Updated: 7d

As a cold front moves into the Northeastern United States from the north and west on Wednesday, it will slice into humid, hot air with high temperatures near 90° and dewpoint temperatures near 70°.  The heat and humidity will result in a large amount of energy available for thunderstorm growth, and clusters of thunderstorms are likely to develop during the middle and late afternoon in Upstate New York, Northern and Western New England.  By evening, these storms move through Central and some of Southern New England.  Given available energy, and a modest wind flow aloft, damaging wind gusts are possible within these storms, and the clusters of thunderstorms should organize into a squall line Wednesday evening, spanning across much of New England, New York and Pennsylvania.  The maps below show forecast high temperatures Wednesday (left) and forecast late-day areas of precipitation (right):


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Updated: 7d

During the warm weather season, one of the most commonly asked questions - and line of complaints - I receive pertains to dewpoint and relative humidity.  Over the last several years, I've come to use both rather sparingly on-air on NECN, largely because use of one always brings complaints and questions why I don't show the other instead.  Our midweek heat and "humidity" in the Northeast U.S., slated to peak Wednesday, brings a wonderful opporutunity to illustrate both, and hopefully explain both in a way that will be helpful, though realistically, I expect that most of the folks who complain about use of the terms won't read this entire post (I hope some do!), which means it'll be incumbant upon those who do finish reading to share the knowledge over lemonade at pool parties this summer, when your friends and family complain, LOL.

In simplest terms:

  • Dewpoint:  Measures the amount of moisture in the air (a dewpoint of 70° means way more water vapor in the air than a dewpoint of 30°)
  • Relative Humidity:  Indicates how close to saturation air is (100% relative humidity means totally saturated air - a cloud...or fog, while 50% relative humidity represents the air still has a long way to go to be totally saturated)

One of the common numbers you'll see a meteorologist refer to in weather broadcasts and weather discussions, is "dewpoint."  As meteorology students at Cornell University, we were always taught, "dewpoint is a fictitious number, in that it cannot be directly measured."  With the aid of electronic measuring devices, your home weather station now can give instantaneous dewpoint readings, though it's true that the dewpoint is not a directly measurable parameter - the technical definition is the temperature at which air becomes saturated if cooled at a constant pressure.

That's not as deep thinking as it sounds - basically, what that definition is saying is, if you cool the air all the way down to the dewpoint, you'll get 100% relative humidity.  So, if the air temperature is 95° F and the dewpoint is 70° F, that will feel really "humid," in everyday terms, because a 70° dewpoint indicates a ton of moisture in the air...but the relative humidity would only be 44%.  If you cool the air down to that 70° dewpoint, your relative humidity rises to 100%, and fog develops.  On the flip side, in winter, the dewpoint may be only 30°...indicating much less moisture in the air...but if the air temperature is also 30°, it surely won't feel "humid" by everyday terms, but your relative humidity with a matching temperature and dewpoint is 100% - you'll have fog that develops.

To ensure we stay focused on real-world application, consider this:

  • In a high dewpoint air (lots of water vapor), it feels "humid" as we use the term in everyday life.
  • In a high relative humidity air (temperature and dewpoint almost the same, regardless of how hot or cold), the air is "moist," but it doesn't necessarily feel humid if the dewpoint isn't high.

So...when I'm asked, "why didn't you show the relative humidity today? (or dewpoint)" the answer is because it's not always relevant.  High relative humidity near 100% indicates fog production, so that's relevant. Low relative humidity indicates rapid drying of clothes on the line, and rapid dehydration of plants, animals and people, so that's important, too.  Anything in between...doesn't really have much impact.  Vice versa for dewpoint - a really high dewpoint means you'll feel the air as "humid" (dewpoints in the 60s starts to feel muggy, above 70 is oppressive), and a really low dewpoint means the air is exceptionally lacking in water vapor.  Again, in between, the applicability to your daily life is rather limited.

As a final example to drive the point home, consider the following forecast maps for Wednesday.  The first map is the RELATIVE HUMIDITY Wednesday just before dawn - the green/blue patches indicate relative humidity near 100%, meaning the air is moist and fog will likely develop:


In the next map, all I did was advance the forecast time to Wednesday afternoon.  What's changed?  The relative humidity has dropped substantially, to between 45-55%.  Why?  Did the total amount of moisture in the air drop?  Nope - the dewpoint stays the same (around 70°), which means it still feels humid to us Wednesday afternoon, but the relative humidity drops because the temperature goes up...farther away from the dewpoint, which means less "moist" air and farther away from making fog.


My hope is that this helps to explain a bit about both the relationship, and the difference, between relative humidity and dewpoint, but if questions remain, I encourage you to pose them and I'll do my best to update the post with some answers.

In the meantime, if you'd like to play around with a dewpoint and relative humidity calculator, here's a great one by clicking here.


