Large storms also arise in the middle and high latitudes where the ocean water is cooler than 25 degrees Celsius (77 degrees Fahrenheit). Warm air flows mainly horizontally toward the low, and there is a lesser but important component that is rising near and within the low. Cold air also flows mainly horizontally toward the low and tends to sink along the way. The occluded front, when there is one, is the boundary between the patterns. At the ocean surface, there are clear boundaries between the air masses along the cold, warm, and occluded fronts, while aloft there are not many sharp differences. Such storms are caused by cold polar air. The denser cold air overtakes the warm front of the depression, creating an occluded front where the warm air is forced upwards by colder air (occluded means “blocked”). Sometimes these occlusions fade away, but at other times they grow into huge rotating storm systems, as great as 3,000 miles in diameter. Extratropical storms differ from hurricanes; there is no warm, clear eye. Also—and most important—is that they have a large area with fairly similar, widespread, strong winds and waves. The storm moves in an easterly direction, traveling at a little more than 90 degrees to the occluded front, which is the boundary between the warm sector and the cold region. An abrupt wind shift often marks the passage of the occlusion, resulting in steep, confused seas.
The barometric pressure drops associated with extratropical storms are not as great as those of hurricanes, and the maximum winds are often less powerful than those of major hurricanes. The great danger from extratropical storms arises from the fact that they can extend over a much broader area and can last for several days. With longer duration and greater fetch, they can produce very large waves and dangerous seas. The resulting swell can travel extraordinary distances, as noted in Chapter 5. A rapid fall of the barometer is a sure sign of an intense storm; however a steady barometer is not a guarantee of good weather. Many a storm has seemingly “come out of nowhere” with no barometric warning.