Marine Weather

I’ve just completed a Marine Weather course, which I found very interesting and I’d definitely recommend it even if you aren’t dependent on the wind to get where you’re going. In addition to teaching you how to read Weather Maps, you also learn: weather and wave predictions, passage planning and storm avoidance all based on various indicators such as cloud formations and barometric pressure and air temperature trends, wind direction and shifts etc etc.

If you are interested in this subject that affects everyone read on. I’ve copied my notes below for future generations of brain cells.

WIND

The wind velocity is often described with the Beaufort scale and  is a function of many factors. There are belts of High (areas of high pressure) and Low (areas of low pressure) pressure systems that circle the globe. The Highs are roughly located at the poles and +/- 30 to 40° latitude (“Horse Latitudes”). The Lows  are roughly located at the Equator (“Doldrums” at +/-10°) and +/- 50 to 60° latitude. These belts shift North as the sun moves to the Tropic of Cancer during summer in the Northern Hemisphere and South as the sun moves to the Tropic of Capricorn during winter. Highs are characterized by light winds and clear skies, while Lows are traditionally areas of bad weather, cloud cover, rain and frequent storms/squalls.

Global Prevailing Wind Pattern
Global Prevailing Wind Pattern

As these pressure belts are semi-permanent, the global prevailing winds have remained fairly consistent ever since the days of the first ocean explorers. The figure above shows the Polar Easterlies from +/- 60 to 90°, the Westerlies from +/- 40 to 50° and the Trade Winds from +/- 10 to 30° filling in the latitudes around the High and Low pressure belts.

Air flows from the Highs to the Lows, but due to the Coriolis effect (caused by the rotation of the Earth), the winds tend to “veer” (clockwise) in the Northern Hemisphere and “back” (counter-clockwise) in the Southern. So in the Northern Hemisphere wind circulates clockwise around a High and counter-clockwise around a Low (opposite in Southern Hemisphere).

Wind Rotation in Northern Hemisphere
Wind Rotation in Northern Hemisphere (with friction)

A consequence of all this is the “Buys Ballot” Rule, which states that with the wind at your back, the High is to your Right (and slightly behind) and the Low is to your Left (and slightly ahead). For us sailors this can be interpreted as, to escape bad weather head off on a starboard tack.

Friction also plays a role. At high altitude the wind is tangential to the isobars but over oceans (i.e. low friction) the wind is 10 to 15° off the isobars, pointing out of the Highs and into the Lows. Overland (i.e. high friction) the wind is 30° off and can be even 45° where there is high friction or steep pressure gradients.

So the winds appear to back (turn counter-clockwise or to the left) as you get closer to shore, due to the increase in ground friction. And the winds appear to veer (turn clockwise or to the right) as you get further from shore, due to the decrease in ground friction. This has a few practical implications:

  • When approaching a leeward island, there will be more wind on the left side (i.e. keeping the island to starboard) as the wind will back and be forced more to the left side. Conversely, when approaching a windward island, there will be more wind on the right side (i.e. keeping the island to port)
  • When close reaching toward land (e.g. a windward port or bay), approach land on a port tack. The wind will appear to back as you get closer to shore and if you’re on a starboard tack you risk ending ending up straight into the wind when close to shore.

There are also many localized effects on the wind, including

  • Diurnal effect: Winds near coast lines tend to be onshore during the day, and offshore during the night. This is due to the heating and cooling affects of the land mass. Watch out for anchorages close to large land masses as the wind can swing 180° when it turns to night. These can have many names (e.g. Katabatic, Williwas)
  • Topographical effects: High ground surfaces interfere with the free flow of winds. They can cause acceleration along coasts and around corners, funneling in inlets and between islands and eddies (i.e. opposite direction to prevailing wind) along coast lines

Lows offer an opportunity of good sailing under a broad reach to cross oceans. In the Northern Hemisphere, when crossing from East to West, one could sail North of the Lows. As the Lows move West to East, you would have to hop from Low to Low. When crossing from West to East, one could sail South of the Low and ride a low all the way across the ocean. The reverse occurs in the Southern Hemisphere (i.e. sail North of the Low when crossing from West to East)

