Can I use an IR distance sensor in rain?
According to the data sheet for this device, located at Adafruit's web site, the frequency used is 870nm. If you required specific attenuation figures, that might be useful information.

Another web site suggests that thermal IR camera imaging systems (which use a different frequency IR) are attenuated by rain. Some of the IR energy is lost by absorption and some by reflection. I was hesitant to suggest that as the answer due to the different frequencies involved.

Digikey has a document covering wide use of IR sensing. Listed in the document:

The disadvantages of IR technology are:


Line of sight: transmitters and receivers must be almost directly
aligned
Blocked by common materials: people, walls, plants, and other objects can block transmission
Short range: performance drops off with longer distances
Light and weather sensitive: direct sunlight, rain, fog, dust, and pollution can affect transmission
Speed: data rate transmission is lower than typical wired and RF transmission


Even though data transmission is not your objective, the considerations apply. A wet surface over which your sensor passes will have the signal attenuated and may also be slightly diffracted. 

I found no direct reference to the magnitude of the attenuation. If your sensor is protected from water intrusion and the surface is only slightly wet, you may have minimal problems reading the distance. If the rainfall is at the level of a frog-strangler, there could be sufficient water to eliminate useful readings.

Another search resulted in a similar question on the physics stack exhange, which led to a more comprehensive answer:



There is another image on the same page, but both show that at the 870nm frequency, there is lower attenuation than much of the IR spectrum, particularly above 3000nm. The above linked document suggests that the affect of rain (water) is low at near-infrared (970-1940nm) but that's still higher than the device's 870nm figure.

For your objective, shielding the surface from rain (umbrella) or clearing the surface of standing water seems to be the best options. If those are impractical, one would have to experiment to determine how much water flow can be tolerated before the signal is lost or attenuated beyond reception limits.