Many voyagers are familiar with common, large-scale (synoptic-scale) features like high- and low-pressure systems, and their impact on weather and sea conditions. But what about troughs, which can cause 50-knot winds on a regional scale with little warning? We’ll explore these systems: what causes them, how to anticipate them and how to avoid their impact. To understand what makes troughs so powerful, we’ll take a brief look at a few weather concepts. Let’s start by summarizing the more familiar large-scale systems (highs and lows) and what drives them.
On a large scale, wind is driven by pressure gradients. Wind blows from areas of higher pressure toward areas of lower pressure. Think of an area of higher pressure as a hill of air molecules that flow outward down the hill in all directions due to gravity. An area of lower pressure can be treated as a crater or bowl toward which air molecules flow. Pressure slope, or pressure gradient, is simply the difference in pressure over a given distance. The steeper the gradient, the stronger the wind.
But air does not flow in a straight line from high to low pressure. The earth’s spin translates to ground-level air moving through friction with a circular tendency, resulting in a clockwise rotation for high-pressure air moving from the North Pole toward the equator. This is reversed in the Southern Hemisphere because air flows in the opposite direction to get from the South Pole to the equator.
What happens to air near the equator? Higher-pressure air moving from the poles is relatively cool, dry, heavy and dense, so it has a tendency to sink toward the ground and remain there. Lower-pressure air near the equator is relatively warm, moist, light and buoyant, so it has a tendency to rise in the atmosphere. When higher-pressure air moves in to displace lower-pressure air, the lower-pressure air rides up and over the higher-pressure air and moves toward the Poles. The earth’s spin also acts on this lower-pressure air, causing it to rotate counterclockwise in the Northern Hemisphere as it makes its way north.
Weather is also affected by movement of heat energy and moisture. Areas of lower pressure support vertical movement of considerable heat energy and moisture over a very short period of time. As this warm, moist surface air approaches the center of the low, the air rises and cools. If it cools to its water vapor saturation point, condensation occurs and visible clouds form. More heat is released through this process, driving heat energy and moisture higher, often leading to squalls.
Convergence lift In a cold front, a large area of cold, dry, heavy air, driven by a high behind the front, wedges itself under relatively warm, moist, light air on the other side of the front. This advancing cold air drives the warm air upward in a mechanism called convergence lift, where the convergence of air masses forces some air up. Convergence lift can happen along the front for hundreds of miles, and since it’s driven by strong and easily observed atmospheric mechanisms, it can be forecast quite well.
High- and low-pressure systems can dominate weather in an area thousands of miles across as they quickly circle the globe. The strong dynamics of such large systems are well understood and can be observed over large areas. Today’s computer models do an excellent job forecasting these strong, large-scale weather events, with wind/wave conditions often matching forecasts out to four or five days or even longer.
On many occasions I have seen a strong low-pressure system and its accompanying cold front arrive within three hours of the time indicated in a 96-hour forecast, along with conditions identical to those forecast for that precise location and time.
Strong lows and cold fronts can pack gale- or even storm-force winds and move swiftly across large areas of the mid-latitudes (between the tropics and the Arctic zones), accompanied by squalls, thunderstorms and large, confused seas. Strong high-pressure systems usually mean fair skies and no squalls; but with a sufficient pressure gradient, high-pressure systems can bring long periods of high winds and large, long-period waves. These effects can often be felt in the tropics, more than 1,000 miles from the strong mid-latitude high.
Thus it seems logical to avoid both extreme high- and low-pressure systems. Given our skill in forecasting these systems, you should have several days to a week of advance warning of their approach. Unfortunately, these systems can spread nasty conditions over an area 1,000 or more miles across, so you may not be able to avoid them even with a week’s notice. What you can do is prepare your vessel and crew for their arrival by catching up on sleep, preparing easy foods and readying heavy-weather gear. Mental preparation is important as well. Try to be ready for anything at any time while at sea. Most voyagers deal better with heavy weather when they expect it and have a feel for its likely severity and duration. We know we can’t control the weather, but most of us feel better if we have some understanding of it, why it’s happening and when it may end.
Surprised by dangerous weather
You might think that if you avoid strong pressure systems, fronts and areas forecast to experience winds more than 20 knots, you’ll have pleasant weather. This is an excellent idea and can result in an enjoyable passage. But I know more people surprised by uncomfortable and even dangerous weather, with sustained storm-force winds and 20-foot seas, in areas of mild pressure gradients and after forecasts for benign weather. Local conditions can always be stronger or weaker than forecast for brief periods of time, but persistent bad weather in areas with mild pressure gradients is often due to the presence of a trough.
Sept. 5, 2004, was a good example of unforecast bad weather due to a trough. All eyes were on Hurricane Frances approaching Florida, but the real story off the East Coast of the United States from Cape Hatteras to New England was a trough. All the various weather forecasts predicted wind in the 12- to 20-knot range and a chance of showers. There was no trough mentioned, but careful study of the wind forecast from the U.S. coast to Bermuda showed a slight arc to the wind direction, with winds along the coast from ENE, but well offshore they were ESE. Barometric pressure isobars showed a similar hump. Actual observations for the area near and to the east of this hump confirmed winds were 30 to 35 knots over an area 200 miles across, with numerous squalls. This was not local convection; it was a trough, and it would be recognized as such in forecast products the following day.
Why are troughs so hard to predict? Most forecasts for wind speed and direction are driven by anticipated pressure gradients on a large scale. Because a trough is a weak pressure system, surrounding gradients are slight, and wind forecasts often predict light winds in all areas of the trough. A trough supports atmospheric lift in the form of convergence lift and/or convective lift, transporting large amounts of energy into the atmosphere, often fueling strong squalls with high winds on a smaller scale (commonly a few hundred miles across). Many forecasts, especially for offshore waters, focus on stronger systems affecting a larger geographical area, and ignoring what can be a significant event for you. The good news is that you can make a good guess about where a trough will form and where bad weather will be concentrated near the trough.
