Polar jet streams are primarily caused by the interaction of warm and cold air masses. These fast-flowing, narrow air currents occur at high altitudes and are typically found at the boundary of polar air masses and warmer air from lower latitudes. The significant temperature differences between the cold polar air and the warmer air to the south create strong pressure gradients, leading to the formation of the polar jet streams.
Other factors like the rotation of the Earth (Coriolis effect) and the presence of the jet stream's meandering patterns also play a role, but the fundamental cause is the contrast between warm and cold air masses.
So, the correct answer is: warm and cold air masses.