Sunday November 24, 2013 | 03:35 PM
Posted by David Epstein

What a cold and blustery day it has been.   In case this is your first winter in Maine, it’s not quite normal to have it this cold this early.  Although there are not any records being set around these parts, it’s going to be a very cold night ahead with wind chill readings below zero at times.  When you awake tomorrow morning temperatures will range from the lower teens right at the water to near zero well inland up across the mountains. 

You can get a sense of how strong the cold is by looking at temperatures across the country.  The cold air has penetrated into Dallas, Texas and the panhandle of Florida.  This is more typical of late December.  Here in southern Maine, the high temperature today was reached at 12:01 this morning as no town surpassed the readings from back at midnight.  The air is also exceedingly dry.  When you get air coming in from the arctic regions it contains no moisture.   As you bring that air inside your home and warm it up, you drop the relative humidity to under 10%.

Humidity Lesson

Relative humidity tells us how close the air, at any given temperature, is to being full of water or saturated.  Today temperatures are in the 20s and there is very low amounts of moisture are present. In this case, the relative humidity outside is about 40%.  As you take that exact same amount of moisture and bring it into your home, then warm the air some 40 degrees the relative humidity drops profusely.  If you don’t have a humidifier to add moisture back into the air, then it’s going to feel very dry the next two days.

As air cools the level it becomes saturated falls.  In other words, cold air can contain or “hold” less moisture.  When I teach relative humidity to my students I illustrate the point by asking them to think of a swimming pool as warm air and a small cup as cold air.  When the swimming pool is full there is a lot of water, a full pool is saturated.  Relative humidity tells you how full the air is at a given temperature.  So, the pool at 50% full or relative humidity still has a lot of water, that’s warm air.  It can feel very humid even if the relative humidity is 50%. 

The cup can be 100% full or saturated, but not have very much water.  That’s why even if the relative humidity is 100% on a cold day, you don’t feel uncomfortable, and there just isn’t enough moisture in the air.

The dew point is another term you often here.   Dew point is actually a temperature and represents the temperature the air would need to be in order to be saturated.  If the temperature is very high and the dew point is very low, the spread is very large and the relative humidity is low.   The term dew point makes sense because when the temperature and dew point are the same, dew forms.


On a clear morning with a temperature of 40F and a dew point the same, the grass would be wet and the relative humidity would be 100%.     On an afternoon with the dew point at 68F and the temperature at 86F the relative humidity would be a seemingly comfortable 55%, but you would feel very uncomfortable from all the moisture in the air.  Conversely, if you were in Phoenix, Arizona  and the temperature was a whopping 100F and the dew point 20F, the relative humidity would be only about 5%.

Upcoming Storm

Our cold dry air is going to stick around for a couple of days before warmer and more moist air returns to the region.  As a storm moves up the coastline later Tuesday and Wednesday a period of rain, some of it heavy will cross the area.   Right now it appears enough warm air will move into the region so all the precipitation will be liquid, not frozen.  This is great news for folks traveling on Tuesday night and Wednesday.   As the storm ends later Wednesday enough cold air could filter back into the area to give the mountains some accumulating snow, but it’s not going to be a big storm.

This is going to be the first major rain we have had in a while.  I expect most areas to receive close to an inch of water.  Some of us could even see closer to two inches of rain before this is over later Wednesday.  This is great news as the ground really needs to be saturated before we go into winter.  As cold as it will be Tuesday, you might want to be sure your gutters are not clogged before the storm begins later in the day and overnight.


Thanksgiving looks sunny, but cold with highs just around freezing.  

Around the House This Week I know some of you are already thinking about Christmas trees. Did you ever consider there are great trees you can add to the landscape that make great Christmas trees? Even if you don't decorate the tree, it will still be a wonderful addition in the landscape.

I'll be updating the forecast for the week on Twitter at @growingwisdom Please follow me there. Feel free to comment or ask questions too.

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About the Author

David Epstein has been a meteorologist for more than 25 years. He spent 16 years in Boston and currently freelances at WGME13 in Portland.

In 2006, David founded, a business producing educational and marketing videos for the green industry. He currently is a professor at Framingham State College, teaches Jan Plan at Colby College and owns Bloomscapes Inc., a landscape design business.

David authored "Gardens Of New England" and his work has been published in Grolier's Science Annual for 10 years. He lives in South Natick, Mass., and has a summer home in Harpswell.

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