Winter's air has the longest tail

Winter's air has the longest tail

Question. Is winter worse every day, or worse on some days?

• Encoding. One density ridge per month on a shared PM2.5 axis and a shared peak height, so a tall, narrow ridge means most days were alike and a low, wide one means they were not. A guide marks China's 24-hour limit of 75 µg/m³.
• Why this form. Twelve box plots would show the medians and hide the shape. Ridges show December's long right tail and August's tight bump together.

What this design gives up

Station identity and the order of days within a month. The calendar in the series shows when the bad days fell.