In reply to sheelba:
Interesting idea. But I feel there are flaws, some of which you acknowledged already.
If 2019 had been up at 7 good days, I think the entire data set would show pretty close to a horizontal line as the best fit, so no real correlation. So your trend might be resting on a single data point.
The tiny number of routes sampled does leave this very open to huge error bars.
You have acknowledged this, but to restate it succinctly: Assuming that recorded sends on UKC is directly correlated to quality of conditions is not something you can really do without some evidence for it.
I think looking at historical weather data from places like Cairgorm Summit station and others would be a more accurate way to track numbers of days with good conditions (you could set temperature, precipitation and wind speed definitions of what would be a "good day").