If I recall correctly, one millibar pressure drop (at around sea level) is about what you get with a 27-foot increase in altitude. So your variation is about one millibar or so.
Don't forget that there are tides in the atmosphere as well as in the oceans. Twice a day there is atmospheric high tide, and twice a day there is atmospheric low tide. The ambient pressure goes up at high tide and down at low tide if you're standing still and the weather is completely constant; there's just more or less air above you pressing down.
If you're riding for more than an hour or two I'd expect to see a visible effect from atmospheric tides, unless you were right around a high or low point.