What kind of sensor to choose?

[Martin_Smith]

I want to measure distance in rainy days.
1.Can I use ultrasonic sensors? Can the two speakers of the ultrasonic sensor be blocked, or else they will be useless?
2. If I cannot use ultrasonic sensor. Is infrared ranging sensors the only choice? Can the infrared sensor be protected by a transparent plastic case? Will the transparent plastic shell reflect and refract infrared rays? ?
3.Or should I try radar sensor?

[Gilles BILLARD]

In my mind: The longest the wave the better the result through the rain;
So: 1st US;
2nd radar;
3rd IR
(may be I'm wrong )

[skywatch]

@Martin_Smith Microwave backscatter can show rain out to some distance. At airports an optical method is used as it also covers fog and snow. It really comes down to why you want to measure distance in rainy days and what sort of distance you intend or need to cover.....