Wind is a critical factor in the pleasure and challenges of boating. But it isn't just the wind at the surface that matters. Observations there are literally only skimming the surface of the atmosphere.

Captains also need to be aware of the air and winds of the middle atmosphere, well above the top of the mast. Based on what's happening up high, you can know how to gauge whether it's smooth sailing ahead or prudent to head for shore.

NOAAThe 500 millibar heights (represents the height above sea level in decameters that the pressure is measured at 500 millibars) show a trough in the west and a ridge in the east for April 10th, 2008. Cooler weather would be expected in the northern Rockies and warmer weather along the East Coast. Storms would be expected in the central Plains. For that reason, the safety checklist for any boater should include consulting a 500-millibar weather chart, long a valuable forecast tool in meteorology and readily available at most weather websites. Its patterns can indicate where storms are lurking and where they are headed, causing potentially dangerous conditions.

THE WIND

High in the atmosphere, above the influence of friction from the Earth's surface, winds blow like rivers of air across the country and around the Earth. The winds blow from west to east because of two forces: the pressure gradient force and the Coriolis force (see "Why the Wind Blows").

In the region of the Earth around 30 degrees north latitude, high pressure is generally dominant, while in the subpolar region around 60 degrees north latitude, low pressure is generally the primary force. The pressure gradient force, therefore, is directed from high pressure to low pressure (simple example: air in a car tire is under high pressure, so when you open the valve, air rushes out of it).

In the continental United States, the pressure gradient force is generally directed from south to north. Meanwhile, because of the rotation of the Earth, the Coriolis force has the effect of turning a moving object to the right. That's why storms and clouds generally move from west to east: Air flows to the north and then turns right, or eastward.NOAAThe 500 millibar heights for Sunday April 13, 2008. Notice the closed strong low pressure center over the Great Lakes. This is called an upper-level low (a low center in the upper air pattern). Meteorologists can expect warming conditions in the west (due to the ridging) and stormy weather in the Northeast (along with cooler temperatures).

That brings us to the 500-millibar chart. The 500-millibar field–the midway point in the atmosphere, where half of the Earth's mass is above and half is below–is about three miles into the atmosphere, and that is where storms track. For comparison, the "jet stream," a term coined in the 1940s when U.S. bombers appeared to "hover" over one area when flying against very strong winds, is about six miles high. The atmospheric pressures there range from 200 to 250 millibars.

The 500-millibar chart is significant for meteorologists and boaters alike because it can help determine not only the direction of storms, but also storm formation and evolution, as well as other weather variables such as temperature and sky cover.

Storm development is determined by the amount of spin in the atmosphere, specifically cyclonic spin (counterclockwise rotation). The meteorological term for spin is called vorticity, and it is created by directional or speed changes in the wind field. Because wind is never uniform over a large area, localized areas of convergence (winds coming together) or divergence (winds spreading apart) create the pressure fluctuations that eventually develop into high and low pressure centers, those H's and L's we all see on weather maps.

TROUGH/RIDGE PATTERNNOAAThis map includes the wind barbs (identifying the wind direction and wind speed). Strong winds (over 60 knots) are located over the Rockies.

The 500-millibar map is also important for forecasting the trough/ridge pattern. A trough is simply a southern "bulge" in height contours that is the upper-air counterpart of a surface cyclone, while a ridge is a northerly displaced "bulge" and is typically associated with calm weather. The trough/ridge pattern determines which regions are having above- or below-normal temperatures and if the wind flow is conducive to storm systems or quiet weather. High-amplitude patterns (deep troughs and high ridges), for instance, indicate stormy conditions are possible just downwind of the trough axis (the "bottom" of the trough).

Air that rotates counterclockwise, such as in low-pressure storms and troughs, is said to have positive vorticity. Clockwise rotating air, such as in high-pressure systems and ridges, has negative vorticity. The presence of positive vorticity at middle and upper levels causes air to rise; negative vorticity causes it to sink. It is this rising and sinking motion that is important in predicting potential locations for clouds and rain. Rising air will cool, causing condensation, cloud formation and possibly precipitation. Sinking air warms, which increases evaporation; evaporation or drying of the air, in turn, promotes sunny skies.

By examining the overall trough/ridge pattern on a 500-millibar chart, a sailor can gain extremely valuable information about what is happening in the atmosphere. Underneath troughs, the weather can bring unsettled weather, cooler than normal temperatures and sometimes severe weather, while under ridges temperatures are warmer than normal and are typically accompanied by clear skies. Why is this? The easiest explanation is the direction of the wind flow. Entering a trough, the air is flowing from the north to the south, which in the United States sends colder Canadian air southward. Entering a ridge, the air is moving from warm southerly regions toward the north.