A experienced measurer did his pre cal rides, with all 4 rides in good agreement, drove to the race course site and performed the two rides as required. Again both in acceptable range. During the drive home a fast moving band of thunder storms blew through,dropping the temperature 6 to 8 degrees leaving the cal course wet. What to do? Calibrate in the rain? Wait 'til the street dries out? Forget about it? Is there a time frame for post cal?
Original Post

24 hours is the time frame. I would do it asp,
!
I've often wondered about such situations. When a front blows through after a measurement, but before the post-cal, it can throw things for a loop. Using the average constant can mitigate things a bit, but when I pre-cal and measure in mild temps, then post-cal when it's significantly cooler (this can also happen, less drastically, late in the day) I wonder if I'm lengthening the course too much as a result.
Here's another situation of a big temperature change. In AZ the morning can be 60,while 4 hours later it could be in the 90's. This can cause problems. Never know what is the best approach. I have calibrated more than twice in this situation and used an average.
This just happened to me. I beat the rain while measuring a race. I had to finish my post calibration in the rain.
My pre calibration count was 17753 CPM @ 38 degrees @ 6:35 Am. Post calibration was 17749 CPM @ 51 degrees and a pouring rain @ 6:21 PM.
What about using the on-site check? Then, if there are extenuating circumstances, the post-cal can be waived?

Do the proper pre-measurement calibration. Go to the course, and, before measuring, set out two pieces of tape, 300 feet, or so, apart (100 meters for you metric-heads ;-) ), or a round number of counts about the same distance apart. Ride it at least twice. Do the measurement. When finished, ride your two pieces of tape at least twice. If your counts are the same, you didn't lose air, or heat the tires too much.

This tells you that your post-cal should be good. You still do the post-cal, but if it is raining, or whatever, you have at least confirmed that your ride was good, which should make the post-cal unnecessary.

Feasible?
Duane, that's a great trick - first time I've heard of it but will make sure to use if I'm measuring in changeable conditions. I'd think riding between 2 fixed points like storm drains might be even better - no worries about your tape getting pulled up by some environmentally conscious passer-by.
I'm not sure any of these responses has answered the question.

Our procedures call for a post-calibration on the same day as the measurement.

The measurer needs to perform the required post-calibration rides. He can take a chance by waiting to see if the temperature recovers and the course dries, but that's exactly what he'd be doing. If the conditions don't improve, he needs to recalibarate under the best conditions available to him.

If the temperature drops and the constant changes in a way that makes the measured course distance short, then he needs to go back to the course and lengthen it. The course may not be as accurate as we'd like- but at least the course won't be short.

I'm a strong proponent of frequent recalibration, but unless you lay a new calibration course in the vicinity of every course you measure (or are measuring in the vicinity of a calibration course you've previously set) you can't always do that- at least not efficiently.

Sometimes feces occurs. This is one of those times.
I have never been happy with having a great variation (more then 2-3 couts/mile)with the calibration. Over the past few years I have measured a new calibration course as close as possible to each race course. It does require a second person (or you can use Pete's single person method).

For most of my measurements in NYC, I measure in the middle of the night which often reduces the chance of variation. Try it sometimes - no vehicles, no runners, no bikers, - just you, the racoons, and yea - sometimes Jim G!
Wow, lay off for a few days and you can miss a great discussion! I agree with David, it's really good to have a cal course pretty close to where you're measuring. I think it's worth the extra effort. With a nearby course you really can finish pre and post calibration, then sit down and figure out what you've got, then make your final markings before driving a long ways and then getting the final verdict.
Middle of the night is also good for laying out a cal course-- not only steadier temps but no sun beating down on your tape.
I think if it rains or something, you look at what you have for an average constant, and if you're not willing to accept that then you've got to go back on another day.
And some RDs think we charge a lot for what we do?