Large Cities May Create Their Own Clouds

sciencehabit shares a report from Science Magazine: Although it's well-known that concrete-rich cities are often several degrees hotter than the leafy suburbs, scientists have now determined that cities are also capable of prolonging their own cloud cover. By studying satellite imagery of London and Paris, scientists discovered that during the spring and summer, the modern megacities are persistently cloudier in the afternoon and evening, by several percentage points, than nearby rural areas. Using ground-based observations in London, the researchers suggest the heat retained by buildings into the late afternoon drives turbulence in the air, feeding moisture to the clouds, they report this month in Climate and Atmospheric Science.

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[aliquis]

Didn't it sound like it made the air there drier? Except for the cloud bit. And if so do that help?

Anyway best would be with people stuck there and then they could live and solve it in whatevrr way they want.

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[Cipheron]

What's new is this data. You can say something has been "known" for years. But that's not the same as saying scientific evidence and measurements, which this article mentions.

"everyone knows" something, so research isn't needed? Come on.

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[Cipheron]

I'll just point out that as often as not, these things "everyone knows" get disproven by research.

Research is equally newsworthy whether it confirms or de-confirms a previous belief. Scientists need to report their findings whether or not they confirm / disconfirm any conclusion. This should extend to science journalism, too.

News for nerds

[jrumney]

I was expecting an article about large cities throwing off the chains and abandoning Amazon, Google, Microsoft etc for ownCloud or some other self-hosted solution. But apparently this is about those white fluffy things that I can sometimes see out the windows when I venture upstairs from my mother's basement.

Re: News for nerds

[K. S. Kyosuke]

Both may still be vaporware, so...

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[umghhh]

Not sure about where you live. In a country I live (Germany) even the place where Greens forced complete ban on diesel this year the measurements in most polluted areas (a traffic junction on most busy street in a city Stuttgart shows decreased pollution to the point where it was on acceptable level even according to rabid green activists. I would imagine London and Paris to be similar although these are very huge cities with lots of exhaust and lost of dust in the air. So while big cities in the world are

Some context for this result

[Anonymous Coward]

The lower atmosphere over London and Paris is warmer and drier than the surroundings. In the large cities, the surface warms faster in the morning and there's more heat available to transfer into the air just above the surface. As the lowest part of the atmosphere is heated from surface energy fluxes, the heat is then transported upward through convection. Ascending air cools to the point of reaching condensation, resulting in fair weather cumulus clouds. The larger and more persistent surface energy fluxes results in more persistent cumulus clouds over the cities, despite them being drier. However, because the cities are drier, this means the air has to rise farther and cool more to reach the point of saturation, resulting in higher cloud bases. This stated in the article, but the rest of the comment is my speculation.

This result seems specific to fair weather cumulus clouds, where stronger mixing would result in more persistent cumulus clouds. It might also result in an earlier onset of cumulus cloud formation in the morning. However, other types of clouds like fog and low stratus decks might mix out faster over cities and actually dissipate quicker. While fair weather cumulus clouds would be more persistent, I wouldn't expect the same result for non-convective clouds.

The convection that produces fair weather cumulus is almost entirely confined to the planetary boundary layer, generally within 1-2 km of the surface. The free atmosphere above the boundary layer tends to be dry, but the bottom of the free atmosphere is also warmer than the top of the boundary layer. The tops of the fair weather cumulus clouds do reach into the free atmosphere, allowing for the exchange of heat and moisture. However, the warm temperatures at the bottom of the layer act as a lid on the convection. Were that not the case, the convection would be much deeper, and the cumulus clouds would have a larger vertical extent.

The cumulus clouds result in a flux of moisture out of the boundary layer and into the free atmosphere. The bottom of the free atmosphere tends to be warmer than the top of the boundary layer, and some heat is exchanged in the process. Over time, this will dry out the boundary layer, assuming that the moisture flux out of the boundary layer is large enough to overcome the flux of moisture from the surface. In cities, there tends to be less evapotranspiration, so the surface moisture fluxes are quite a bit smaller than in surrounding areas that have more vegetation.

When mixing is particularly strong, persistent convection in the boundary layer can dry it out to the point that convective cumulus clouds no longer occur. Because the convection is stronger and more persistent over large cities, that should actually result in convective cumulus clouds dissipating earlier in the afternoon than over surrounding areas. If the boundary layer is going to mix out during the day, it should happen faster over cities, and the cloud cover should dissipate earlier. I'm assuming that didn't happen often in the period of record that was studied, because that should have the opposite effect on cloud cover.

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[Evtim]

Few years ago I noticed something interesting.

I live in A'dam, relatively close to the international airport which is rather large (3rd in Europe or some such). There are certain weather conditions under which the vapor trail from the plains forms a sort of low-altitude cirrus(sp?) cloud. They spread very thin but since the composition is like small icicles it reflects light quite well. Sometimes, when this happens there are even few photos in most newspapers; people treat it like art.

So, some years ago, wh

sorry

[Evtim]

spellchecker blunder (planes != plains)

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[Solandri]

In the large cities, the surface warms faster in the morning and there's more heat available to transfer into the air just above the surface. As the lowest part of the atmosphere is heated from surface energy fluxes, the heat is then transported upward through convection. Ascending air cools to the point of reaching condensation, resulting in fair weather cumulus clouds. The larger and more persistent surface energy fluxes results in more persistent cumulus clouds over the cities, despite them being drier.

Could also be different amounts of dust

[Anonymous Coward]

Clouds form around condensation nuclei. [wikipedia.org]

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[Anonymous Coward]

You're correct dust is one type of cloud condensation nuclei, and that there's more dust in cities. The article [nature.com] actually addresses this:

However, for low, non-precipitating clouds such as shallow cumulus the effect of enhanced aerosols on cloud lifetimes is observed to be very small or aerosols might even decrease their lifetime. The mechanisms responsible for linking urban surface fluxes to altered clouds and convection need to be confirmed using numerical simulations. Idealised simulations without synoptic forcing or aerosols have seen more persistent cloud cover over urban areas.

The authors are suggesting that for the types of clouds in this study, larger CCN concentrations in urban areas don't explain the observed result.

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[PPH]

It could also be due in part to the numerous air conditioning cooling towers on office buildings dumping humid air into the atmosphere.

Good?

[Krakadoom]

So, since cloud cover lowers the overall temperature on Earth, maybe it would be better for the climate if we blanketed the whole place in urban sprawl Coruscant-style.

Re: Good?

[reanjr]

If the sprawl was just large enough to fit the entire human population, it would have a huge positive impact on the environment. No one hurts the environment more than suburbanites and their need to obsessive need to avoid the poors.

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[account_deleted]

Comment removed based on user account deletion

Can't they just use AWS?

[jenningsthecat]

Or Azure? How about Digital Ocean? That ought to have a pretty good cooling effect!

Even some small cities internalize computing :-)

[chipperdog]

I'm technology director for a small city, and we run our own cloud on our own fiber between sites. Virtual server hosts are not that difficult to run and keep redundant at multiple locations..


Oh, not that type of cloud...