Guest Post by Willis Eschenbach
I grew up on a remote cattle ranch surrounded by miles of forest, far from street lights. The nights were large and silent and very dark when there was no moon. But when the moon was full, the forest at night was full of life. It was clear that the moon had a huge effect on the animal life. The farmers in the area often planted by the moon. Whether that did any good I don’t know. I do know that the moon rules the ocean, though. When I fished commercially for anchovies off of Cannery Row in California, we’d take a week off during each full moon. You couldn’t catch the anchovies during that time, they were too skittish. And the difference between night diving with and without the moon is startling.
When I was younger, I read in several places that the moonlight doesn’t influence the weather. What the sources said was that it was just too weak to affect the temperature. Heck, you can find people making that claim today. There was no scientific evidence for a detectable effect of moonlight on temperature until 1995, when an article in Science magazine called “Influence of Lunar Phase on Daily Global Temperatures” (paywalled, as usual) said that their comparison with lower tropospheric temperatures showed a temperature difference between full moon and new moon of 0.02°C.
What does that have to do with the moon wind? And what is a “moon wind” when it is at home, anyhow? Well, when I was younger, I once sailed across the Pacific as the first mate on a fifty foot (15m) gaff-rigged staysail schooner. The skipper and I split the navigation duties. This was well before GPS, so the navigation was all done with a sextant and a chronometer.
The skipper took the noon shots, the ones that firmly establish the latitude (north-south). I took the star shots at dawn and at dusk. The navigation wasn’t done while someone else ran the boat. In the morning watch, I was usually the only one awake. I’d set the sails to where the ship would self-steer, and then take the sextant shots. In the evening, the other three guys would usually all be up, but the drill was the same. Self-steer, and sextant.
Sailboats are sloooow, a typical day we would make maybe a hundred miles, less when there’s no wind, and the Pacific is very wide. As a result, I got to watch lots of sunrises and sunsets, plus taking in a lot of late-night skies sitting on deck talking story with the boys.
Including the obligatory breakdowns, we were four months at sea. As day followed day, I became more and more attuned to the cycles of the weather. One of the things that I noticed was what I later found out was called a “terminator wind”. Great name, it sounds ominous. I didn’t know that at the time, so I just called them the “dusk wind” and the “dawn wind”.
A “terminator wind” is a wind that blows across the “terminator”, the moving line on the earth that divides day from night. The sun heats the air on the day side of the terminator line. The heated air rises, and cold dense air from the night side of the terminator flows in to replace the rising heated air on the day side. As a result, the terminator wind always blows from night to day. This leads to a morning/night difference. In the morning the dawn breeze blows from the dark areas further to the west of my location, and towards the sunlit areas east of my location where the sun has already risen. In other words, the dawn breeze is always and forever a west wind.
In the evening, on the other hand, the sun has set in areas to the east of my location. So an east wind blows from the nighttime there, towards the west, where the sun still warms the earth. As a result, the dusk breeze is always and forever an east wind.
The opposite direction of these two winds leads to a curious phenomenon. This is that for relatively steady overall winds, the dawn and dusk winds will alternately oppose the underlying wind, or it will increase the underlying wind. This is most visible when there is a light constant east wind. At the dawn breeze is a west wind, so it opposes the light east wind and leads to a short period of calm around dawn. At dusk, on the other hand, the terminator wind blows from the east, so the dusk breeze reinforces the underlying east wind and leads to a brief gusty period around dusk … and if there is a light underlying west wind, the opposite is true.
Now, here is where the moon came in. After I’d spent enough nights at sea, watching the comings and goings of the moon, I noticed that the moon has a terminator wind just like the sun. I started calling it the “moon wind”, I didn’t know from terminators, I was on a boat in the middle of the sea, I made up a name. I first noticed the moon wind in the doldrums, where the air is often quite calm, with no wind of any kind. In those peaceful conditions, with the boat not moving at all, the terminator wind from the moon is quite apparent … at least it is to sailors hoping for any kind of wind in the doldrums. It obeys the same rule as the dusk and dawn wind in that it always blows from areas with no moon to areas with the moon. Of course, it is much weaker, and only detectable on calm nights. On a calm night it is a sliver of a breeze, just enough to send small wavelets shimmering in the emerging moonlight.
During the time before the full moon, when the moon is waxing, we have only moonsets at night. As a result, before the full moon, the moon wind is always an east wind. After full moon, we only have moonrises at night, so during that time the moon wind is a west wind.
Since that time, I’ve occasionally noted the moon wind on land as well. You need near full moon, preferably a large flat open area, and fairly calm conditions to be able to detect it. It helps to know what you are looking for. In light east wind conditions after the full moon, for example, the west wind at moonrise opposes the underlying east wind, and is seen as a brief period of calm around the time of moonrise. But if the underlying wind is from the west, the wind at moonrise will reinforce that west wind and lead to a brief gusty period around moonrise.
