Which weather signs can you trust?

At noon on November 8, 1870, a weather bulletin went out from Chicago to observers along the Great Lakes. Its first piece of news concerned the wind in Cheyenne the day before. Cheyenne is a long way from a lake freighter. Anyone standing on a Chicago dock could have offered a more recent, considerably wetter view. What could yesterday’s wind tell a sailor that the water in front of him couldn’t?

Increase Lapham, the Milwaukee scientist responsible for the warning, had spent years on that question. You can try your hand at his forecast below. First it’s worth stepping outside with some older signs, because each has much the same puzzle tucked inside it. You see something here. How far from here does it entitle you to look?

The red sky needs a compass

Picture a red evening sky. Where did you put the color? Most of us would aim a camera straight at the setting sun. For the sailor with an interest in tomorrow, it pays to turn around. If low sunlight from the west is catching the undersides of clouds to the east, there must be a gap in the west for the light to get through. In a place where weather systems generally travel from west to east, that open horizon is on the side tomorrow’s weather often comes from.

This is the geography inside “red sky at night, sailor’s delight.” The long path that sunlight takes through the atmosphere near sunset scatters much of the blue light out of the direct beam. The remaining warm light can paint a bank of clouds handsomely. The color tells you something about the light. The position of the illuminated cloud tells you something about where there’s still cloud and where a clearing may be coming from.

Run the scene at dawn. Low light now comes from the east. If the clouds catching it lie to your west, the cloud bank is on the incoming side of a west-to-east pattern. That gives the morning warning a reasonable foundation in much of the middle latitudes. A photograph cropped close around a red sun throws away almost everything you need to use the saying. You’d want the time of day, the horizon and some idea of which way the weather was traveling. A compass can recover only one of the three.

Two forecasts from the same porch

Now suppose a pine cone kept on the porch has drawn its scales closer overnight. There’s a good physical reason for the change. The tissues in a scale take up moisture and bend. Cool the air overnight and its relative humidity can rise, even if no extra water vapor has come into the neighborhood. A clear night can set the cone moving all by itself. The few inches around a sheltered porch rail are quite a small territory to ask about an afternoon shower.

From the very same porch you might see something at the opposite extreme. A pale ring stands well out from the Moon, so far away from its bright center that there’s room for stars in between. Moonlight passing through ice crystals can make that broad halo. Thin, high cloud sometimes spreads ahead of a front, before the thicker clouds that bring rain or snow. Here the sign really can give advance notice. The catch is that you’ve sampled one patch of high cloud. The ring gives you neither the front’s track nor the time it might reach your roof.

There’s even a sign that can fool you after the rain has begun to fall. Watch a distant cloud with gray streaks trailing from its underside. Follow a streak down. Sometimes it dwindles away and leaves daylight between its tip and the ground. Meteorologists call this virga. Precipitation has fallen from the cloud into air dry enough for it to disappear on the way down. Someone watching the streak from a hillside and someone standing below with a dry sleeve can both give an honest account of the weather.

What did the next town see?

Lapham had a way of getting beyond the view from the dock. Take a look at his map of a storm that crossed the country in March 1859. Working afterward from collected observations, he traced its progress across several days. He put his argument right in the map’s title. People on the lakes, he reckoned, could have known about that storm a whole day before it reached them. The inland observers had already done the looking. The news needed to travel faster than the weather.

By November 1870 the U.S. Army Signal Service could move observations by telegraph. On the eighth of the month Lapham got responsibility for warnings on the Great Lakes. He issued one that same day. Below are the observations in the Chicago bulletin, with the forecast held back. Imagine you have room for one sentence to send east. What would you tell the people on the lakes to expect?

The place names do much of the work. A person in Milwaukee could feel the wind and read a falling barometer. With news from Omaha, the Milwaukee wind acquired a history. With pressure falling as far east as Rochester, New York, it had company well beyond Lake Michigan. The reports gave Lapham something no particularly observant sailor could supply alone. He could put an observer yesterday and an observer today side by side, hundreds of miles apart, while the information could still do somebody some good.

Give a saying a fair trial

There’s a small version of Lapham’s experiment you can do with a friend who lives west of you. The next time a line of clouds moves through their town, ask them to send the time and the view. Before it reaches you, write down what you expect at your place and by when. A picture of dark clouds in the west is a fine observation. “Rain in my yard before supper” is a forecast you can actually check. If the clouds slide north and your yard stays dry, keep both entries. You may have read the approaching cloud perfectly well and missed its track.

And if all you have is one window, keep the horizon in the picture. The next time the clouds turn pink at sunset, turn your back on the sun for a moment. Over there in the east you might catch the last light on the underside of a cloud that’s already on its way out.