A few years ago I was in Ocean City, New Jersey at the end of September. The temperature was probably low to mid 20s centigrade, nothing that would be remarkable on a British summer’s day. Walking around, it didn’t feel any more humid than I’m used to. However in the late afternoon/early evening, many outside surfaces were literally dripping with condensation. This is not something I have noticed or heard of anywhere in Britain.
Yesterday in Scotland it was about 25C. ‘Humidity’ was reported on the BBC website as being in the 80s and 90s % (Ocean City was about the same). It was definitely sticky, my chest was damp with sweat. But no condensation at any time during the day. What is the difference?
I’m guessing there may be some difference in the ways that humidity is measured, that 90% humidity in Britain fails to report some other aspect of humidity that is measurably higher in parts of the USA. So I’m interested to find out what that would be. I’m also interested to find out why humidity is different in places like Ocean City. Sure, it’s next to the sea and it’s near lots of swamps/marshes, but we have all that in Britain too.
To be clear, we get dew falling in Britain at night once the temperature drops. But it doesn’t coat surfaces (like glossy painted fence posts) with water as much as the condensation in Ocean City did.
In the former scenario, the temperature was lower so you weren’t uncomfortably warm - thus there wasn’t significant sweating (or at least not enough sweating to exceed the evaporation rate). The dew on solid objects indicates that those objects were cooler than ambient air, causing a local microclimate (within a fraction of an inch of those objects) in which the humidity hit 100%, forcing condensation to occur. This (objects colder than ambient air) can happen when a warm weather front with very high humidity rapidly moves in; it more commonly happens when the sun goes down and objects cool via radiating heat away into a cloudless sky, resulting in them having a temperature significantly below that of the ambient air (this is why dew commonly happens at night even when there’s no fog, and its’ why light frost can develop even when the air temp is a little above 32F).
Nope. Humidity as reported by meteorologists is relative humidity, which is simply the amount of moisture in the air as a percentage of the maximum possible amount of moisture that could possibly be in the air at that temperature. That’s it. There is no other aspect.
The fact that you witnessed an extremely heavy dew is a testament to how much moisture was in the air in the first place, and how far the temperature dropped. The maximum possible moisture content for air is greater at higher temperatures. So imagine three different scenarios:
-23C, 90% humidity, cool it down by 3C
-23C, 90% humidity, cool it down by 8C
-10C, 90% humidity, cool it down by 8C
The first case will create a lot more condensation on solid objects (or a lot more fog) than the second or third case.
Thank you Machine Elf, a lot to chew over there. I’m working on the assumption that this extremely heavy due is quite common in parts of the USA, but rare to non-existent in Britain, correct me if I’m wrong?
I think in the Ocean City example it must have been the warm weather front; I don’t remember it getting colder (though my memory could be faulty as I was hungover from booze brought in from Out of Town). Dew in Britain is associated with clear skies at night as in the second example but it’s never as heavy.
Obviously those are dashes, not minus signs, but I don’t quite follow this. Why does a drop from 23C to 20C cause more condensation than a drop from 23C to 15C?
So if we assume that the extremely heavy dew was caused by a very humid warm weather front, why would I have encountered this only in New Jersey, having spent all my life in places in Britain that have similar coastal/estuarine locations? (As I say, if it’s much less common in America than I’m assuming then let me know).
Temperature and humidity levels were much the same on the days in question. New Jersey gets more annual precipitation than many parts of Britain, but it’s not a remarkable difference. New Jersey has a giant continent to its west and a giant ocean to its east. Britain has a giant ocean to its west, with small seas and a smaller continent to its east and south. How do these or other factors combine to make the differences I’m talking about?
Another lurking variable here is that “relative humidity,” which is the percentage number that you’re seeing and discussing, is not always a useful way to determine “does it feel humid out?”
The reason for this is that warmer air can hold more water vapor in it than colder air can. And, thus, “50% relative humidity” at 35 degrees Celsius will have far, far more water vapor in the air than when it’s “50% relative humidity” at 10 degrees Celsius. It’s primarily the absolute amount of water vapor in the air which makes it feel humid to us, not the relative humidity.
A more meaningful term for understanding the amount of water vapor in the air is the dew point. This number is expressed as a temperature, and what it means (and what Machine Elf is alluding to) is the temperature at which, given the current amount of water vapor in the air, the point at which the atmosphere would be saturated with water (i.e., the point at which the relative humdity would be 100%).
So, for example, if the current air temperature is 25 C, and the current dew point is 15 C, if the air cooled to 15 C (as it might overnight), the relative humidity (which would have been somewhere below 100% at 25 C) would reach 100% when the temperature cooled to 15 C. Note that a corellary to this is that, in most cases, the lowest that the air temperature can go is the dew point; it can’t cool off any more unless the amount of water vapor in the air is reduced.
