Monday, April 29, 2013

Illuminating the Fog

I've waited almost a month to post this. Mostly because I've been savoring the feeling of having put together information from various sources and all on my own coming up with an explanation for an observed natural phenomenon.

Okay, really? I've waited so long to post this because one of you will probably tell me my explanation is incorrect.

I've been inordinately proud of myself, and I didn't want to pop that bubble.

So here's the deal: Several times over the past six weeks or so -- most recently two mornings ago -- I've noticed some very localized fog over Gulf waters.

In the West Delta field, one drilling rig might be completely in the clear as the sun rose, while another rig two miles away is shrouded in gray.

On the previous boat we carefully logged the dewpoint, relative humidity and temperature each day; I assumed this was to aid in the prediction of fog during the coming 24 hours.

Of course those factors most directly impact the formation of radiation fog, which is mostly a land-based phenomenon.

Of greater concern to mariners is advection fog, or sea fog, which forms when warm, moist air moves across cooler water, causing the warm air to contract and condense.

My question was: why over here, and not over there?

Seawater, particularly deeper water in a relatively large basin such as the Gulf of Mexico, change temperature only slowly. It's one reason coastal South Texas enjoys more moderate temperatures year-round than, say, Dallas or even San Antonio.

One day as we were plunging into the gray, I noticed a "rip," or line of foam that denoted a current in the water. On one side, the opaque green of the nearshore Gulf; on the other, a silty brown. I thought about this, and decided that the brown stuff was Mississippi River water, floating on top of the heavier salt water.

It also occurred to me that these eddies or currents of river water were probably cooler than the surrounding Gulf water. (I could find nothing on the interwebs about river water in the Gulf, or surface water temperatures for the areas I was looking at, by the way.)

And sure enough, that fog bank started on the other side of the rip and appeared to cover only the brown water.

So, that's my explanation for localized advection fog in this part of the Gulf: river water which started as snowmelt in the continent's heartland remains cooler than the Gulf of Mexico as it flows into the sea, and fog forms over it even when the surrounding Gulf waters are not cool enough to support the phenomenon.

Did I get it right?


  1. Said it once, and saying it again. You're living in a whole, 'nother world! So interesting!

  2. I will send this to Dick. He probably knows. I think you are right. It is known about the separation of river and gulf water, of course. But he has been very interested and actually photographed many of the rips we see while crossing the causeway. He has wondered why on one studies those rips.

    1. Aaron, that on one should be NO one.
      Aunt Ruth from the herb blog.

  3. You'll love this:

    1. Thanks for the thesis, Jeff. I've downloaded it and can't wait to read it!