Wind and Current on the Water
Learning Goal 9g:
- Describe what different weather systems (ie. High and low
pressure, warm and cold fronts) look like when you’re on the water; and
- Describe the effects that wind, tide and current can have on your
travel speed and access to certain areas.
Now you know about the global circulation systems, how the
atmosphere and the currents interact, and about some of the more
dangerous characteristics of wind. The trick is going to be using this
knowledge to plan your trip. Weather is hugely important to sailors. An
unexpected change in wind or seas can greatly reduce how efficiently
you travel across the water or even threaten your safety. You need to
know what the winds and currents are going to look like ahead of time,
and how to look for changing conditions once you’re out on the water.
Sailing near a High-pressure center, with light wind and mostly clear skies. Source: Samantha James.
Surface Pressure Systems and Winds
If you know roughly where the high- and low-pressure systems are
going to be during your trip, you can navigate through them more
effectively, selecting for optimal winds and fair weather.
High-pressure systems are typically
associated with mostly clear skies and light to calm winds (see photo above) because you have warm, dry air
descending.
Low-pressure systems are
typically associated
with unfavourable weather such as rain, storms, and fronts, because you have
warm, humid air rising and causing the water vapour to condense to form
clouds.
Sailing Tips
- Faster winds and higher waves are often found closer to the Low
center, and also near the fronts. Strong lows can be dangerous
for sailors.
- A
wide band of stratiform clouds with associated drizzle or snow
is often found AHEAD of warm fronts. Very low clouds and possibly
fog can occur at the warm front. After the warm front passes, the
skies clear and the winds are warmer. But often cold fronts will follow
behind warm fronts.
- A narrow band of thunderstorms (cumuliform clouds with lightning,
gusty winds, very heavy rain, and downbursts)
is often found along or just ahead of cold fronts. These squall lines were discussed in more detail in Learning Goal 9f. As the front passes, the temperature and humidity drops,
pressure rises, and skies eventually clear. The winds usually shift
from southwest to northwest (in the Northern Hemisphere), and you can
expect a period of clear but cooler weather to follow.
- Away from fronts, the air can be clear or partly cloudy, but
winds can still be strong if there is a strong pressure gradient
(change of pressure across a distance) between High- and Low-centers.
- High-pressure centers have light to calm winds - - bad for
sailing.
- For long-distance sailing, it is good to pick a route around lows
and highs so that you have a tailwind, as discussed in the next
paragraphs.
Using synoptic-scale pressure patterns to good advantage
In the figure below, suppose you wanted to sail across the North Pacific
from Vancouver (point A) to Asia (point B).
BAD route: The
shortest route would appear to follow the green dashed line straight
from A to B. However, soon after starting from point A, you would
approach the center of the first High-pressure (H), where you would be
becalmed. So you would need to turn on your engine to get through
this region because there is no wind to fill your sails. Then, as
you approach the Low (L), winds would get so strong and stormy with
high waves that sailing would be very difficult, and you would need to
use reefed sails (reduced sail area) to not destroy your sail or your
mast or sink your boat. Then on to the next High (H) where you
would be stuck in calm winds again.
Conclusion: the shortest route is not necessarily the best or
fastest or safest route.
Sketch of synoptic-scale highs (H) and lows
(L) over the North Pacific Ocean. Wind directions are shown with the
thin black lines. Adapted from R. Stull, 2017: Practical
Meteorology.
GOOD route:
However, if you sailed following the thick dashed purple line, then
you would have nice tailwinds around the highs and the lows, allowing
you to "run" downwind. Between the highs and lows would be nice
crosswinds ("beam reach", see "Points of Sail" in the sailing basics
web page) that would provide fast and fun sailing. You would stay
out of the calm centers of the Highs, and you would stay far enough
away from the Low center to avoid most of the bad weather. Your
trip would be fun and fast.
Thought exercise: If you wanted to sail from Asia
(point B) to Vancouver (point A) as plotted in the figure above, what path would you want to sail for the best speed and safety?
Cross-ocean and "round-the-world" skippers take good advantage of
the synoptic-scale weather systems (lows and highs), while trying to
avoid the difficult sailing near fronts. They also try to take
advantage of planetary-scale winds such as the trade winds at low
latitudes if they are traveling from east to west, or they take
advantage of the westerlies at mid-latitudes if they are sailing from
west to east. They are also avoiding
planetary-scale problem areas of calm winds such as the doldrums (near
the equator) and horse latitudes (near 30 degrees latitude). When
sailing, it is important to know where the highs and lows are and where they are moving, in order to know
how the wind is moving around them.
We will look at how to read these systems on weather maps later in
Topic 11 on Marine Weather Services.
Wind and Waves
It’s important to remember that winds also affect waves. If the
pressure gradient between high and low pressure areas is large, the
winds will also be large. Large winds can create large waves and
dangerous sailing conditions (remember ‘8e
Beaufort Scale’). Winds travelling in the opposite direction of the
waves can also create large choppy waves, or a confused sea state.
Sailing toward a front, with increasing clouds, wind, waves, and rain. Source: Samantha James.
Tides and Current
Missing the right tide or current can mean not getting to your
destination when you planned or even entering dangerous waters. It is
immensely important that you are familiar with the daily tides and the
local currents of where you will be sailing (recall Learning Goal 9d).
For longer trips, sailors often use the global ocean currents to
help carry them long distances. The global currents are steady and
consistent. If you are sailing across the Atlantic, ride the Gulf
Stream up to the North Atlantic Current and you’ll end up somewhere in
Western Europe. If you were to aim straight across the Atlantic for
northern Africa, you would be fighting the North Atlantic Equatorial
Current and the trade winds the entire way. If you ever want
to cross a current, you will need to account for the amount of drift
you will experience (see ‘School of Sailing- Currents’ in the
Additional Resources for an example of crossing the Gulf Stream).
For smaller trips, the tides will be more likely determine the
direction of the current than the larger circulations. Timing your
travel with the tidal current will make your trip much more enjoyable.
We will look at local tides in greater detail in Learning Goal 10d, however for now, know that tidal
currents vary in speed and direction, and travelling with the tidal
current is favourable to travelling against it.
Travelling with the current can also help you save fuel if you’re
driving under motor power.
This is a basic introduction of
things to consider when planning a
route. In the next set of Learning-Goal topics, we will introduce you
to a few more
pieces of the puzzle and provide you with some hands on tools to help
you plan the optimal route. Finally, Sailing topic 11 will show
where to get weather, tide, and current forecasts and how to interpret
them.
Sailing at a location between high and low
pressure, with partial clouds and enjoyable winds caused by a moderate
pressure gradiant. Source: Samantha James.
Additional Resources: (non-required material)
School of Sailing – Currents: http://www.schoolofsailing.net/currents.html
Video: (non-required material)
Sailboat struggling against fast current in Deception Pass: https://www.youtube.com/watch?v=4B3sd0XRFBE