More airflow means less sweat
Rain capes are quasi-ponchos, smaller and longer on the front.
They're supposed to go over the handlebars and seat and provide a big waterproof tent with lots of ventilation beneath it.
Elastic loops built into the front of the rain cape that go over your wrists, and in the back there's a waist loop to keep the rain cape in place in windy conditions.
The great benefit of a rain cape versus the traditional rain suit is that the rain cape keeps you dry and ventilated,
whereas in sustained exertion, rain suits tend to build up heat and sweat.
When you ride a sustained distance in a rain suit you still get wet from just your own moisture.
The highest-regarded manufacturer of rain capes is Carradice in Britain.
They make a one-size-fits-all rain cape that's high-viz yellow,
and a size-specific version raincape in dark green.