What causes westerlies?

Answer 1

The westerlies are the prevailing direction whence winds and weather move in the northern hemisphere.

It starts with the fact that the sun hits the Equator more directly than the poles (in this most basic, simple description), making the surface air near the Equator relatively hot and the air near the pole relatively cold. Hot air being less dense and more buoyant than cold, convection causes the warm Equatorial air to rise while the colder polar air sinks.
Among other effects, this leads to higher air pressure in the middle levels of the atmosphere near the Equator (because air is buoyant and lifting into, or being added to, this part of the atmosphere) and lower pressure at mid-levels near the poles (because air is dense and sinking away from mid-levels of the atmosphere). So now you have, off the immediate Earth's surface (but even to some extent at the surface), higher air pressure near the Equator and lower air pressure near the poles. Nature and the laws of physics try to eliminate this pressure difference, meaning that air flows from areas of higher pressure toward areas of lower pressure to even things out - at least that's its intent.
This is where the Coriolis effect kicks in. On a non-rotating surface, air moves from higher pressure to lower pressure, directly perpendicular to the gradient (change in pressure with distance). But because of the Earth's rotation, this air flow is spun, in a sense, such that it is almost but not quite parallel to the gradient, keeping lower pressure to its left and higher pressure to its right (it is actually we and the Earth that spin beneath the air, but that is not the way humans on this rotating surface experience it).
Because of the direction the Earth spins, and because of the air movement dictated by all the factors above, the over-simplified result is a band of air that encircles the middle latitudes of the northern hemisphere, with winds that flow from west to east; high pressure near the Equator to its right (south in this case) and the lower polar air pressures to its left. These winds in their more complicated realistic state - stronger here, weaker there, buckled here, warped there, with waves interacting in often complex ways - drive weather systems, and generally keep them moving from west to east - that's the PREVAILING situation anyways - hence "prevailing" westerlies.

Answer 2

The Westerlies are caused by high pressure systems in the horse latitude. The tend toward the poles and are the strongest in the Western Hemisphere.

Answer 3

The westerlies are found between 30 and 60 degrees latitude and blow west towards the poles.

Answer 4

They are the prevailing winds in Canada

Answer 5

100!