How/why do the effects of humidity vary from place to place?

Factual Questions
21

Remember that Ocean City is just above 39 N, and Edinburgh is just about 56 N. Here in Scotland we almost never see a sufficient temperature drop at 8pm (and sufficient previous heat and humidity etc) that would cause a significant dew to form - unless there’s a thunderstorm passing through or something. In our warm and humid months, the sun is still up hours after 8pm.

Add in the temperature smoothing of being on an island where nowhere is far from the sea, it’s no surprise that the most visible dews are at dawn after a still, cloudless night.

22

I missed Machine Elf’s exact wording in browsing through the previous replies and responded to your own dismissive arrogance in stating “If you don’t know the answer to the question, then perhaps wait for someone who does?” to a post that had no inaccuracies, but was pointing out that you are pursuing a red herring with your insistence that the exact same atmospheric conditions in Britain will have a different outcome.

No one has said that your whole question is invalid based on anecdotal failure to observe the same thing. What people are pointing out is that you’re making massive assumptions in your opening and replies about the accuracy of your perception of weather conditions.

23

Well, maybe you should read the whole of a thread before wading in with such an unhelpful, obnoxious attitude. I quote from another messageboard: “If you observe in the desert you may hardly ever encounter dew. If you observe in Florida like I do you may often encounter dew so heavy that water is dripping off the scope.” If you’re going to question the premise of my OP on the grounds that you do in fact get such heavy dew in Britain, or that that Florida poster is mistaken, then please provide some evidence.

Your derisive use of the words “insistence” and “exact” is not justified here, given that I listed similarities and differences in the geography and climate of NJ and the UK and was asking for people with greater knowledge to break down the differences and explain why you get a particular weather phenomenon in one place and not another (I notice that you also have nothing to actually contribute here).

The “post that had no inaccuracies” stated dismissively that “Geography isn’t going to make a difference” and went on to imply that I was too stupid to know whether I had ever previously encountered the weather phenomenon I was talking about in my home country. It implied that you don’t in fact get the type of dew that had already been described by Machine Elf. It is absurd to claim that this isn’t about geography, and to do it while patronising me is going to get a snarky response.

24

Alright, so, to sum up:

The OP noticed heavy condensation (“dew”) on a number of outdoor surfaces during the late afternoon / evening, on a day that didn’t feel like it was particularly hot nor humid. The OP had never seen something like this before, at that time of day, back home in Scotland, and wondered if this phenomenon was typical in the U.S., or if humidity is measured differently (or functions differently) in the two countries.

Short answer: heavy condensation / dew during daylight hours is atypical here in the U.S.; there was likely an unusual phenomenon going on that day.

Generally, there are three conditions that are going to cause condensation like that:

  1. The ambient air temperature drops to the dew point, causing the air to become saturated, and condensation to form. This is the usual mechanism for dew to form in the overnight hours, though in the OP’s example, if a cold front had gone through, and the air cooled enough for it to lower to the dew point, that may have caused it.

  2. The air becomes more humid (i.e., a moister air mass moves into the area), without an accompanying increase in temperature. With more water vapor suddenly in the air, the air reaches its dew point, and condensation suddenly starts to form.

  3. The surfaces on which condensation is forming are cooler than the ambient temperature (and cooler than the dew point), causing condensation to form on the surfaces.

As the OP notes that the surfaces which had condensation were probably not being artificially cooled (e.g., fence posts), I suspect that it wasn’t condition #3.

The OP also notes that he doesn’t remember it suddenly feeling cooler at that point, so that points away from condition #1. And, unless the air had already been close to the dew point, it would have taken a substantial decrease in the air temperature to start dew forming, which likely would have been noticeable.

My educated guess, therefore, is that it was condition #2 – a more humid air mass had moved in, causing the air to suddenly become saturated.

