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):
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:
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:
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.
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.