From the Coolest Things I’ve Ever Seen file, comes this photo of Condo Effect Fog (CEF) – yes, I just made that up. The photo was taken by JR Holt of Panhandle Helicopters (website, Facebook) on Sunday, February 5th, 2012 at Panama City, FL. (Thanks to my friend, Jennifer Provo Joyner, for pointing this out)
The article from which I got the photo does a decent job of explaining the phenomenon, which is cool, moist air off the Gulf of Mexico flowing over the buildings and condensing as it is lifted. But I notice a few more things in the photo and want to inspect the science a little more closely (Note: if you don’t feel like geeking out, stop here and just marvel at the photo!).
I am skeptical by nature (a good thing for a scientist to be!), so the first thing I wonder is whether this is a hoax. Way too many photos are faked these days, so we need to know if the conditions in Panama City, FL on Feb 5th even support such an occurrence. For that, I turn first to the surface observations.
The weather that morning at Tyndall AFB showed saturated air at the surface and east-southeast (ESE) winds of up to 5 mph. I would have expected a little stronger winds for the effect we see in the photo, but ESE winds are roughly off the gulf in Panama City, and conditions can vary widely over small distances, so I’ll give it the benefit of the doubt. So far, so good. One other note about the obs: Notice how the winds turn to the north, offshore, in the early afternoon (12:55 pm)? This means the photo must have been taken before that, since the phenomenon would not have occurred with offshore flow.
Now, what about the conditions aloft, ie above the surface? Unfortunately, there is no sounding taken in Panama City, but there is one from Tallahassee, FL about 100 miles away. Tallahassee is inland, so conditions will be different at the surface, but conditions aloft should be similar. The sounding from the morning of 05 February showed a lifting condensation level (LCL – the level at which air lifted from the surface will reach saturation due to expansion and cooling) of 971 millibars, which corresponds to around 400 meters (around 1300 feet) above the surface*. Estimating from the cumulus over land in the photo, that looks about right. Adding support to this is the fact that, later in the day, Tyndall AFB reported clouds between 1200 and 1500 feet.
So, if the LCL is 1300 feet, why is the water vapor condensing as it rises over buildings that are nowhere near 1300 feet tall? The following annotated version of the photo will help to clarify (I’ll talk about each point in order, as shown by the numbers on the photo):
1. The land mass is being heated by the sun which heats the adjacent air. This air then rises and creates low pressure over inland areas. 2. This air, having different properties (higher temperature and lower humidity, for example) than the air over the water, forms clouds only when it reaches the actual LCL (1300 feet). 3. The cool, moist air over the water is drawn toward the low pressure and is forced over the buildings. 4. Based on observations from local land stations as well as buoys in the gulf, the ambient air over the water is nearly saturated (see Pensacola tide station plot^ for example). Since it is already near saturation, the LCL of the air adjacent to the water’s surface is near zero. In other words, it takes almost no lifting to saturate this air. This is evidenced by the fact that the cloud/fog forms within a hundred or so feet of the surface, well before it encounters the actual obstruction. 5. The air continues to flow toward the low pressure inland but, now free of the obstruction, it returns to its former level behind the buildings. As it descends, it compresses, warms, and dries and the cloud evaporates.
6. One interesting question is why this doesn’t happen down the entire length of the beach. I don’t know other than to say something is different down there. Maybe the winds are at a different angle to the buildings or the air is somehow drier. This just shows the chaotic nature of weather and is an example of why it can be so hard to predict. What happens in one area may not happen just a few miles away despite nearly identical conditions. Go figure!
Now that I’ve beaten this to death, I’ll just say that the photo is pretty cool, and, from what I can tell, its legitimacy is supported by the data.
* Which I calculated using the hypsometric equation. Note to self: when the airmass is near saturation, don’t waste time calculating the virtual temperature….the actual temperature is close enough. It worked out to a grand total difference of 0.1 meter…hardly worth the 8.5X14″ page of calculations and hand waving I did. Deep down, I already knew this, but sometimes you just have to do the math.
^The dewpoint of the air near the water’s surface is typically close to the temperature of the water itself. Notice that the air temperature dropped to the water temperature on the morning of 05 February. This indicates the air over the water was likely saturated as it moved inland.