While walking to school one wet morning, my daughter wanted to know why we get so much rain in the fall and winter.
Eight other weather reporting stations in Canada are rainier than Vancouver, the top being Prince Rupert. Still, that's pretty rainy. And it rains twice as much in the winter compared to the summer. From October to March, an average of 870 mm falls, compared with 369 mm from April to September. The snow is hardly worth talking about.
ENERGY FROM THE SUN heats up the air at the equator most because of the curvature of the earth. This tends to rise up then head toward the poles where it cools and moves closer to the surface and then more or less back toward the equator.
THE ROTATION OF THE EARTH makes the movement of air relative to the surface of the earth seem to deflect. This Coriolis effect doesn't affect your toilet flushing, but does influence large scale wind patterns and hurricanes.
THE LATITUDE OF VANCOUVER roughly half way between the equator and the north pole positions us so the large scale wind patterns tend to bring us winds from the west.
THE PACIFIC OCEAN sits to the west. Winds pick up moisture from it toward shore. Large bodies of water also tend to hang on to energy that keeps our temperatures more moderate.
THE COAST MOUNTAINS push the moist air upward so it cools off and can't hold on to as much moisture, so it rains.
THE TILT OF THE EARTH'S AXIS relative to the plane of our orbit around the sun means we are tilted away from the sun during winter, so it's cooler. This results in a greater temperature gradient between the equator and the north pole, and the winds get stronger. The cooler temperatures combined with more moisture-ladened wind brings more rain during winter.
If you're keen on this subject you might want to read Living with Weather Along the British Columbia Coast by Owen Lange or The Climate of Vancouver by Tim Oke and John Hay.