Home > Mares' Tails and Mackerel Skies

Mares' Tails and Mackerel Skies

With one eye on the sky and the other on the fish-oil barometer, the old salts could make some stunningly accurate weather predictions. Lives and livelihoods depended on it, so it stands to reason that study of the weather spawned a rich supply of rhymes and tricks to help with forecasting.


It's easy for modern boaters to forget the basics as we become more reliant on high-tech sources for weather forecasting. But if you really want to know what the conditions are likely to be in your immediate area, there's no substitute for learning the essential skills of weather reading. All you need to start is a barometer, sight of the sky, and the habit of observing. Sophisticated reports generated by fax, NOAA weather radio, and your satellite pictures are best used as a back-up to your own observations.


Reading the Clouds

There are four basic cloud classifications. Their names are a little confusing to remember, but understanding what the different types of clouds portend is a necessary part of good seamanship.

The highest altitude clouds, cirrus, are your long-range weather forecasters. These clouds come in various shapes and sizes, including the "mare's tail" variety, but they are always thin because they are formed by a thin layer of ice crystals. Getting familiar with cirrus formations is important in forecasting weather. Once you begin to notice and classify clouds, you'll notice that high-altitude cirrus is responsible for a blue sky gradually turning into a milky haze and thickening, or "lowering" weather.


Cirro-cumulus clouds are the "mackerel skies" which develop from cirrus clouds beginning to lower and clump together. Due to their relatively high altitude, they have a dappled look, and a silvery sheen.


Cumulus clouds, or "fair-weather clouds," are the middle range of cloud which are characteristically white, fluffy, and lend themselves to imaginary shape-shifting. These are the happy-go-lucky clouds of the trade winds and high-pressure systems. If uncomplicated by further development, a parade of these simple cumulus against a true blue sky, absent any cirrus or cirro-cumulus background, is a good indicator of decent or calm weather ahead.


Cumulo-nimbus clouds result when cumulus build up into the shape of a blacksmith's anvil. The heat of a summer day often causes morning's innocent cumulus fluff-balls to develop into towering anvils with very white tops and very dark lower edges (squall lines) by late afternoon. The good news is that cumulo-nimbus developments (if uncomplicated) tend to be very localized, though potentially extremely powerful in their vicinity. Because of their tremendous height from top to bottom, you can spot them a long way off on the water.



You can also estimate the distance to the cumulo-nimbus by counting the number of seconds between the lightning flash and the sound of thunder and dividing by 5. An old sailor's rhyme which applies to most weather moving from west to east (in the northern hemisphere) says:


Beware the bolts from north or west;

in south or east the bolts be best.


In other words, lightning squalls will probably advance on you from the north or west, and cumulo-nimbus developments at a distance to the south or east probably won't threaten you. Along the same lines, a rather obvious rhyme reminds the nervous boater not to struggle into rain gear unnecessarily:


Rainbow to windward, foul fall the day;

rainbow to leeward, rain runs away.


High and Low Pressure Systems

The reason worsening weather is referred to as "lowering" is that the lower altitude of clouds is associated with localized surface conditions, or bad weather here and now, rather than long-range developments. Since high-pressure systems generally bring clear skies or clumpy white cumulus, low-pressure systems bring dark, low (in Latin, alto) clouds - which is really what we experience as fog, rain, and hail.

Let's assume you have a barometer aboard, or perhaps you're noting the readings given over the radio. A falling barometer (diurnal differences of up to .15" or 5 millibars notwithstanding) tells you that a depression, or low-pressure system, is nigh. As far as wind prediction goes, strong winds are a given. But you can go further yet! You can predict the directions of your local winds over the next few days if you know where the center of the depression is, because (in the northern hemisphere) winds "pour" into the center in a counter-clockwise direction. Expect to see "chaotic" cloud conditions (a mix of every kind of cloud at different altitudes) before the gray sets in. High-pressure systems can have fickle or light winds, but they are generally spiraling in a clockwise direction into the center of the high.


To figure out where you are in relation to a low, find its center by facing into the wind and visualizing the center as being to your right and a little behind you. This is also known as Buys Ballot's Law. Some folks prefer to put their back to the wind and look for the center on their left hand. Either way, it works.


Keep in mind that lows typically travel at about 17 miles per hour, and can take two or three days to pass over a stationary spot. Let's say that you notice in this case that your barometer suddenly begins to rise again - but don't breathe easy yet:


                        Quick rise after low

Often portends a stronger blow.


If you're going to see a cranky gale, it'll be on the quick rise of a low barometer. Expect the fierce and gusty variety of wind with this reading.


Barometers are moody instruments, and you are well advised to ignore any advice written around the face such as "Fair" or "Foul." What's really significant is the change itself, not the numbers. Digital barometers have built-in change monitors. A rapid and large change in your barometer usually indicates the same for the weather. It may happen that you'll diligently watch your barometer for a couple of weeks and find no relationship to weather reality; in this case, get that barometer calibrated and re-checked. A properly working barometer is probably your best ally in league with knowing your clouds.


Putting It All to Work

It's Thursday morning, and you consult the big blue chart in the sky about your weekend outing. You look up and it's basically blue. A few little wisps of white high up, which you might recall are cirrus clouds, don't seem to interfere with the bright sunshine. By the afternoon, the mare's tail is streaming out of the west, and by late afternoon, you notice the accumulation of a silvery dappled effect called mackerel skies, or cirro-cumulus.


You can be sure that within a couple of days, strong surface winds will arrive, probably from the west. Depending on how quickly the cirrus thickens and lowers, the blow could arrive within 12 hours.


You also notice that the gentle southerly breeze has gradually moved to the southeast. This is known as a backing wind, because it's shifting in a counter-clockwise direction. How does this fit with your observation of the cirrus? There are at least two old sayings to help with this one.


A backing wind says storms are nigh,

Veering winds will clear the sky.


Based on what you've learned about cirrus, you can safely make a bet with your neighbor about the sunset and sunrise, remembering that old favorite,


Red skies at night, sailor's delight;

Red sky at morning, sailors take warning.


On this hypothetical Thursday you probably would be well advised to get your chart out and plan a weekend boating that gives you plenty of options to run in to shelter from strong, wet winds that will back through south and east, which will bring clearing when they reach north. By Friday, you ought to be able to tell whether it'll clear by Sunday!


To learn the fine points of weather prediction, you need to study the local weather patterns over a period of time. It's a very complex picture when you consider the contours of the landmass, shorelines, and water. The particulars of the area interact with larger weather patterns to produce particular trends. After you've been observing and checking your educated guesses against reality for a while, you can reach a surprisingly high level of accuracy. It works, it's fun, and it can have a great impact on your boating pleasure.