Advantage of this sensor is fast and easy installing by the non qualified personal. The sensor is designed for large agriculture facility with many greenhouses where is needed to be controlled temperature and humidity, the atmospheric pressure is for control the motor of air inlet and outlet. With help of the sensor installed outside the greenhouse provide information about positive/negative pressure inside the greenhouse.
Finlay the software read accurate temp/humid value of the Si7021, and only atmospheric pressure of BMP180, the temperature sensor inside the BMP180 is used only by the sensor itself to provide accurate atmospheric pressure measurements.
The sensor can be used also for home automation, and drone applications where is needed correction of the UAV barometric sensor compared to the ground barometric sensor, also humidity measurement can help the pilot to calculate more accurate flying time.
Hmmm. Looking up scintillometers, I see they are advertised as "large aperture", so you may need an array of photo[diodes,cells], which adds a lot of complexity, matching, summing etc,but maybe you could get away with a condenser lens.