In my experience, in most environments, if the dew point is at 50 F (10 C) or below, you really don’t notice the humdity. When the dew point creeps up to the high 50s F (say, around 14 or 15 C), it starts to become noticeable. When the dew point reaches around 65 F (18 C) or so, it definitely feels humid, and anything over 70 F (21 C) feels tropically moist.
Yes. There are parts of the US that can have high humidity along with large diurnal temperature swings. If you park your car outside overnight, in the morning you may find that there’s a wet ring on the ground where water has been rolling off of the sides of the car during the night. If you start driving, and wipe the dew off of your windshield with the wipers, fresh dew will immediately begin forming, again blocking your view.
:smack: My bad. Let me restate:
[ul]
[li] 23C, 90% humidity, cool it down by 8C[/li]
[li] 23C, 90% humidity, cool it down by 3C[/li]
[li] 10C, 90% humidity, cool it down by 8C[/ul][/li]
The first case will create a lot more condensation on solid objects (or a lot more fog) than the second or third case.
Ok, thanks everyone. So it seems that the dew point that day in New Jersey would be significantly higher than what one would encounter in Britain. If altitude and air temperature are the same, which geographical factors account for this difference?
It’s just going to be temperature and water content in the air. Geography isn’t going to make a difference.
One question: how sure are you that the temperature and humidity were actually the same in both cases? The human body makes for a terrible thermometer, and travel exacerbates inaccuracies. That could explain the difference all by itself.
ETA: also, how sure are you that your body was in the same shape a few years apart. It may feel the same, but may not actually be reacting the same in the same circumstances. Also, for all the surfaces you saw covered in condensation in New Jersey, how many were windows? Is it simply the case of more air conditioning in the US than Scotland?
Others have already confirmed that this phenomenon of extreme dew is a regular feature of parts of the USA. It is not something that happens in Britain, on windows or any other surface, even in this week of high humidity and record breaking temperatures. So geography clearly makes a difference. If you don’t know the answer to the question, then perhaps wait for someone who does?
No one in this thread has posted that “extreme dew” is a regular feature in parts of the US. We have no way of knowing the actual pertinent details of your experience in the US and your impression of “normal dew” in Britain. And your complete inexpertise when it comes to basic meteorology does not instill confidence that you experienced something that never happens in Britain if there’s a similar weather pattern.
The basic fact is that dew comes down to warm moist air cooling down and cool surfaces for it to settle on and it is impossible to guess exactly what the weather was like that day in Ocean city as humans are very bad at sensing either temperature or humidity.
Unless of course you can give us date and time and there’s a public data set around with sufficient granularity.
As noted above, you get the phenomenon of dew (or, in colder conditions, frost) forming when the air temperature cools to the dew point temperature. Conditions that lend themselves best for this are a clear nighttime sky (which facilitates greater radiative cooling) and calm or nearly calm winds.
The British Isles are stereotypically known for cloudy skies, though, of course, that’s not always the case. But, if a cloudy night is more typical in Scotland, it may be that the overnight air temperature doesn’t commonly cool to the dew point, and, thus, the phenomenon of dew forming might be less common. (If calm winds are also uncommon in Scotland – and I have no idea if they are or aren’t – that may also be a factor.)
WTF are you on about in your extremely patronising post, Naita? I have noticed this phenomenon many times on the SDMB; somebody posts a question about something in nature or human life that they have noticed. Some people discuss the question in a respectful way. But then someone else chips in, appearing to believe that their anecdotal failure to observe the thing in question is somehow evidence that that the whole question is invalid.
We get both - high pressure and low pressure alternate frequently in Britain. Dew is associated with cloudless, still nights across Britain. But it doesn’t happen on a warm day at something like 8pm as it was in Ocean City, and it’s never as dripping wet as it was there.
I live in west central Ohio. We get a lot of dew just about every morning in the summer. This morning my truck was covered in dew, and I had to use the windshield wipers and defrost to clear the window.
Late afternoon condensation is not typical in the US, either.
Condensation sheeting walls and windows in late afternoon in warm weather is going to be associated with air conditioning or a weather front moving through.
I wasn’t processing that it was happening that early in the evening. Given that, I agree with Great Antibob – that is a very unusual condition, even here, and it’s probably the result of a sudden change in the amount of moisture in the air mass, and/or a sudden drop in temperature, due to a cold front.
I’ve already mentioned that it was noticeable on shiny fenceposts that would not have been affected by air conditioning, and I’ve already asked whether/why such a weather front would be more common in NJ than the UK. Do you know the answer(s)?