Guest Post by Willis Eschenbach
As I’ve mentioned in my previous posts, I’ve been using a home-made thermometer setup to measure the air and water temperatures of the waters around Fiji. To start with, I’ve been thinking about just why I did it.
The answer, as with many things in my life, is “I’m not sure.” (As an aside, I generally laugh when people ascribe motives to my work, like the ludicrous claim that I’m in the pay of big oil, or the false idea that I am prejudiced against the myriad sunspot claims. In fact, all of us do things all the time without being certain just why we are doing them, and my choice to take scientific measurements was another example.)
One of the strongest motives was simple—I’ve never done it, and the undone is always fascinating to me. One of my lifelong rules of thumb is “If there’s a choice between something I’ve done before and something new, always choose the new”. I see part of my purpose on this planet is to go do strange and unusual things, and then report back on what I’ve seen. I let my eyes feast on the unknown, and then I try my utmost to put that into words and pictures for my friends.
Next, there has been recent coral bleaching in Fiji, and I wanted to look at the water temperatures. Also, I wanted to work toward a deeper understanding of what the fluctuations in temperature might actually relate to in the real world. Another point impelling me to take the measurements was that for reasons only know to himself, my friend Dave sent me a NIST high-accuracy thermometer, and it was getting lonely doing nothing.
I also wanted to answer a couple of questions. One was about the accuracy of bucket measurements versus actual measurements of the ocean itself—how fast does a bucket of water change temperature due to evaporation and cooling? Also, I wanted to look at the vertical temperature difference over the top few metres of water. Another was, people often use nighttime marine air temperature (NMAT) as a proxy for sea temperature, so I wanted to look at that relationship. Finally, I wanted to see if the rain cools the sea and by how much. Of course … not all questions in this world get answers …
Finally, I wanted to get a better understanding of the pains and problems of doing data collection … at least that part was a success.
Here is my experimental setup. The thermometer is inside a piece of plastic pipe that I drilled holes in for throughflow.
Inside the pipe, the thermometer is held away from the pipe walls by a very thin plastic case that it was originally shipped in. I cut the thin plastic to allow free flow around the thermometer, especially the bulb. I put duct tape over the ends of the pipe to prevent the thermometer from being washed out the end, and punched holes in the duct tape to allow circulation.
My procedure was to tie the thermometer in a spot where it was exposed to the wind but shaded from the sun (red arrow).
In between measurements of the sea temperature, the thermometer dried out and after about fifteen minutes or so it equilibrated with the air temperature. At each observation I first took the air temperature, and then the sea water temperature. After several experiments with the bucket I saw that I could avoid the fun and danger of filling a bucket from a moving boat, because I got the same temperature by just trailing the thermometer behind the boat where it stayed mostly beneath the surface.
I chose to make measurements generally at two-hour intervals. In part this was to allow the thermometer to dry completely between measurements, although I later found it dried much more quickly than that. And in part it was to give me time to attend to my other duties aboard the ship, like adjusting the sails, drinking beer, standing watch, squinting at the horizon in a most heroic fashion, and swapping lies with the crew … at the end I found that I had taken forty measurements over the ~ eighty hours of the voyage, about one every two hours.
So what did I find out? Well, to begin with I came away with an increased admiration for those folks who do this kind of data collection. I had initially thought I would take measurements around the clock, but the lack of sleep soon put paid to that plan. And I marveled to think that I was doing this in the most pleasant conditions possible … what’s it like for others doing it in driving sleet and the like? I also found out how easy it is to miss something, like the time I tossed the thermometer in the water without first measuring the air temperature … grrr. And then, of course, I was reminded that you should never, ever use raw data without quality control … after a run from 175° East to 174° East, I wrote down the longitude as 147° … grrr again.
Overall, I found out that as expected, the air temperature porpoises above and below the water temperature. Here are the results of the forty measurements:
A few comments to begin with. The first and last measurements were taken inside of the reef, in inshore waters. The rest were offshore, generally in deep water (1 – 2 km depth). All measurements have been converted from Fahrenheit to Celsius. The blue dotted vertical line marks when we turned around to go back to Fiji. The gray dashed vertical lines mark two rainstorms, the first lasting fifteen minutes, and the second lasting half an hour. The first and last days were clear, but on Saturday afternoon we sailed through a large area of a fully developed cumulus field with a number of thunderstorms.
