Days With Desert-Like Humidty In The 21st Century
In April of 2016 New York experienced eleven days in a row with clear or sunny skies (4/12-22). In addition to this abundance of blue skies, the streak was also characterized by extremely low humidity. While typical late afternoon humidity levels in April are generally between 45% and 50%, during these eleven days it was mostly between 15% and 20%. And at 5 PM on 4/20 it fell to 9%. Since 2000, only one other day before 4/20/16 had a humidity level that was lower - March 31, 2007, which reported 6% humidity late in the afternoon (and on 4/6/21, the humidity dropped to 7%). It was April 2016's streak of arid days was the inspiration for this post, which looks at hourly and monthly relative humidity reported in Central Park since 2000, with a focus on extremely low levels, which for the purposes of this analysis is 25% or lower.
- Since 2000 5% of the days have had at least one hour with a relative humidity of 25% or lower (about 19 days/year). The number of such days ranged from six in 2000 to 32 in 2010. When I began this project I guessed that January and February might have the most days with the lowest humidity, but it turns out that two-thirds of the these days occurred in March or April.
- February, March and April have the lowest humidity, while September has the highest (followed closely by June).
- Not only were the two years with the lowest average humidity consecutive (2015, 2014), so were the years with highest average humidity (2003, 2004). And while the year with lowest humidity had significantly lower than average precipitation, the second driest was well above average. In fact, the total precipitation for the year with the highest humidity was very similar to this second driest year (58.56" vs. 57.79").
- During a typical day, the lowest and highest humidity levels occur about nine hours apart, with the highest occurring around daybreak (averaging 72%), and lowest levels during mid-afternoon, averaging in the mid-50s.
- There have been close to 50 days with at least one hour of humidity at 15% or lower (thru the end of April 2022). All were in March or April, until 2018 when the humidity fell below 15% twice in May and once in November, and then again in Dec. 2021. The lowest humidity to occur in the other months has been 16%, and it's happened on four dates: Feb. 26, 2014; May 3 and May 13 in 2007, and Sept. 11, 2001.
- The months with the least and most precipitation don't always have corresponding low and high average humidity. For example, the most humid October and February had only 1.18" and 1.66" of precipitation, respectively. The months with the least humidity don't have outliers quite that extreme, but the driest June and January had more than four inches of precipitation, which is close to average.
- In this century, months in the years 2000-2004 tended to have the highest average humidity while those with the lowest were clustered in the years since 2010.
Thanks for doing this study, very interesting results. I wasn't really all that surprised at April being the driest month, since wide discrepancies between morning low temperatures and afternoon high temperatures, which April is noted for, seem to go hand in hand with low humidity. But I had not idea how rare a low humidity reading in the last 6 months of the calendar year was. I would've never guessed that. Thanks again.
Posted by: Ken K. in NJ | 05/20/2016 at 03:28 PM
Hi Ken, thanks for suggesting the topic in the first place! I'm trying to discover what it is about air masses in March and April that make days with super-low humidity more prevalent. Those two months certainly get their share of storms (rain and snow). Does it have something to do with air temperature? Or how the air descends and ascends that is different from the other months? Any thoughts?
Later in the year I may add to this post with additional years to see if humidity patterns in the 20th century are similar - and how many other instances of humidity below 10% there have been.
Posted by: Rob | 05/20/2016 at 05:40 PM
Hi Rob. I've wondered the same thing as you. I really don't have any theories, other than to guess that it has also something to do with the fact that Late Match thru April is also generally the peak period for tornadoes in the Midwest. Too much of a coincidence to not have a relationship.
I don't know any actual Meteorologists who might be able to answer the question. Maybe you could share your data with one of the TV Meteorologists to get their opinion. Lee Goldberg, Janice Huff and Bill Evans all seem to me to be quite data-oriented, for example. Just a thought.
Anyway, thanks again for a very interesting study.
Posted by: Ken K. in NJ | 05/21/2016 at 05:31 PM
I have come here after a pleasant period in the early spring of 2021 in which the first half of March and the first full week of April had dry weather, crystal blue skies, and exceptionally low humidity (which fell to 7% on 4/6). Days with low humidity below 25% most often occur in afternoons between late February and early May. On clear days during this time, the sun is gradually climbing up higher in the sky and shining very strong while the ground is still cold from the winter and has a low dew point. (Let me know if I have answered the author and Ken K’s speculations.)
If you observe closely, you will often see red flag warnings issued during periods with very low humidity. The low humidity is often accompanied with gusty winds at least 15-30 mph. These two factors are favorable enough to start a ground fire. The most notable red flag warnings that come to mind are those in the first half of April 2012, during a very dry period there. On the 13th of that month, I think, a brush fire broke out in Central Park. That fire was observed by spectators at Yankee Stadium in the Bronx. It took the whole afternoon to extinguish that fire.
Posted by: Henry | 06/16/2021 at 11:51 AM