Observing the clouds.
A red sky at night is a sailor's delight A red sky in the morning is a sailor man's warning.
Red sky at night, sailors delight. Red sky in the morning, sailors take warning.
Red sky at night, shepherds delight. Red sky in the morning, shepherds warning.
The evening red and morning gray Are sure signs of a fine day, But the evening gray and the morning red, Makes the sailor shake his head.
Evening red and morning gray will set the traveler on his way; but evening gray and morning red will bring down rain upon his head.
Evening red and morning gray. Two sure signs of one fine day.
Evening grey and morning red, put on a hat or you'll wet your head.
A setting red sun means it'll be hot tomorrow.
If the sun goes pale to bed 'Twill rain tomorrow, it is said.
If red the sun begin his race, Be sure the rain will fall apace.
If red the sun set in gray The next will be a rainy day.
If the morning sky is red, the ewe and her lamb will go wet to bed.   A child's interpretation.
An excellent range of links to red sunset photos and  a test report
If clouds are gathering thick and fast, Keep sharp look out for sail and mast, But if they slowly onward crawl, Shoot your lines, nets and trawl.
When clouds are gathering thick and fast, Keep sharp lookout for sail and mast; But if they slowly onward crawl, Out with the dories, nets, or trawl.
The higher the clouds the better the weather.
High clouds indicate fine weather will prevail lower clouds mean rain.
Mackerel sky, mackerel sky - never long wet, never long dry. Herringbone sky, neither too wet nor too dry. (clouds have a herringtone pattern) Herringbone sky, won't keep the earth 24 hours dry.
Mare's tails; storms and gales. Mackerel sky; not 24 hours dry.
Horses' manes and mares' tails-- Sailors soon shall shorten sails.
Mackerel sky and mares' tails make lofty ships carry low sails.
Mackerel scales and mare's tails make lofty ships carry low sails.
Mackerel skies and mares tails, make ships carry short sails.
Backing winds and mares tails make tall ships carry small sails!.
If clouds look as if scratched by a hen. Get ready to reef your topsails then.
When clouds appear like rocks and towers, the earth's refreshed with frequent showers.
When clouds look like rocks and towers, the earth will be refreshed by showers.
If wooly fleeces deck the heavenly way, Be sure no rain will mar a summer's day
Yellow sky at sunset, wind in the morrow.
Clouds known as mares' tails by sailors of old, are fallstreaks. They are fibrous, hooked-shaped clouds that are composed of ice. Although not uncommon, the fallstreak is somewhat of an intriguing mystery.

 

Observing the wind.
When the wind shifts against the sun, Trust it not, for back it will run.
When the wind is from the south the rain's in its mouth.
When rain comes before the wind, halyards, sheets and braces mind, But when wind comes before rain, soon you may make sail again.
When rain comes before the wind, Dories, gear and vessel mind; When wind comes before the rain, soon you'll make the set again.
When the wind is blowing in the North, no fisherman should set forth, When the wind is blowing in the East, 'Tis not fit for man nor beast, When the wind is blowing in the South, it brings the food over the fish's mouth, When the wind is blowing in the West, that is when the fishings best!
Wind from the east fish bite least, wind from the west fish bite best.
With rain before wind, stays and topsails you must mind. But with the wind before the rain, your topsails you may set again.
If the wind is in the east, they bite the least. If the wind is in the west, they bite the best. If wind in the south, throw bait in their mouth. If wind in the north, stay home by the hearth.
Wind before the rain, turn and shoot again. Rain then the wind, pick up and go in.
Every wind has its weather.
The wind in the west Suits everyone best.
No weather is ill, if the wind be still.
A little rain, stills a great wind.

 

Observing rain, dew, snow, fog and rainbows.
If it's foggy in the morning then it'll be a sunny day.
When the fog goes up the hill it takes the water from the mill. When the fog comes down the hill, it brings the water to the mill.     The explanation.
Rain before seven, clear by eleven.
When dew is on the grass, no rain will come to pass.
A little rain, stills a great wind.
Snow like meal, snow a great deal.  A test report
Wind before the rain, turn and shoot again. Rain then the wind, pick up and go in.
When rain comes before the wind. Halyards, sheets and braces mind, But when wind comes before rain, soon you may make sail again.
When rain comes before the wind, dories, gear and vessel mind; When wind comes before the rain, soon you'll make the set again.
If there is dew upon the ground, no rain that day will be found, Unless it rained the night before and then you had better keep score.
Rainbow in the morning, travelers take warning; rainbow at night, traveler's delight.
Rainbow at night, sailors delight; rainbow in the morning, sailors warning.
Rainbow in the eastern sky, the morrow will be dry. Rainbow in the west that gleams, rain falls in streams.