I bring all of this up for several reasons. One is to point out that the earth responds to a very slight change in conditions. We routinely overestimate the strength of the light coming from the moon. The light from the moon is about a million times weaker than the light from the sun (with a full-moon peak at about 0.006 W/m2). The infrared from the moon’s surface is stronger than that, it’s somewhere around 0.03 W/m2. The sum of the two is only a bit above 0.03 W/m2, that’s thirty milliwatts per square metre, a very tiny amount in terms of the global energy budget.
And yet despite that energy being so small, you can still feel the moon wind at the moon’s terminator line, a wind that arises from that tiny energetic addition. What an astounding system. It is so stable that the global temperature hasn’t varied more than ± 0.5% in the last millennium, and despite that amazing stability, it is also so delicately balanced that thirty milliwatts of energy are enough to make the moon wind come up and shiver the silvery ocean with its breeze …
I also bring this up to point out that there is still a whole lot that we don’t understand about how the weather and the climate work. Let me quote the conclusion of the 1995 Science article:
The existence of an identifiable relation between lunar phase and global temperature begs the question as to its fundamental cause. Presumably the causal factor is lunar, but, as pointed out by researchers examining the relation between precipitation and lunar phase (3, 4), this cannot be demonstrated by statistical analyses alone. Other scientists who have examined the lunar influence on various climatic variables have suggested several causative linkages.
For example, increased thunderstorm activity near the time of the full moon may be related to lunar distortions of Earth’s magnetic tail (5). Another hypothesis advanced to explain the precipitation-lunar phase relation involves the lunar modulation of meteoritic dust (2). Others have speculated that lunar tidal changes could influence Earth’s basic atmospheric circulation patterns, in particular,the position of the subtropical high- pressure belts(24).
Also, with respect to global temperature variations, a full moon results in an increased solar load due to the moon’s reflection as well as to an increase in infrared emission from the moon’s surface. The infrared flux to Earth is five orders of magnitude less than the direct flux from the sun, whereas the shortwave flux is six orders of magnitude less than the direct flux from the sun (10, 25); the 0.02 K modulation in temperature identified in this study is correspondingly five orders of magnitude less than the mean lower-tropospheric temperature. Feedback responses of global temperature to potentially lunar-related variations in other climatic parameters, such as precipitation, cloudiness, and thunderstorm activity, may also account for the lunar effect on global temperatures.
Our analyses show a significant empirical relation between lunar phase and daily planetary temperature over the past 15 years. The lunar phase appears to produce a modulation of approximately 0.03 K in the lower troposphere, with the warmest daily temperatures over a synodic month coincident with the occurrence of the full moon. The results not only confirm the suspicions of many past scientists but also suggest that the daily global temperature measurements are quite accurate. Most important, lunar influence is identified as another potential forcing mechanism to consider in the analysis of variability in the short-term, global temperature record.
Finally, I bring all of this up to remind myself of why it is that I took up the study of the weather and the climate—because I greatly enjoy being out in the weather, experiencing its multifold phenomena, and struggling to understand why it does what it does when it does it.
And so that is my wish for all of you— calm starry nights outdoors, good friends, and a glass of grog, with the moon a pirate’s silver coin rising out of the ocean, and the moon wind to blow your ship of life to the port of your dreams …
I have calculated the strength of the moonlight by multiplying the moon’s albedo times the TSI times the cross-sectional area of the moon. This gives something approximating the total luminance of the daylight side of the moon, in watts. I then divide this by the area of a hemisphere whose radius is the earth-moon distance, in square metres. This gives 0.006 W/m2 as an estimate of the strength of the moonshine.
For the IR a similar procedure is followed. The illuminated side of the moon has an average temperature of around 60°C (this is calculated as the fourth root of the average of the temperature to the fourth power). I converted this to W/m2 using the Stefan-Boltzmann equation with an emissivity of 0.95, and multiplied it by the cross-sectional area of the moon to give something approximating the IR luminosity of the moon in watts. Then I followed the same procedure as with the sun, dividing the total IR luminosity by the area of the hemisphere with radius equal to the earth-moon distance. This give 0.028 as an estimate for the IR from the moon.
These figures are in agreement with the estimates that I have found in the literature for those two variables, moonlight and moon IR. Note that there are a variety of simplifying assumptions in the calculations, as I am only interested in a rough calculation. Here are the figures I have used:
Moon Polar radius, 1736, km Moon Bond albedo, 0.11, units Solar irradiance, 1367.6, W/m2 Moon Cross-section, 9,467,805, sq km Moon Cross-section, 9.46781E+12, sq m Moon's Shortwave Luminosity, 1.42472E+15, watts Earth-moon distance, 378000, km Earth-moon distance, 3.78E+08, m Hemispheric area with earth-moon radius, 2.24E+17, sq. m. Lunar reflections at earth's surface, 0.006, W/m2 Moon surface temp., 60, °C Moon emissivity, 0.95 Moon's day-side average radiation, 663.54, w/m2 Moon's longwave luminosity, 6.28225E+15, watts IR at earth's surface, 0.028, w/m2 Total energy (short + longwave), 0.034, w/m2
… from Willis’s upcoming autobiography, entitled “Retire Early … and Often” …