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I live in New Jersey. Now that summer is coming on us, temperatures and outdoor relative humidity are both increasing. I've noticed the few years that I've lived here that there seems to be a rough correlation with a day's high temperature and the relative humidity. I've always found this odd, since for a fixed amount of moisture in the air, relative humidity will go down with increasing air temperature. However, the hottest days (as the thermometer reads, not just as I feel) also seem to have the highest relative humidity.

I lived in Hong Kong for two years and the same thing seemed to happen there. The hottest days also had the highest humidity (and were usually accompanied with the formation of a typhoon).

What makes relative humidity go up with hotter days?

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    $\begingroup$ Perhaps because both New Jersey and Hong Kong are areas which have a lot of water nearby, so a lot of it can evaporate on hot days. If you lived in Phoenix or Las Vegas, you would be likely to see the relative humidity decrease with increasing temperature. $\endgroup$
    – jamesqf
    Jun 12, 2017 at 19:12
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    $\begingroup$ It's more likely that you simply don't pay attention to relative humidity when temperatures are moderate or cooler because relative humidity has little bearing on comfort in those temperature ranges. Relative humidity has a significant bearing on comfort when temperatures exceed 80 F (27 C), and a huge bearing on comfort when temperatures exceed 90 F (32 C), so that's when you pay particular attention to relative humidity. As DavePhD's answer shows, there isn't all that much variation in NJ in relative humidity over the course of a year. $\endgroup$
    – David Hammen
    Jun 12, 2017 at 22:18
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    $\begingroup$ Source for the claim of relative humidity being correlated to temprature in the way described, because it sounds like simple attention bias. $\endgroup$
    – John
    Jun 13, 2017 at 4:53
  • $\begingroup$ @John absolutely ! $\endgroup$
    – user1066
    Jun 13, 2017 at 6:50
  • $\begingroup$ I'm still working on this (github.com/barrycarter/bcapps/blob/master/STACK/…), but once the temperature exceeds 21C, temperature and humidity are negatively correlated for weather station KTTN (Trenton/Mercer County Airport), the nearest long-term reliable weather station near Lawrenceville. There is, however, a fairly strong correlation between temperature and dewpoint. $\endgroup$
    – user967
    Jun 14, 2017 at 2:14

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In coastal New Jersey, relative humidity is maximal in the early fall when the ocean temperature is warmest, and minimal in early spring when the ocean is coldest. However, the monthly variation is rather small:

Atlantic City, New Jersey:

70 January
69 February
67 March
66 April
71 May
73 June
74 July
76 August
76 September
75 October
73 November
71 December
72 Annual

Newark, New Jersey

65 January
63 February
60 March
58 April
62 May
63 June
63 July
66 August
68 September
66 October
67 November
67 December
64 Annual

source: https://www.currentresults.com/Weather/New-Jersey/humidity-by-month.php

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  • $\begingroup$ any details for Lawrenceville NJ? $\endgroup$
    – NeutronStar
    Jun 12, 2017 at 18:44
  • $\begingroup$ @Joshua nothing $\endgroup$
    – DavePhD
    Jun 13, 2017 at 10:37
  • $\begingroup$ If someone REALLY wants to research the issue, opendata.stackexchange.com/questions/10154/… has pointers to lots of data. $\endgroup$
    – user967
    Jun 13, 2017 at 14:19
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I do not have the data to back this, I present this more as possible contributing points that you may be able to verify with data. DavePhD presents an excellent and backed answer for local sea effects. For non-local contribution, I would start with the point that warm air has the ability more total water than cold, and that if the air holds the same amount of water, but you raise its temperature, by definition, relative humidity drops. Also, if you lower the temp, relative humidity rises, and the tendency to precipitate out that water increases. Those are well know, just restating as a base.

As a coastal location, NJ will tend to have its temperature somewhat moderated by the ocean, that is be a bit warmer in winter than it otherwise would be, and cooler in summer. Not a lot, but a few degrees makes a difference in relative humidity. Winter weather patterns will have a high percentage of the air mass heading at NJ coming across the continent, and a good share of the systems being arctic. We thing of these as being wet systems, but really, they are cold systems and result in snow because they cannot hold as much moisture. When they get to the coast, the temperature may be low as compared to summer conditions, but it may also be a bit warmer than the conditions to the West. That rise of a couple degrees equals a drop in relative humidity for that air mass much like heating that air in your house creates very dry conditions in the winter.

