| |
Crystalize the Forecast
Listen in RealAudio
Email your weather question
Relative humidity, or RH, is a measure of the water in the air. At one hundred percent humidity the air is completely saturated and fog forms. But, at a lower RH this moisture is invisible.
Hi, I'm Bryan Yeaton and this is the Weather Notebook.
One of the most reliable signs of rain is a ring around the moon. If you see a ring around the moon, or a lunar halo as it's sometimes called, then you can pretty much count on a steady rain falling within the next 24 to 36 hours.
What's needed to produce a ring around the moon is a thin layer of clouds, up about six miles high. Up at those heights, it is very cold and the clouds are made of crystals of ice instead of droplets of water like most clouds. The seemingly infinite number of tiny ice crystals that make up the clouds each act like a prism by bending moonlight as it travels toward you and the earth. Because the ice crystals are all the same structure, the light bends the same amount when it passes through them, making a circle around the sun or the moon, a circle of light.
OK now, the high clouds that produce this effect are riding strong upper level winds that blow from west to east. The clouds come from an approaching storm which is well to your west, and will be dropping rain or snow on you in 24 to 36 hours, in most cases.
A halo can also form around the sun, since we don't spend a lot of time looking at the sun, seeing a ring around it is less common than seeing the ring around the moon. So, either way, around the sun or moon, if the light does huddle, you'll soon be stepping, in a puddle.
The Weather Notebook is produced by the Mount Washington Observatory and is funded by Subaru and the National Science Foundation. Do you have a question or comment. Write to us at questions@weathernotebook.org and thanks today to executive engineer, Sean Doucette.
|
