If you measure humudity Down on the ground it need not be and could be any humidity when the rain is falling

Back on point to this entry: I also note that after summer rains, the humidity seems to drop. I always thought it was a drier air mass ‘moving in’ but could just be the dehumidifying effect of the rain itself. Thanks for the great blog.

I’m Civil not Mechanical, so
I’m Civil not Mechanical, so I’ll just say the water “features” have been operating for over a decade now. I do recall there was some initial tweaking of the lobby waterfalls (a hydroelectric utility’s lobby, these are spillway analogues, two, about 10 m wide, 5 m high). The 24m tall features are not waterfalls exactly, but water trickling down mylar ribbons within six storey atriums into which the 18 storey building’s tower ventilation air is introduced on its way to an undefloor distribution system. It’s 100% fresh air, passthrough ventilation. One could see lots of opportunity for legionella in the past decade. John Straube has been in the building and provided an absorbing critique of its double wall exterior, but I do not recall him commenting on the ventilation system 🙂

Good points, David.  Also,

Good points, David.  Also, from what I understand, those conditions in the cloud can be significantly higher than saturation before nucleation of water droplets occurs.  The vapor pressure can keep increasing until nucleation sites appear.

And yes, the ground RH can be significantly lower than the 93% I showed.  Last night it was raining again and I saw on my weather app that the RH was 75%.  Here are the screenshots:

[[{“fid”:”2408″,”view_mode”:”default”,”fields”:{“format”:”default”,”field_file_image_alt_text[und][0][value]”:”Rain in Tucker, Georgia”,”field_file_image_title_text[und][0][value]”:”Rain in Tucker, Georgia”},”link_text”:null,”type”:”media”,”field_deltas”:{“1”:{“format”:”default”,”field_file_image_alt_text[und][0][value]”:”Rain in Tucker, Georgia”,”field_file_image_title_text[und][0][value]”:”Rain in Tucker, Georgia”}},”attributes”:{“alt”:”Rain in Tucker, Georgia”,”title”:”Rain in Tucker, Georgia”,”height”:”268″,”width”:”450″,”class”:”media-element file-default”,”data-delta”:”1″}}]]

[[{“fid”:”2409″,”view_mode”:”default”,”fields”:{“format”:”default”,”field_file_image_alt_text[und][0][value]”:”75% relative humidity during the rain in Tucker, Georgia”,”field_file_image_title_text[und][0][value]”:”75% relative humidity during the rain in Tucker, Georgia”},”link_text”:null,”type”:”media”,”field_deltas”:{“2”:{“format”:”default”,”field_file_image_alt_text[und][0][value]”:”75% relative humidity during the rain in Tucker, Georgia”,”field_file_image_title_text[und][0][value]”:”75% relative humidity during the rain in Tucker, Georgia”}},”attributes”:{“alt”:”75% relative humidity during the rain in Tucker, Georgia”,”title”:”75% relative humidity during the rain in Tucker, Georgia”,”height”:”519″,”width”:”450″,”class”:”media-element file-default”,”data-delta”:”2″}}]]

The point I’m trying to make is that rain is produced when the air (cloud) reaches saturation, but once released, the rain will then pass through whatever air happens to be between the cloud and the ground.

The dryer the air the rain passes through, the more likely it will fully evaporate before reaching the ground, as often happens in arid climates. But in general, there’s no requirement that the air through which the rain passes must also be at saturation. In fact, the local RH could be significantly lower than the 93% in Allison’s example.

BTW, when the air near the ground is at or near saturation, then what we get is fog. What happens inside a cloud is above my pay grade 🙂

You’re welcome, Mark.  It’s

You’re welcome, Mark.  It’s one of those things that seems obvious…until you really start thinking about it.

Thanks, Peter!  But after

Thanks, Peter!  But after seeing a presentation at Building Science Summer Camp on Legionella, I’m wondering how safe it is to have a “several 24 meter waterfalls” indoors.  The speaker, Jack Springston, discussed how common it is in places where water is sprayed out into the air like that, including in showers.  You can download his presentation here: 

Legionalla, by John P. Springston (pdf)

https://buildingscience.com/sites/default/files/legionella_jack_springston.pdf