One last point: several of us have noted that what the OP observed is, generally, very unusual, even here in the U.S., at that time of day. That said, there are such things as microclimates – small areas which, due to the surrounding geography, weather patterns, etc., may regularly experience weather conditions which aren’t typical elsewhere. (San Francisco is an example of a place with multiple microclimates.) It is possible that Ocean City has a microclimate, and that late afternoon dew isn’t that uncommon there; without talking to locals, or doing some research, I don’t know. Even if it is the case that Ocean City regularly gets late-afternoon dew, it’d have to be via one of the mechanisms I describe above – it just means that such conditions happen more often there.

25

Geography as in “being in a different place with a different climate” will make a difference, because this is a question of weather. But given the same atmospheric conditions, being in a different place will not. That is clearly what Great Antibob was getting at.

There was no implying you were stupid, just an attempt to make you realize that humans are not good measuring devices for either temperature or humidity, that human memory is fallible, and a request for further detail.

26

[bolding mine]

No, as I’ve tried to make clear several times, you don’t get this type of dripping wet dew anywhere in Britain (or Western Europe as far as I’m aware) at any time of day or year. It is the dew, not the time of day that interests me, so if it was a freak occurrence in Ocean City then unfortunately that has confused this thread. The dew might actually have condensed later than I thought and the temperature drop might have been greater than I recalled (but believe it or not I am able to read weather forecasts and reports and I tend not to misread 38C for 24C or 60% for 90%). If the thread had not become derailed then I could have clarified this sooner.

I had heard of this type of dew and high humidity in the USA before, in the same vague way as I’ve heard of extensive beef farming in Argentina, or cold nights in the Sahara, and been interested in why it occurs. I had though it was associated with places further south like Louisiana or Florida, so I was a bit surprised to encounter it in New Jersey. But talk of dew point in this thread led me to this map which suggests that the NJ coast is in one of the higher bands.

This website gives current dew point temperatures across the world. High dew points deem to be a feature of equatorial areas; the southern USA seems quite far from the equator for such a large area of high dew points, China and Japan are similar. Places on similar latitudes in Africa, South America and Australia are mostly much less humid. Enclosed seas like the Gulf of Mexico, the Bay of Bengal and the Sea of Japan seem to make a difference. I can guess at some of the reasons this might be but I’d be interested to hear from people who know what they’re taking about.

27

Still no appreciation of this and the posts in questions are actually patronising, insulting and lacking in anything that makes a useful informed contribution to the thread.
And when you write:

not only do you misrepresent me (which is not surprising since you didn’t bother to read the thread properly), I begin to suspect that your own understanding of meteorology may not be as sophisticated as you believe it to be.

28

Of course it’s winter in Australia right now which might account for it… I’m trying to find year round or average dew point stats for places other than the USA but they don’t seem to be as easy to find.

29

The Stafford Cripps - very interesting question. I did glance through the earlier posts and sorry if I am repeating something already said.

So I looked through the weather data for Atlantic City , New Jersey September 25, 2017 and Glasgow July 25, 2019. Converted the temperatures to Celcius since the OP was in Celcius. I did not see the temperature ranges mentioned by you - but that maybe because I have the exact location or dates wrong. It does not matter though to the analysis.

The raw data is available here for Atlantic City and here for Glasgow.
Here are the plots I made for temperature and dew point throughout the day for the above dates

Glasgow
Atlantic City

As you can see from the plots, the temperature as well as the dew point goes up and down in the morning hours. So something with thermal mass may still be at the low previous temperature while the dew point of the air lowers and then you see the precipitation.

For example, look at the last temperature dip before 9:36 AM and the dew point changes after that. Although the air heats up, the metal post will take a while to heat up and will see condensation at around 9:15am

30

Also - please don’t discount the ground / soil temperature. Metal posts with concrete anchors will tend to have a thermal inertia and will resist quick Temperature changes.

31

Looks like tinypic is not showing the plots.:

Here’s the updated location Glasgow and Atlantic City

32

Thanks am77494, that’s a great site. I think the night in question might have been the night of the lunar eclipse in 2015, 27th September (it was that or one of the nights on either side). The site shows that at 7:51pm temperature was 63 F, the dew point temp was 63 F and the humidity was 100%. I’ll have to have a look to see if I can find examples of similar stats in Scotland at times when I can remember what the weather was like, to make comparisons.