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To me extreme weather covers rain. We all have probably had a day when rain happened during our measurement. Once, I went back and checked to see if the 5k I measured in a rain shower was accurate. I found that there was a slight difference, but was OK with my original measurement.

I know wind can be a real factor. Just ride a calibration course with the wind and against the wind and compare your cnts. I would not want to ride a course with lots of wind or rain.
I've always taken a relaxed attitude regarding rain. If it's raining at the outset, I'll generally put it off, as I don't enjoy riding in the rain. If rain starts in the middle, I usually tough it out to the end of the measurement.

It is possible that in doing so I may introduce inaccuracy, but I suspect it is small.

It would be unreasonable, in my view, to ask people to stop measuring if it starts to rain.

Just go ahead and measure.
This interests me because I've always assumed there is considerably less accuracy in rainy conditions. (Didn't our manual used to advise measurers to measure in dry conditions?) My thinking about it was that you sometimes have a very wide range of conditions that affect tire pressure-- perhaps in some places riding through standing water, in others no standing water, etc.

Anyway I've gone ahead and measured in the rain many times-- but whenever possible I'll try to repeat the measurement when it's dry. Well there's a practical problem here too-- it's hard to put a paint mark on a wet road.

Once I did find a pretty big discrepancy when I remeasured a course under dry conditions after first measuring during a light rain. I blamed the rain at first-- but later decided that the tires were at fault-- they had very thick treads and seemed to be much less consistent than the tires I was used to.

Very unscientific, all of this I admit-- does anyone have some experimental evidence about accuracy in wet conditions?
I once found myself measuring a course in a summer downpour. I kept going until, on a short, steep section, my rear tire spun out a few times. This caused a wobbly ride up the hill. Maybe it didn't make any difference in the measurement, but the lightning convinced me to bag it until the storm passed...

I calibrated and measured a 5K in a steady rain a couple of years back. The measurements came out very close on a relatively flat and turn-free course. I did not experience any ride anomalies. I had confidence that the measurement was good. The challenge was recording the data and keeping it dry while writing in my notebook while keeping it inside a clear plastic bag.
During the second ride of a 5K measurement, there was a sudsy 10' gutter / road puddle right in my path and I got 20 JO counts fewer than the first ride.

Spraying my 95 lb 26" tires for 90 seconds with city water this April and then riding about 500' 8 times extrapolated to 13.5 JO counts more per mile but I rode about 100 yards from my driveway to where my "course" was.

Lynwood Wagner
I can usually detect tire slippage and road hazards. My major worry with weather had been with potential silent changes in wheel circumference.

For years I had gone to great lengths to avoid measuring if there were standing puddles or rain. Without specific evidence I had presumed that water on the tires would make them colder and therefore the wheel diameter smaller. I theorized that if it happened mid-measure then the measure was ruined.

My view changed suddenly one day. I was measuring a 5K. The job had to be completed very soon. Mid-measure, a sunny day turned to torrential downpour. Disgusted, I aborted the measure and hurried back to the calibration course to generate the new rainy-weather "wet tire" working constant. To my astonishment, the new working constant was identical to the earlier one! The wet tires and rain, by themselves, had no detectable impact!

My new theory: Tires are not like human skin. They are not warm-blooded, hence, they do not experience a lot of evaporation caused cooling effect that I may have been sub-consciously ascribing to them. The ambient temperature and air pressure remain the key factors in the dynamic nature of the wheel's diameter.

Today, providing there is no tire slippage, I too harbor a very relaxed attitude about rain when measuring. But I keep my eye on the thermometer...

I agree with the general sense of the conversation, that rain or standing water doesn't preclude a good measurement. I used to think otherwise but I've had the same kind of experience Jeff describes.

I even found that a cal course with DEEP standing water over long portions gave me pretty much the same constant I was getting without the water. Durn it, there went my excuse for not measuring in the rain-- now I have to just say I don't like doing it, as Pete said.

Very tricky to take good notes in a downpour though. If you can keep your smartphone dry then dictate to it. Making marks on the road, that's tough too although you could put nails and washers, maybe tie flagging to something nearby.

Considering all the extra trouble, maybe there is a better way to spend a rainy day (if you have a choice)!
Evaporative cooling happens for tires just like it does for skin. But it may be that when it was raining the humidity was very high, so there wasn't much evaporation going on. Or maybe the effect in general just isn't big enough to make that much difference.

I think maybe there isn't much test data on this because nobody really wants to conduct tests in the rain. And nobody cares all that much because they don't want to measure when it's raining anyway!

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