 

Observing plants, birds, animals and insects.
Ash before oak in for a soak, Oak before ash in for a splash. See the contributors comments.
Seagull, seagull sit on the sand. It's never good weather when you're on the land.   A test report.
Seabirds, stay out from the land, we won't have good weather while your on the sand.
An Australian rain prediction that works in winter, "When the kookaburra's call, the rain will fall".
When the trees begin their dance. Of rain there is a great big chance.   An explanation
When the ass begins to bray, be sure we shall have rain that day.
When leaves show their back, rain we won't lack. A test report and another and more detail.
Moss dry, sunny sky, moss wet, rain we will get.
Bees do not swarm before a storm.
When the rooster goes crowing to bed, he will rise with a watery head.

 

Observations at night.
A change in the moon brings on a change in the weather.
If all stars are out at night it will be a nice day tomorrow.
When halo rings the moon or sun, rain's approaching on the run.
If a circle forms around the moon, 'twill rain soon.
If the moon holds water it wil be dry. If water from it can leak rain is nigh.      Other moon observations

Observing the Barometer
It is interesting to note that only a few of the verses mention the glass (barometer). My research, using The Popular Encyclopedia (published about 1896, over one hundred years ago) shows that the barometer was invented in 1645 as a three foot (900mm) glass tube. The aneroid barometer was invented in 1844. For many years these instruments remained the province of the scientists.
When the glass falls low Stand by for a blow; When it slowly rises high All the light sails you may fly.
When the glass falls low Prepare for a blow; When it slowly rises high, Lofty canvas you may fly.
Long foretold, long last Short notice, soon past, Quick rise after low, Sure sign of stronger blow.
Quick rise arter low Indicates a stronger blow; Long foretold, long last, Short notice, soon past
Fast rise after low, Foretells a stronger blow.
At sea with low and falling glass Soundly sleeps a careless ass, Only when it's high and rising Truly rests a careful wise one.

 

Other observations
When Windows won't open, And the salt clogs the shaker, The weather will favour the umbrella maker!
When you hear the train, it will rain.   An explanation
Thunder before seven, rain before eleven.

 

Observations for the Seasons
All of the above relate to forecasting the following days weather. There are also verses that attempt to forecast the coming seasons weather. They would appear to relate more to the farmer / shepherd than to the sailor or fisherman. The majority of the seasonal forecasting adages apply only to the northern hemisphere as the seasons in the southern hemisphere commence six months before (or after, which ever way you look at it). See Christmas in Australia for an example of this divergence.
When March blows it's horn, your barn will be filled with hay and corn.   And more
January brings the snow, Makes our feet and fingers glow.   February brings the rain, Thaws the frozen lake again.   March brings breezes sharp and chill, Shakes the dancing daffodil.   April brings the primrose sweet, Scatters daisies at our feet.   May brings flocks of pretty lambs, Sporting round their fleecy dams.   June brings tulips, lilies, roses, Fills the children’s hands with posies.   Hot July brings thunder-showers, Apricots, and gilly-flowers.   August brings the sheaves of corn; Then the harvest home is borne.   Warm September brings the fruit; Sportsmen then begin to shoot.   Brown October brings the pheasant, Then to gather nuts is pleasant.   Dull November brings the blast-- Hark! the leaves are whirling fast.   Cold December brings the sleet, Blazing fire, and Christmas treat.
Ice in November to bear a duck, The rest of the winter'll be slush and muck.
April showers bring May flowers. March winds bring April showers.
March comes in like a lamb, goes out like a lion. March comes in like a lion, goes out like a lamb. Which is correct? or are both correct depending on your location?   Details.
These are the childhood memories of another supporter.   Details. If March comes in like a lion, it will go out as a lamb; if it comes in like a lamb, it will go out as a lion.
St Swithin's day, if thou dost rain, For forty days it will remain; St Swithin's day, if thou be fair, For forty days 'twill rain na mair. See history