In the Northern Hemisphere air flows inward, upward, and counterclockwise in a low pressure system and outward, downward, and clockwise in a high pressure system. According to Buys Ballot's law, if you stand with your back to the wind in the Northern Hemisphere, the low pressure will be on your left and the high pressure will be on your right. The Coriolis effect causes winds aloft to flow parallel to the isobars (lines of constant pressure shown on a weather map); this force is weak in the tropics and the pressure gradient force dominates, causing air to flow across the isobars from high pressure areas to low pressure areas. At lower altitudes, surface friction also causes winds to flow across the isobars at an angle. Reported winds: If wind direction is written, it's true. If spoken (as in an ATIS broadcast), it's magnetic.