In April of this year 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 5PM on 4/20 it fell to 9%. Since 2000 only one other day has had a humidity level that was lower - March 31, 2007, which reported 6% humidity late in the afternoon. This April's streak of arid days was the inspiration for this post, which looks at hourly and monthly relative humidity in New York 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 was guessing 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 25 days with at least one hour of humidity at 15% or lower. All were in March or April. The lowest humidity to occur in the other months has been 16%, and its happened on five dates: Feb. 26, 2014; May 3 and May 13 in 2007, Sept. 11, 2001 (aka 9-11) and Nov. 12, 2010.
- 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.