You can connect this chest strap heart rate monitor to your iPhone and Android. Since the model is waterproof, you don't have to worry when it's raining outside.
From Amazon
Polar Verity Sense - Bracelet with Optical Pulse Sensor - ANT+ Dual Bluetooth - Pulse Sensor for Sports with Just One Button - Compatible with Peloton, Endomondo, Zwift and Others Amazon Exclusive
365rider
Polar Verity Sense Optical Heart Rate Sensor