Weather proverbs are an ancient part of human society; there are even a couple in the Bible which are still used today, although they are sometimes re-worked. Some weather proverbs are quite accurate, depending on where one is, while others appear to be a load of hogwash. When considering weather proverbs, it is important to remember that weather patterns move very differently in the Northern and Southern hemispheres, and that within each hemisphere, there is a great deal of variation. Regional weather proverbs tend to be more accurate, because they come from years of experience with the weather of a specific area.
Perhaps one of the most well known weather proverbs is “red sky at night, sailor's delight, red sky in the morning, sailors take warning.” According to meteorologists, a particularly red sunset or sunrise is caused by large amounts of particulate matter in the sky, which filter the rays of the sun before they reach the earth. The longer red wavelengths make it through, while shorter blue and green wavelengths do not, causing the sky to appear red. During periods of high pressure, particulates are trapped close to the earth; at sunset, they will make the sky turn red, and in the Northern hemisphere, where weather generally moves West to East, this suggests that fair weather is coming. If the sky is red at dawn, on the other hand, the high pressure system has passed, and the weather may turn foul. A related proverb is “rainbow in the morning gives you fair warning,” because it indicates rain and a low pressure system to the West.
The saying “clear moon, frost soon” is applicable to all regions of the Earth, because when the sky is clear, there is no insulating cloud cover to prevent frost, so during the winter, a clear moon can be a strong indicator of frost. “Halo around the moon, rain or snow soon” is a bit less reliable; the halo is caused by crystals in the sky, which may or may not develop into wet weather. You may also have heard weather proverbs about stars huddling together before poor weather; these proverbs reference the fact that when heavy cloud cover obscures much of the sky, it can look like the visible stars are clinging to each other.
A particularly colorful weather proverb is “mackerel scales and mare's tails make tall ships take in their sails.” This proverb references the appearance of the sky in advance of a major storm. The cirrus clouds which often drift in front of a low pressure system do look sort of like lumpy fish scales. Closer to home, many people believe that smoke curling downward is a sign of poor weather, and they are right, as it indicates a low pressure system.
You don't have to look to the skies for weather proverbs. Many regions of the world have some variation of this proverb: “seagull, seagull, sit on the sand; when you're on shore, poor weather's at hand.” As biologists are well aware, many birds roost in advance of a storm to ensure that they stay safe and sound, so when seagulls hunch on the shore, it can be a sign of an incoming storm.