Many of you may have noticed a beautiful rainbow or two this morning.  Being in a dark, windowless studio in a basement downtown (ironic for a weather center, I know) I got to see a lot of pictures of rainbows on twitter.  Here are just a few from @beantwnbrawler in Brookline and @Emjay1028 in Boston…


Why this morning?!?  Good question!  In order to see a rainbow, the sun needs to be low in the sky, the sunlight needs to pass through a raindrop, and you need to be in between the sun and the raindrop.  In other words, from your vantage point the sun and the rain must be on opposite sides of the sky in order to see the rainbow.  With the sun rising in the east at 7:14 am this morning and some raindrops moving in from the west at the same time, it was the perfect set up!

Drawing show the sun on the left and a rainbow on the right (under some rainy clouds) and a person viewing the rainbow in the middle.

Drawing shows the path of a light beam as it enters a spherical drop of water and reflects off the inside. It is separated into all its colors as it exits the droplet.

(graphics courtesy of NASA)

How does it work?!?  As sunlight enters a raindrop, the light slows down because water is more dense than air.  That causes the light to bend, or refract.  Different wavelengths of light bend more than others.  Consequently, the ray of light spreads out and separates into its components.  Our brains interpret the shorter wavelengths of visible light as purple and blue and the longer wavelengths as orange and red.   The colors of light then bounce off of the opposite side of the raindrop and back toward your eye.  Side note…if you see a double rainbow, the color patters will always be reversed.  Double rainbows are mirror images of each other with one going from purple to red and the other going from red to purple.

So by now you’ve figured out that if you see a rainbow in the morning that means there is rain approaching.  Enough of that, will it rain all day?!?  No!  Today should be a slight improvement over yesterday.  It won’t be bright and sunny but it won’t be a washout either.  You might notice a few sprinkles from time to time.  More so if you are in VT or NH.  In between there will be some clouds and some peeks of sun.  Temps will be a few degrees warmer than yesterday but more importantly it will be a lot less windy so it should feel a bit more comfortable.

Temperatures hold steady this evening.  So whatever you are comfortable wearing as you head out for dinner you will be comfortable wearing coming home later this Saturday evening.

There is another chance of rain Sunday afternoon and evening, but mainly south of the Pike.  The bulk of the day will be dry.  If I had to sum up the weekend…seasonable temps, not a lot of sun, but not a lot of rain either.

Halloween will be clear with little to no chance of rain.  So your costume doesn’t need to be waterproof.  However, it will be the coolest day of the week with high temps only in the upper 40s to low 50s.  That means temps will be in the mid 40s shortly after sunset.  So your costume does need to keep you warm.

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