Odd Cloud Over Oroville Dam (Spray Cloud)

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I noticed some
Berry Creek is way behind the lake, so I doubt that's spillway related. Lots of people closer.

I actually noticed the cloud around 7 AM this morning while heading back up the hill, about 1.5 miles east of the spillway. Checked the webcam a bit ago and it isn't visible anymore. At the time I couldn't tell if it was smoke or a cloud.
DWR has several photos of it at the bottom of the spillway. Calls it a mist cloud.


Fog forms when the relative humidity = 100% which happens when the dewpoint equals the temperature. The WeatherUnderground charts show the green dewpoint line approaching 45° F. If the greel dewpoint line goes above the red temperature line, then you potentially have fog. Note that the air temperature was barely 5° F over the dewpoint except for the warmer hours between 10 AM to 6 PM - and that was at the airport sensors.

We don't know the exact water temperature over the spillway, but the water below the Thermalito diversion dam (FRA) was around 47° F.


This kind of fog rarely happens in cool temperatures that are not otherwise producing fog - you need the right combination of dewpoint, humidity, air and mist temperatures. It's different because it wil slowly rise and blow away - there is no inversion 'holding' it down like regular radiation or advection ground fog. Nuke cooling towers sometimes produce the same kind of fog which persists for miles and can rise to the base of low clouds.
Fog forms when the relative humidity = 100%

But this particular instance is not fog. It is liquid moisture falling rapidly, aerating and misting, cooling, condensing, and then lifting into a low cumulus type cloud.
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It is liquid moisture falling rapidly, aerating and misting, cooling, condensing, and then lifting into a low cumulus type cloud.

There's a lot of energy in the water pouring down that spillway, and much of it converts to heat. If conditions are right, that helps with cloud seeding -- as with cooling towers, which give off both moisture and heat. Without the heat it wouldn't rise, and there wouldn't be cloud. The cloud forms where already moist air rises, and then cools.
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But this particular instance is not fog. It is liquid moisture falling rapidly, aerating and misting, cooling, condensing, and then lifting into a low cumulus type cloud.

Here's my caveman understanding of the phenomena:

There is always a mist 'cloud' from the water's impact. That mist cloud either falls to the ground if the droplets are large enough or they float around for a few tens-of-seconds and evaporate. The floating mist is composed of water droplets hundreds- or thousands-of-microns in size or larger, orders of magnitude larger than droplets in fog or clouds.

When the droplets evaporate, the liquid water absorbs heat from the surrounding air to transform to gas vapor. Cold mist needs to absorb more heat from the air to evaporate than warm mist. Now the surrounding air is both cooler and has a higher relative humidity. That always happens anytime water droplets of any size evaporate - no matter what the temperature of the droplets and surrounding air - as long as the air temperature was above the dewpoint.

The difference here with these particular temp/dewpoint conditions is that the evaporating mist absorbs enough heat from the surrounding air to lower its temperature below the dewpoint. That means 1) some mist droplets will no longer evaporate right away, and 2) some humid air from evaporating mist will condense back out into droplets of a few microns or so in size and produce fog (which in this case will also not immediately evaporate).

Fog and clouds are both micron- or tens-of-micron-sized droplets. Ground fog tends to keep condensing fairly quickly and has proportionally more larger (50-100 um) droplets. The fog produced from the spillway mist does not continue to condense - the droplets remain smaller micron- or tens-of-micron-sized. They can't evaporate because the temperature in their fog/cloud bubble is still below the dewpoint.

Cloud droplets continue to condense over time, but very slowly (hours/days) and most droplets are usually micron or tens-of-microns in size. The temperature/dewpoints in a cloud are nearly the same and often rely on some kind of sub-micron-sized particle to act as a nucleus. That is not always the case though - a cloud is droplets whether they formed spontaneously or around a nucleus.

Whether you call the spillway phenomenon fog or clouds, it is still composed of micron- or tens-of-micron-sized condensation droplets that are floating in localized air whose temperature is below the dewpoint. They will remain in liquid form until the surrounding air temperature rises above the localized dewpoint, or the dewpoint drops below the localized temperature. When either of those conditions is met, the droplets evaporate into transparent gas.
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