Every cruising boat should have a reliable barometer on board. Invented by Italian physicist and mathematician Evangelisata Rorricelli in 1643, a barometer helps a weather-wise boater stay abreast of developing weather patterns. The approach of a strong low pressure system with high winds, a squall line, or a hurricane is typically signaled by falling barometric pressure. The steepness of the decline in pressure provides valuable insight into the strength of the approaching system. In contrast, rising barometric pressure usually heralds the arrival of fair weather and light winds. The ability to measure the rise and fall of barometric pressure only scratches the surface, however. What is barometric pressure, and what does it represent?