jacket to swap, and the car seat answer is the same as the plain one.
Once the car is warm, take the hat and the blanket off. A baby in a warm car under a winter blanket gets hot quickly.
0–3 months, 2°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps, warm hat and warm mittens. 3–6 months, 2°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps, warm hat and mittens. 6–12 months, 2°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps, warm hat and mittens. 1–3 years, 2°C: long-sleeve bodysuit, warm pants, warm socks, warm sweater, blanket over the straps, warm hat and mittens.
Indoors, or in a warm car, bus or train, take the hat and outer layers off.
Check the neck — warm but not sweaty means just right.