I need to get value on cloud when there is less visibility outside due to fog

Hello everyone i am new to iot.
I need to get value on cloud when there is less visibility outside due to fog.
Kindly tell me the hardware(sensor required to connect with nodemcu) i need and what type of logic could be used

I’d suggest that you do some research on meteorological forums for suitable sensing systems (I hope you have deep pockets unless you’re going to build your own sensor) then get it working with a suitable microprocessor (a NodeMCU should probably be your first choice).
Once you have this set-up and working to output visibility readings to the serial monitor then adding the Blynk parts is easy; but if you can’t figure that out then share the details of your sensor and your working code and we’ll point you in the right direction.

Pete.