Physical Properties of Air
Hot air expands, and rises; cooled air contracts--gets denser--and sinks; and the ability of the air to hold water depends on its temperature. A given volume of air at 20°C (68°F) can hold twice the amount of water vapor than at 10°C (50°F). The relationship of how much water a given mass of air actually holds compared to the amount it can hold is its relative humidity. When air holds as much water vapor as it can for a given temperature (100% relative humidity), it is said to be saturated. If saturated air is warmed, it can hold more water (relative humidity drops), which is why warm air is used to dry objects--it absorbs moisture. On the other hand, cooling saturated air (said to be at its dew point) forces water out (condensation). This is why a can of cold soda sweats: it cools the air next to it and moisture from the air condenses on the outside of the can.
So, air warmed by ocean currents picks up a lot of moisture. As the heated air rises, it expands, which is measured at the surface as low air pressure. Expanding air cools, which forces it to lose its moisture as rain or snow.
The opposite is true for sinking air. Such air compresses and warms. In a zone of high pressure like this, moisture is absorbed by the air from its surroundings.