In the summer though, as temps go up, there will also be changes in weather patterns, with a higher portion of you air masses coming from the gulf or even straight up the coast. These will be wetter air masses. In addition, now you are getting a cooling effect from the ocean. Yes, temps are higher, but still a few degrees lower than they likely would have been without the ocean effect, and cooler than what they likely are to your West. Drop the temp of the air mass a couple degrees and the relative humidity rises.

One more factor in that line, when the air mass is coming more directly from the West, or from the NW, it crosses a line of mountains before it gets to NJ, which will tend to cause more of the moisture to precipitate out. With the summer pattern, with a bit more of the air being gulf and coastal, this misses some of the effect of NJ being on the lee side of a range.

Not saying these are the cause, but logically, they likely seem to contribute to a correlation between seemingly higher humidity as temps increase.

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I have two ideas, you may need to test them out. They may not be mutually exclusive.

  1. The hottest days are clear days, and relative humidity peaks at night/dawn.

Relative humidity can approximately measure the ratio of condensation to evaporation. Therefore, during the day (especially clear days, where you will get the warmest days), water is evaporated off the ground, leading to higher mixing ratios of water vapor. When night occurs, the rate of condensation increases, and the rate of evaporation decreases, leading to high values of RH. In some instances, this may lead to fog.

  1. The Gulf of Mexico is both a source of moisture and a source of maritime tropical air.

Many times, the hottest days occur due to clear skies. Clear skies, generally are associated with areas of high pressure. High pressure areas have a clockwise circulation in the Northern Hemisphere, which brings both warm air and moisture from the Gulf of Mexico.

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Water from open water bodies will evaporate more rapidly at warmer temperatures but, more importantly, transpiration of moisture by vegetation increases unless the temperatures get so high and the plants get water stressed that they shut down the stomata in their leaves to slow the transpiration. So you basically have green vegetation pumping water into the atmosphere.

The relationship between temperature and transpiration is not linear and depends on the available water. In dry climates the evapotranspiration (evaporation + transpiration) is water limited. The evapotranspiration approaches a maximum value called the potential evapotranspiration if there is enough water.

Even though it takes more water to reach a given relative humidity in warm air, there is enough water available to increase the water vapour in the air and relative humidity. You have probably noticed nights where it cools down and the air becomes saturated forming mist.

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[I've decided to severely abuse the wiki feature by posting what I've found so far and hoping that someone else, possibly me, will do more and more work on this problem until it asymptotically approaches completion without ever quite reaching it; as with any wiki answer, please feel free to mercilessly edit and re-arrange what I've written -- for my first attempt, I am writing mostly in chronological order]

What I've done so far:

USA,US,United States,NJ,Mercer County,Trenton|Altura,Mercer County Airport,,USa KTTN,KTTN,TTN,,14792,TTN,m,KTTN,A,ICA12 ICA10 ICA09,TTN,A,FAA13 FAA12 FAA11 FAA10,,,,724095,,14792,,,40.27669111,-74.81346833,N,FAA13 FAA12 FAA11 FAA10,64.6,A,FAA13 FAA12 FAA11 FAA10,59,,,America/New_York,US-08628,,,,,ADJ/20120717,40,-74

The two key numbers here are "724095" and "14792". If I look in https://www1.ncdc.noaa.gov/pub/data/noaa/isd-lite/1999/ for example, the data I seek will be in the "724095-14792-1999.gz" file.

  • Curation: This station has data from 1973-2015 (2016 data exists, but I'm using my personal collection, which doesn't have it), but some of the data is missing entirely (no line of data for a given hour) and some of the data has a missing temperature or dewpoint. I exclude these lines.

  • This data includes the temperature and dewpoint, but not the relative humidity. To compute the relative humidity, I used the formula from http://icoads.noaa.gov/software/other/profs_short (which is based on the Clausius-Clapeyron equation).

  • I then rounded the temperature to the nearest degree Celsius and plotted the average humidity for each rounded temperature:

enter image description here

This "trilinear" function fits the data fairly well:

$-0.0135991 \left| x-21 \right|-0.00131261 \left| x-1 \right|-0.0075821 x+0.899765$

enter image description here

I still need to look at standard deviation for each point and do many other things, continuing work at https://github.com/barrycarter/bcapps/blob/master/STACK/bc-nj-temp-hum.m

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