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Updated: 7d

A light southwest wind Tuesday will continue to slowly blow increasingly moist air into New England through the overnight Tuesday night.  Measured by the dewpoint temperature, the amount of moisture in the air directly contributes to how "humid" the air feels, and with dewpoints forecast to be in the 60s Tuesday night, a mild night of rather humid conditions is likely.  While dewpoint measures the amount of moisture in the air, relative humidity indicates how close to saturation the air is - that is, a relative humidity of 100% means the air is totally saturated (fog), while a relative humidity of 50% indicates the air is halfway to saturation.  In the image below, I've plotted relative humidity for the predawn Wednesday timeframe - note the relative humidity values of near 100% in Central/Eastern Maine (indicated by the blue/green area), and also in Pennsylvania...these are areas where some dense fog is likely to develop Tuesday night, hindering travel into Wednesday morning, and likely leading to a low deck of gray clouds early Wednesday morning.  Patchy fog is possible in any of the green shaded areas, and fog is less likely in the lower relative humidity areas in yellow.


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Updated: 7d

Your New England 10-day Forecast shows building heat, then a great start to the weekend:


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Updated: 23d
Maine Landmark took a hard hit & needs help -
See story below

 Manchester New Hampshire reported High Temperatures of 90 on July 1, 91 July 2, and 94 July 3rd - The first heat wave of 2014. At the same time a weather front from Canada inched across New England July 2-5 wreaking havoc with severe thunderstorms knocking down thousand's of trees in New England. That front then merge with Category 1 Hurricane Arthur on the night of July 4th causing major flooding from southeastern Massachusetts to eastern Maine and Eastern Canada. The wind also ripped dozens of boasts from there mooring on Nantucket and Downeast Maine where winds gusted 60-70 mph on the backside of hybrid Arthur (Arthur transitioned from warm core tropical to a cold core baroclinic Nor'easter very quickly Friday Night July 4th. July 5th goes into the books as the coldest and wettest July 5th on record in Eastern Maine. The news was even worse in Canada where Post Tropical Storm Arthur created wind gusts past 85 mph and knocking out electricity to nearly a quarter million customers in New Brunswick and Nova Scotia.

I am compelled to tell this story after reading an Email in our NECN viewer feedback inbox. It's a sad story of damage to an historic landmark... now in need of our help. Here is the unedited message.

From: Tim Harrison [Timh@lhdigest.com]
Sent: Sunday, July 06, 2014 8:21 PM
To: Tim Harrison
Subject: Arthur Sinks Lighthouse Boat Press Release 2 photos attached FMI call 207-259-3638 or 207-259-3833

Arthur Sinks Lighthouse Boat and Damages Island
This past Saturday, Little River Lighthouse in Cutler, Maine was hit especially hard by Hurricane Arthur as high winds literally flipped over the Nautico catamaran boat while it was tied up at the dock at the lighthouse and left it submerged in the water upside down. Additionally the high winds toppled many trees, which fell over the boardwalk that leads from the boat house, up and across the island, to the lighthouse.  One gigantic tree fell next the boat house said Tim Harrison, founder of the Friends of Little River Lighthouse, the nonprofit that manages the historic lighthouse. He said, “It just missed the boathouse. If it had fallen a few feet slightly the other way it would have crushed the boathouse.”
Harrison said the loss of the boat, which had been donated several years ago, is a devastating blow to the group, which had planned to use the boat to transport visitors to and from the island for its first Open House of season, which will be held this Saturday, July 12 from 10am to 2pm.  Harrison also said it was the best of the three small boats that the lighthouse owns and was the best equipped vessel to use in transporting guests and supplies to and from the island lighthouse.
As soon as it was realized that the freak weather event had flipped the boat over, the local lobstermen immediately launched a salvage operation in an attempt to save the boat. However, once on dry land it was realized that the boat and its 50hp Mercury outboard engine were damaged beyond repair. Harrison said that for a while it looked like the whole town turned out to help or offer moral support.
Harrison said that he’s not sure what the insurance company will cover for the loss of the boat, however he did say “I doubt that we will get anything near what the boat and engine is really worth. The loss of this boat is devastating to our entire season and the ongoing restoration of the lighthouse.”
Harrison said it could take anywhere from several days to several weeks to cut up the trees that fell on the island. “It all depends on how many volunteers are willing to work with chain saws and weather conditions.” One large tree fell on the gravesite of two shipwrecked sailors whose bodies washed up on the island in 1898.  The group has posted a number of photos on its Facebook page of the boat recovery and the tree damage at the lighthouse.

Anyone wishing to make a donation to the lighthouse can do so on-line at www.LittleRiverLight.org or by mail to Friends of Little River Lighthouse, P.O. Box 671, East Machias, ME 04630. You can also call them at 207-259-3833.


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