So … what was of interest in the results? One thing I noted was that the sea runs about a degree warmer than the air during most of the twenty-four hours, but the air is warmer for a short period in the afternoon. However, this was not true in areas of active thunderstorms. After the storms on Saturday afternoon, the air temperature was a couple of degrees below the sea temperature.
I also noted something I’ve discussed before, which is that the coolness precedes the storm. What happens is that air from the condensation level where the clouds form is entrained in with the rain, making a vertical wind. When this wind strikes the surface it spreads out in all directions, cooling the surrounding area well before the rain hits. This cool wind has a characteristic smell which of course I can’t describe as our language doesn’t really cover smells … but it has a fresh clean smell that is quite distinct once you notice it.
Before and after the storms I took temperatures every fifteen minutes or so. I couldn’t see any effect of the rainfall on the ocean temperature, which surprised me until I thought about the relative volumes of fresh and salt water … even a hard rain doesn’t add much water to the ocean.
However, the air temperature changes were quite dramatic. After the first storm, which was in the morning, the air temperature warmed back up in a couple of hours. The second storm, however, was in late afternoon, and after that the air was very slow to warm. In any case, it leads me to think that I might be able to identify storms in e.g. the TAO buoy data by using a combination of sea and air temperature measurements. It also makes the use of NMAT as a proxy for sea temperatures something I need to investigate further.
Overall, sea temperatures in Fiji are still quite high. The peak open ocean temperature was on Sunday afternoon, when it got to 30.25°C (86.5°F). Both of the high temperatures were not far from the main island, which leads me to think that the islands heat the sea around them. This would happen through the warming of the water in the extensive barrier reef system around the islands, where water is held in shallow areas. Seems obvious in retrospect, but I had never considered that the lagoons were affecting the open ocean.
Regarding the other questions, I measured the sea temperature right at the surface, down two metres, and down three metres at a couple of points. One was in the open ocean, and one was in the lagoon. In neither case was there any detectable difference in temperature with depth, showing that the “mixed layer” is more than just a name.
Regarding the bucket question, I first measured the air and water temperatures. The air at the time was about a degree F (5/9°C) cooler than the water, with about a six knot wind. I put a bucket of seawater on deck, and took the temperature every fifteen minutes for an hour. I fully expected that with the smaller volume, the evaporation would cool the water down … not so. At the end of the hour, I could detect no change in the temperature of the water in the bucket. My conclusion was that despite the larger volume of the mixed layer, the evaporation factors (wind speed, air vapor pressure, air temperature) were the same for the mixed layer and the bucket … and that the mixed layer was already at equilibrium with the prevailing conditions. As a result, the bucket was already at the temperature dictated by evaporation.
Anything else? Well, we came across a “tide rip”, a line demarcating two different bodies of water. They are a great place for fishing, animals and plants generally like the edges of ecozones. So I checked the temperature on both sides of the rip … nada. They were identical.
Now, the limitations of this data are glaringly obvious. Not anywhere near enough measurements, the thermometer is moving, you can’t swim in the same ocean twice, and it is in the tropics. For example, I suspect (although I don’t know) that bucket vs. in-ocean temperatures in the polar regions might show very different results. However, given all of that … I know I learned a lot.
Today, I’m in the hotel restaurant drinking a latte. I have Tom Waits on my computer, he’s singing:
“Leavin’ my family, leavin’ my friends, my body’s at home but my heart’s in the wind
Where the clouds are like headlines on a new front-page sky, tears are salt water, and the moon’s full and high …”
Turns out our bar tab is the same as the hotel bill, and the rooms weren’t cheap … bad swabbies, no cookies. In a couple hours I’m off from Fiji with the rest of the crew to the land of Oz and the Aussies, one of my favorite places. Mike has invited us all to hang with him for a week or so. I have an appointment to meet the tattoo god and his good lady (also an awesome artist) on Wednesday. Other than that, it’s likely fast motorcycles and sloe gin for the next week, and then back to the gorgeous ex-fiancee and my beloved hills and forest …
And for all of you, I can only wish that your lives be as full of sun and moon and rain and clouds as mine,