WEATHER FRONTS

Fronts are interaction between two air masses of different characteristics (e.g. pressure, temperature and humidity). Fronts are characterized by thunder storms and strong winds (especially Cold Fronts) as well as rain and confused seas. Fronts rotate around a Low pressure system in a counter-clockwise direction (in the Northern Hemisphere). A Warm Front, is the surface separating relatively light warm/wet air and the mass of heavier cold air in front of it. The warmer air tends to slide over the cold air and creates clouds of horizontal development over a large gently sloping area. The entire front can stretch over thousands of miles and might take days to get by.

warmfrontgraph
Warm Front cross section

A Cold Front is the surface separating relatively dense cold/dry air and the mass of lighter warm air in front of it. The front has a very steep surface and creates clouds of vertical development over a narrow steep area. The front is only perhaps a few miles and takes a few hours to get by.

Cold Front cross section
Cold Front cross section

The Warm Front always proceeds the Cold Front but the Cold Front tends to move faster and catch up with the Warm Front and can form occlusions that can look like a warm or cold front. As a warm front approaches, the barometric pressure drops and the temperature rises; both slowly at first and then at a higher rate. Between the warm and cold fronts, pressure and temperatures remain relatively stable. Just before the cold front hits, there will be a sudden drop in pressure. After the cold front, pressure increases and temperature drops, both rather rapidly.

Sailing North of the Low (in the Northern Hemisphere) avoids crossing the Weather Fronts and will have least amount of wind. You can tell if you are North or South of a Low by with your back to the wind whether the wind appears to be backing (North of Low) or veering (South of Low) and this works for both Hemisphere’s.

CYCLONES AND SQUALLS

Cyclone systems (Hurricanes, Typhoons) form near the equator and are caused by temperature differences between hot/dry air and warm/humid air. The systems develop on the East side of water bodies and typically move West (whereas Lows move W to E) and veer North in the Northern Hemisphere and back South of the Equator. They are forecast by the National Hurricane Center in Miami and the Naval Western Oceanography Center in Honolulu.

Typical seasons are: Atlantic (Jun to Nov, peak mid Sept); N.E. Pacific (May to Oct); N.W. Pacific (Year Round, but fewer in Feb and most in Aug and Sept)

As cyclone trajectories tend to move away from the Equator, the “Safer” side of a cyclone is closer to the Equator (e.g. South of the cyclone path in the Northern Hemisphere). Again you can tell whether you are North or South of the cyclone by with your back to the wind, whether the wind appears to be backing (North of cyclone) or veering (South of cyclone). If you are on the “Safer” side, to avoid the cyclone just Broad Reach on starboard tack. However, if you are on the more “Dangerous” side (e.g. North of path in Northern Hemisphere), then you need to decided whether to try and cross in front of the path and get to the “Safer” side or to go on a starboard tack and try and get away from the cyclone path. However, this last option, risks the cyclone path curving and catching you in the middle.

Squalls are short lived and the result of unstable conditions (difference in temperatures). For example in the evening, when the water remains warm but the air cools rapidly. Squalls are frequent in the Equatorial Doldrums (ITCZ – Intertropical Convergence Zone). The highest squall activity occurs in the beginning of the night. Most of the time, temperature drops rapidly just before a squall and tall black clouds and white waves will form.

WAVES

Waves are linked to the wind speed and height will build as the wind speed increases. It also depends on the available fetch (i.e the longer the fetch or length of open water for the waves to build, the larger the waves). Waves are the most dangerous when they become breaking waves, which occurs at very high wind speeds (Storm Force) or when against a current. The current doesn’t have to be much, only a knot or two, to have sufficient momentum to cause steep and/or breaking waves.

CURRENTS

The Coriolis effect also has an affect on the movement of water across oceans. Ocean currents tend to turn clockwise in the Northern hemisphere and counter-clockwise in the South.

Ocean Currents
Ocean Currents

RESOURCES

For surface, 500 mb and wave analysis and prediction charts:

Now it is time to put all this new knowledge to use and see if I can start predicting the future!

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