To understand why troughs can generate such bad weather, let’s examine the mechanisms at work, starting with pressure gradient and how it generates convergence lift. This will sound familiar. A trough is an area of slightly lower pressure. Making a topographical metaphor, a high is a mountain of air; a ridge is a long but not too tall ridge of air; a low is a deep crater; and a trough is a long but not too deep valley. If we draw a line along the bottom of the valley, that would represent the axis of the trough. Air will flow gradually in from both sides toward the trough axis.
Modifying prevailing wind
If the trough axis is oriented in a north/south direction, you might expect converging winds to blow from the east and west toward the axis. But the pressure gradient associated with a trough is too weak to cause wind to blow opposite the prevailing direction, so the trough modifies the prevailing wind direction just a bit. Let’s assume prevailing winds are from the east. The trough creates a very weak convergence. So the trough modifies the east-to-west-flowing prevailing wind, with some wind flowing toward the trough from the northeast and some from the southeast.
What happens when air flowing from the northeast meets air flowing from the southeast? Air does not compress easily. Air will not penetrate the sea surface, so it can’t go down. Much of the air from the northeast and southeast combine and result in an increased breeze from the east. But some is deflected upward in the atmosphere, transporting heat and water vapor into the cooler air above. The warm air rises in a process called convergence lift. When the rising air cools sufficiently, moisture condenses to form clouds and eventually rain; even squalls if there’s enough lift. The illustration on page 40 shows wind flags you would see on your fax chart or grib reader, as well as the clouds and rain this rising air can generate.
If the trough persists, it can strengthen, dropping in pressure and pulling in more air at a greater velocity, which will converge with more force and push higher into the atmosphere, creating taller clouds and stronger storms. Low-pressure systems often form near the southern end of a persistent trough. You can identify a trough on a synoptic chart and assess its strength by the degree to which it bends wind direction and pressure isobars (see Risk avoidance on page 44).
Convection, or convective lift, is the lifting of warm, moist, light air and transportation of heat energy and moisture from the sea surface upward into the atmosphere as a result of warm air rising. Warm air will continue to rise as long as it remains warmer than surrounding air. Both the rising warm air and the surrounding air mass cool at higher altitudes, but they do so at different rates. We need a way to predict when the temperature of the rising air will equal the temperature of the surrounding air mass, at which point it will stop rising and stop carrying heat energy and moisture higher. We can do this by quantifying the temperature profile of the surrounding air at various altitudes and predicting whether sufficient convection exists to support squalls even in the absence of a trough.
Two common measures describe the atmosphere’s temperature profile and support for convection. Lifted index (LI) is a direct measure of temperature decrease with altitude increase. Higher negative numbers indicate a greater decrease in temperature of the air mass with altitude and more support for strong squalls. Convective available potential energy (CP) quantifies dynamic support for convection. A higher CP number means more convection. (See Passage planning on page 42 for significant values for each of these indicators, as well as how to obtain the data.)
Most models predict strong, large-scale weather events with a great degree of precision. But as we’ve seen a trough is a weak pressure system on a regional scale. While models identify a trough’s wind and pressure gradients and often account for showers due to convective lift, they usually fail to account for the effects of convergence lift. The reason may be that this convergence lift is restricted to certain parts of the trough.
Another important difference between weak weather systems such as troughs and strong lows and cold fronts is how long their effects linger in an area. The effects of strong pressure systems passing through an area last for a number of hours to a couple of days, while a trough can linger for days, weeks or even longer. For each of the past few summers we have had persistent troughs off the U.S. Southeast coast.
In 2002 there was a strong trough from Nicaragua and Honduras through Cuba and the Bahamas toward the Carolinas for most of May and June. This caused severe squalls in much of the Bahamas, across Cuba and throughout the northwest Caribbean, which is a spawning ground for early season tropical storms.
In 2003 the persistent trough sat from Cape Hatteras to the Florida Keys and along the Gulf Stream region off the U.S. Southeast coast from April through June. This frequently combined with an upper-level trough and a split in the jet stream to create nearly three solid months of squally weather in the coastal and offshore waters, including the Gulf Stream, from Cape Hatteras down the entire east coast of Florida. Vessels leaving the Bahamas for points north encountered squalls and winds of 40 to 50 knots.
And in 2004, a persistent trough lay farther east, generally between Bermuda and the U.S. East Coast in June and July. Vessels traveling from the eastern Caribbean to the U.S. coast from the Carolinas northward, or between Bermuda and the United States, encountered 60-knot winds for a day or two during their journey, and these conditions were confirmed by satellite wind observations.
The significant element is that these conditions were often not forecast. Even when they persisted for days or weeks, forecasts tended to treat these troughs as showers or squalls in areas of light forecast winds. But when gale or storm conditions persist for days over regional areas, they are usually due to convective lift and/or convergence lift generated by a trough, which can be identified in forecast products, usually a couple days before conditions deteriorate.
Troughs can occur almost anywhere you’re likely to sail, though not frequently in the Arctic. Troughs often linger in one area for long periods near the boundary of mid-latitude weather, where cold fonts dissipate. In the tropics, a trough that moves from east to west is called a wave and can occur year-round. In the summer, many tropical storms and hurricanes begin as waves.
Given their speed and geographical coverage, you will often be unable to navigate clear of a strong low and its cold front. But given sufficient warning and a flexible plan, you can often avoid a smaller developing or slow-moving trough. In many cases where vessels encountered unexpected bad weather for prolonged periods, a trough was to blame.