Radar guns inaccurate in rain?

[hoitoider]

I had always heard that the police can't use radar guns in the rain because it gives an inaccurate reading. Then today I saw a cop parked in the median of the interstate using one in a pretty good downpour. Perhaps I believed a UL all these years? Googling provides answers both ways. This vendor's site has on their FAQ:

(Q)Does weather effect radar?
(A)Yes. Weather mainly effects radar detection range. X band radars operate better than K or Ka band radars in bad weather. Water in the atmosphere (humidity and rain) effects K band radars the most, Ka band radars transmissions are also effected. X band radars to a lesser degree.

I can't really interpret that as per the law but I'm wondering if the cop today was wasting his time.

[UEL]

This is just a quick thought off the top of my head. I do work with radars, however, not the type used by police. But emitted radar energy behaves the same largely independent of the type of radar emitting it.

There are some wavelengths that are horrible in rain, or snow, or sand. Others are fine.

In one of our radars, our detection range is reduced by about 12% (range and radar not identified for my own reasons). Everything up to the reduced maximum range is still as accurate as ever.

Your observed police officer may very well have reduced range, but unless his maximum range was reduced to mere metres, I would submit that he would still have enough space to get his required readings to issue a ticket.

[erwins]

Also, a significant portion of the "radar" guns you see police using these days are actually laser based, not radar based. I don't know if those are affected by rain either, but seeing an officer using a speed measuring gun doesn't automatically mean they are using radar.

[crocoduck_hunter]

What's the normal effective range of one of those devices?

[geminilee]

Originally Posted by hoitoider

I can't really interpret that as per the law but I'm wondering if the cop today was wasting his time.

If the radar gun was visible, like held out a window or something, his main purpose might not be to ticket people but to get them to slow down in dangerous conditions, for fear of getting a ticket. IOW, he could just have been bluffing.

[Not_Done_Living]

in my neighborhood, as you enter the "school zone" there is a large sign that reports your speed back to you a a reminder -- it normally tracks for about 500 meters. in rain, in snow, night or day -- i have not seen a difference -- as soon as i can see it, it can see me --

i am pretty sure its not some super high tech device ---

[jimmy101_again]

Originally Posted by Not_Done_Living

in my neighborhood, as you enter the "school zone" there is a large sign that reports your speed back to you a a reminder -- it normally tracks for about 500 meters. in rain, in snow, night or day -- i have not seen a difference -- as soon as i can see it, it can see me --

i am pretty sure its not some super high tech device ---

Actually, it probably is a super-high tech device. But super-high tech devices can be made for a couple hundred bucks (see iPhones, laptops, ...)

[Hero_Mike]

What UEL said. Different atmospheric conditions will affect different wavelengths. Fog or mist might be the worst because of its density, but that all depends on how "thick" the air is with moisture. FWIW, my radar detector consistently warns me of only laser and Ka band down here in Arizona when near police, though X and K are found in automatic doors in parking lots and the detector reports those faithfully as well. I have no proof, but I strongly suspect that police here only use Ka band (with respect to "radar"). That being the case, I got a warning for speeding on the 4th of July last year, on a rare rainy day. The radar performance was, apparently, not affected enough to matter to the deputy who have me the warning. I did not have my detector that day, and was not watching my speed on the long, straight road into my subdivision (speed limit 45).

[Meka]

Originally Posted by erwins

Also, a significant portion of the "radar" guns you see police using these days are actually laser based, not radar based. I don't know if those are affected by rain either, but seeing an officer using a speed measuring gun doesn't automatically mean they are using radar.

I can't speak to laser speed guns specifically, but I have had experience with lasers being affected by rain, specifically heavy rainfall inadvertently triggering the timing lights at autocrosses. However, those lasers are intended to function over distances of just a few yards, so I suspect an officer's laser would be more powerful, and therefore more capable of penetrating a rainstorm.

[Troberg]

My guess is that the policeman simply did not know the limitations of his tool, or simply didn't care. At least, it's common enough here for them to behave like that.

[hambubba]

The reason weather radar works, is based on the reflection of the water droplets. A droplet reflects the signal, some drops reflect more, all based on the wavelength of the signal.

This varies with type of radar, etc. Higher frequency devices need much smaller drops to get a full reflection. A big drop, oh, say "car-sized" wouldn't be much of a problem even with the rain. fog would be the worst culprit.

For you math geeks out there, this might be some help.

[TB Tabby]

So I can get away with speeding when visibility is poor and the roads are slick! Awesome! I see no downside to this!

[DawnStorm]

Would this apply to speed cameras? Just recently the Baltimore Sun did a big study on how accurate Baltimore City cameras were and the results were not pretty. The company claimed that environmental conditions affected the accuracy and that the cameras worked fine. Naturally the City defended the cameras.
Here is one article about the issue.
And another one.

[Wintermute]

Originally Posted by erwins

Also, a significant portion of the "radar" guns you see police using these days are actually laser based, not radar based. I don't know if those are affected by rain either, but seeing an officer using a speed measuring gun doesn't automatically mean they are using radar.

Maybe it's a regional thing but LIDAR isn't THAT common. It's a very expensive tool and the RADAR units have already been purchased. Working in the east bay the only time I saw a LIDAR unit was when I went to the training course for it.

LIDAR is affected by rain. It's a light beam. So fog, rain, smoke, etc will shorten the effective distance.

It's ironic that people always try to fight speeding ticket based on RADAR by googling things on the internet. Most errors cause by RADAR technology actually show a DECREASE in speed and not an increase in speed.

The best way to beat a RADAR ticket is ask for the logs for the RADAR gun. It's common the officer either does not test the unit or does not document the testing of the unit. At least in California it's expected to be done at the start of the shift and at the end of the shift.

[RichardM]

The logs are not generally available throughout the country nor is the officer likely to have them available at the time the ticket is being issued. The logs can be subpoenaed by your lawyer if needed.
Richard (who is known to drive fast)