Why You Can Trust “Red Sky at Night” and Other Old Sayings

“Red sky at night, sailor’s delight”…“When swallows fly high, the weather will be dry”…“When dew is on the grass, rain will never come to pass.”

Before there were professional ­meteorologists, people depended on signs such as the color of the sky and the behavior of animals to forecast the weather. That folk wisdom, which often was condensed into catchy, rhyming sayings, might seem dated in this modern world of smartphone weather apps, but some of it actually has merit.

Here is the science behind seven weather-related folk sayings…

When dew is on the grass, rain will never come to pass. Dew forms when overnight temperatures drop to the “dew point”—that’s the temperature at which water vapor in the air condenses into droplets. This is most likely to ­occur on cool nights when a high-pressure system is in the area. High-pressure systems tend to bring clear, dry weather, so morning dew does indeed suggest that rain is unlikely.

It’s too cold to snow. Yes, it truly can get too cold to snow…but only if it is phenomenally cold. When temperatures drop below around 30°F, snowfall ­becomes virtually impossible.

But there is a degree of truth to this folk wisdom even at less extreme temperatures. Heavy snow is unlikely, though not impossible, when ­temperatures dip below 0°F—most major snowfalls occur when temperatures are in the 20s or 30s. Why? Snow requires not just freezing temperatures but also moisture in the air, and very cold air tends to be very dry air.

Red sky at night, sailor’s delight…red sky in morning, sailor take warning. This famous weather proverb can be traced all the way back to the Bible—the book of Matthew contains similar guidance. And while a red sky is not a 100% reliable indicator of upcoming weather, science generally does back this saying.

When the sky appears red, it often is because sunlight is passing through air thick with dust particles. Those particles scatter much of the visible light spectrum—but red light has a long wavelength that is relatively unlikely to scatter, allowing it to reach our eyes. High concentrations of atmospheric dust particles tend to occur during high-pressure systems, and as noted above, high-pressure systems usually bring nice, clear weather.

In the US (as well as in Europe and certain other parts of the world), weather systems tend to move from west to east, so if there is a red sky at night, when the sun is setting in the west, that could mean there’s a high-pressure system to the west that will soon arrive, bringing nice weather with it.

If the red sky is in the morning when the sun is in the east, however, a high-pressure system might recently have passed, suggesting that a low-­pressure system could soon arrive, bringing cooler, rainy weather.

This is not guaranteed—for one thing, a red sky could be the result of air pollution rather than a high-pressure system—but as a rule of thumb, it is reasonably effective.

When swallows fly high, the weather will be dry. Several folk sayings claim that birds of various species fly high when good weather is on the way and that they fly low when bad weather will soon arrive. Some researchers have argued that these birds are adjusting their flight patterns to maximize flying efficiency in response to changes in barometric pressure—studies have determined that some birds can sense these changes.

But the real reason is likely less complicated—low-flying birds are probably looking for a meal. When rain is on the way, the air tends to be heavy with moisture. That moist air sticks to insects’ wings, making them fly lower or discouraging them from flying at all. Birds such as swallows that dine on flying bugs probably fly lower because their food is lower.

Doors stick before a rain. Wood tends to expand slightly when humidity rises, and high humidity often does presage rain. Thus a wooden door or drawer that opens easily most of the time might be more likely to stick shortly before it rains. A modern convenience has cost this piece of folk wisdom much of its usefulness, however. Most Americans now have air-conditioning, which removes moisture from the air, reducing the odds that wooden doors and ­drawers will expand.

A ring around the sun or moon means rain or snow is coming soon. The most likely cause of a ring or halo around the sun or moon is that the light of the sun (or the light of the sun reflecting off the moon) is passing through ice crystals in cirrus clouds—wispy clouds very high in the sky. Cirrus clouds can be an early indication of an approaching low-pressure system, which might indeed bring precipitation. This folk wisdom is far from perfect, however. For one thing, cirrus clouds do not ­always signal approaching low-pressure systems.

The cricket is a thermometer. Researchers have confirmed that cricket chirps really do increase in a predictable way as temperatures increase. If there’s a cricket outside your window, count the number of chirps in 15 seconds (some people recommend 14 seconds) and then add 40 to estimate the current temperature in degrees Fahrenheit.

However, this won’t work if temperatures fall below 55°F…if you cannot single out the chirps of a single cricket…or if you are listing to the wrong type of cricket. This formula is based on the chirps of the field cricket, which is the most common cricket in the US but not the only one. Snowy tree crickets chirp at a faster rate…katydids more slowly.

Weather Folklore to Ignore

Not all traditional weather wisdom is rooted in truth.

Do not believe that…

Lightning never strikes twice in the same place. Lightning can—and does—strike the same spot repeatedly. Some tall buildings and trees get struck many times each year, for example. Standing where lightning has already hit definitely will not keep you safe in a lightning storm.

Put on your coat (or scarf…or hat), or you’ll catch a cold. Whenever temperatures drop, it seems that someone warns us that we’ll get sick if we go outside without sufficient outerwear. It is not true—germs make people sick, not temperatures. Colds and flu are more common in winter mainly because that’s when people spend the most time cooped up together in tight spaces, increasing the odds that they will transmit germs to one another. This does not mean that it’s a good idea to go outside in cold weather without proper apparel, of course—doing so could lead to ­hypothermia or frostbite.

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