The last few days, I've had a weak GPS signal, as ridewithgps reports. Using google maps for navigation, the app falls behind in knowing where I am and which direction I'm moving in. Last night was bad, and I couldn't see how dense the cloud cover was. We are having snowfall this morning, so maybe the clouds were dense last night. The path the ridewithgps says I've taken is much more zig-zaggy than reality.

Is the problem likely to be attributable to clouds? Do other things cause this? Is it possible that the GPS receiver in my iPhone might be starting to fail?