Is there a simple way to measure, record, and use wind resistance or tail winds? Since moving to a new office where my commute is a bit longer I have been obsessively recording my cycle to work hoping to see a gradual improvement. (N.B. I've stored them on Strava but you need to be a member to see aught but a summary page). It looks like any increasing fitness is not the dominant factor, wind is. We know that wind resistance has an enormous effect but is there a way to factor it in to my data? How might I measure, record, or just note wind speed through my ride and somehow factor it out when calculating performance each day?
Arguably, just the time it takes you to get from point A to point B is a perfectly good measurement of improvement (same with average speed)

As you say, wind influences this, as does countless other factors - traffic lights, weather (wind, wet roads, snow and ice, etc..)

However, unless you are remarkably lucky or remarkably unlucky, these variables will largely average out over time.. E.g a head wind on the way will become a tail wind on the return journey (or a windy day followed by a calm day, or the prevailing winds changing throughout the year)

It's also worth remembering that elapsed time wont just be skewed by negative factors, but also positive things like your bike-handling skills, confidence cornering, confidence at drafting other vehicles etc

The main point is: You should not judge fitness improvement based on specific rides ("todays ride took 2 minutes longer than yesterday") but instead look at trends over time. For example, here is a plot of average speeds for all rides during my first year of "proper" cycling:



Day-to-day it was impossible to see any improvement in speed, it was hugely influenced by factors like the terrain - but, over time there is a clear trend.
Speed and time alone aren't a good indication of fitness

Power output as measured by a device like a Quarq, PowerTap or SRM power meter is far more representative.

Aside from being prone to external influences, increases in speed become exponentially harder. Going from 10km/h avg to 20km/h is far easier than going from 20km/h avg to 30km/h (and then 30 to 40 is even harder)

Part of this is because air resistance approximately increases with the cube of your speed ((ground speed)*(air speed)^2) - in other words, air resistance increases exponentially compared to your speed

I would speculate that the other part is, increasing your power-output becomes exponentially harder the closer your are to your "genetic limit" (e.g increasing your 1-hour sustained power from 100w to 200w is much easier than from 200w to 300w, increasing from 300w to 400w requires far more training)

What does all that rambling nonsense about watts and exponents mean?

If you look at my average-speed plot, there's a large increase in average over the first few months (the easy 10km/h->20km/h increase), then the speed appears to plateau, despite the fact I was increasing in fitness over that time...

This is where power meters become especially useful: It more directly measures a useful metric of fitness, essentially how hard you can pedals for a given duration.

I bought a power meter near the beginning of Dec 2012, where the avg-speed line is quite flat. I knew I increased in fitness since then, however the average speed line doesn't really reflect this.

The best-efforts power curve far better reflects my improvement than avg-speed:



As you can see, despite no large increase in avg speed, my best effort 1-hour power increased from 197w to 254w, which is a substantial change (dividing by 60kg and using the easy-to-misinterpret "watts per kg chart", it's an increase from "moderate" to "very good")

Power meters produce great measurements, but, they are expensive.
There are ways to measure wind speed on a bike, but not for the reasons you describe

iBike sell several versions of a bike computer which measures wind speed.

It's sold for two reasons, the first is to estimate power output without a power meter (the iBike Newton is quite expensive, but cheaper than a direct-force power meter like Quarq, PowerTap etc)

The second reason is if you have a power meter, it can use the power output, ground speed and wind speed to estimate your aerodynamic drag (since your speed on flat terrain is primarily based on power output divided by aerodynamic drag)