I was getting my hair done this afternoon, and the hairdresser and another customer were discussing the weather. With the drought we’ve been having, the lawns are turning brown and no one has needed to mow their lawn for weeks. (While I know we need rain, personally I don’t mind at all not having to mow.) The sky was overcast, and they were discussing whether we would get rain today.
The customer said she thought the forecast said 20% chance of rain – meaning 80% chance of no rain. The hairdresser said that there had been dew on the grass this morning, which meant there would be no rain today. I had never heard of predicting the rain based on dew, and I asked her about it. She said it was an old wives tale, but it seemed to be accurate more often than not.
I know there is a lot of lore about predicting the weather (among many other topics) that is very useful, though I’ve never learned any of it myself. (Not being a farmer, and working indoors, it has never seemed all that important to me to know ahead of time what the weather will be.) I tried to think what scientific principles might explain the success of this particular bit of lore at predicting the weather.
I couldn’t think of anything.If anything, it seemed that dew should be an indicator that there was a lot of moisture in the atmosphere, which would lead to rain before long, though not necessarily the same day. Why, I wondered, would a lack of dew be an indicator that it would rain that day?
So naturally I made up my mind to check it out on the internet later – and post the explanation on my blog. The answer I found makes perfect sense – but it isn’t about how much moisture is in the air, but rather the temperature during the night.
On a clear night the ground cools, radiating its heat away into space. When the ground gets cool enough, dew forms, like beads of condensation on a can of cold soda.
If the sky is cloudy at night, however, the Earth’s surface doesn’t cool as much. Some of the heat radiates into space, but much of it bounces off the cloud layer and goes back into the ground. If there are lots of clouds, the ground won’t get cool enough to form dew. The saying works because, chances are, all those nighttime clouds might also cause a rainstorm during the coming day.
As I said, it makes perfect sense. But it’s not a perfect predictor. By the time I left the hairdresser, it was raining. The cloud cover must have been somewhat scanty, however, because by the time I got home the rain had stopped and the sun was shining. So I suppose the dew was partly right – just like most weather forecasters.