Saturday, December 28, 2013

Sea Smoke

Note: This program first aired on December 28, 2013.

We’ve had some very cold weather so far this winter, and beyond the obvious experience of stepping out the door on an arctically cold morning, I can tell it is cold when I am driving to work and I see sea smoke on the harbor. Sea smoke is the wispy mist that forms on cold days just above the surface of open water, and when you see that, you know its cold out.

Sea smoke is like fog, it results from the difference in temperature between the air and the water. I wrote about this many years ago on this show, but I think it is time for us to revisit it. The fog we encounter in the summer comes from warm air hitting cold water, or a rapidly cooling landscape. Warm air can hold more water vapor than cold air and when the warm air gets cooled, it can’t hold as much water vapor. The water vapor in the air has to go somewhere, and it condenses out as fog.

What we are really talking about here is relative humidity. Air of any given temperature can hold a certain amount of water molecules in the gaseous phase—water as a true gas. We can’t see water in this phase—its different than steam, or seeing your breath. The maximum amount of water vapor the air can hold at any given temperature is called the saturation point. When air is holding as much water vapor as it can at its given temperature, it is said to be 100% saturated. Its relative humidity is 100%, because, relative to its temperature, it is holding as much water vapor as it can. As air cools down, its relative humidity rises. The amount of water the air may contain doesn’t change, but the ability of the air to hold that water does, when air cools down to a temperature where it is 100% saturated, we call that temperature the dew point. If the air cools below that, liquid water will condense out of the air on any surface it contacts. In the winter when this happens, it forms frost.

Sea smoke is different than the regular summer fog we can see on land and in river valleys as well as on the ocean. In the regular fog scenario, the water vapor is in the warm air mass, and only condenses because the warm air gets cooled when it encounters cold water, or cools as it travels up over a ridge line or pools in a valley. The source of the water vapor in sea smoke is the sea, not the surrounding air mass. Very cold winter air can’t hold very much water. Its relative humidity may be high, but 100% of a tiny amount is still a tiny amount. That is why everything dries out in the winter, your skin, your wood flooring, your shingles, you get the idea. Sea smoke forms when a very cold air mass rests over warmer water, though again, its all relative. The water only has to be warmer than the air. For example, during this recent period of cold, the air temperatures were around zero many mornings; according to real time buoy data water temperatures are in the low 40’s, Fahrenheit. That is much warmer than the air was, but hardly what any of us would call warm.

The warm water warms the air directly above it, on a large scale this is the reason that the immediate coast tends to have milder temperatures in the winter. That slightly warmed up air can hold more water vapor, and water is constantly evaporating from the surface of the ocean. Warm air also rises, so this slightly warmed, more heavily laden with water vapor air rises up into the much colder air around it, where it cools off. When it cools, it can’t hold as much water vapor, and that water vapor must condense out into liquid form, or as often the case with sea smoke, as ice crystals. As the sun rises further and warms the air mass more uniformly, the sea smoke dissipates, so when you see it, you know you are seeing a special and ephemeral combination of elements; here one moment, gone the next. You also know, its cold.

From a class at the University of Illinois, a nice diagram showing the non linear relationship of temperature and the overall amount of water in the air mixture.


Great old article from the Woods Hole Oceanographic Institute:

Here’s a much better use of your time than Facebook, its real time buoy data from the Northwest Atlantic:

Thursday, December 26, 2013

La Luna: The Lunar Environment

Note: This program first aired December 21, 2013.

As we wrap up our lunar series with today’s program we’ll take this time to think about what its like on the moon. If we were to go there, what would we experience? Only 12 people can answer that question from first hand knowledge, here’s what one of them had to say: “It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn’t feel like a giant. I felt very, very small.” Those are, of course, Neil Armstrong’s words, reflecting what I believe would be the near universal humbling of seeing our home planet from outer space.

If we fast forward forty four years, we find a renewed interest in the moon. China has just successfully landed a landing craft and robotic rover on the surface of the moon, giving the world fresh images and insight on the lunar landscape.

When you watch the images of the Chinese lunar rover leaving the landing craft, or of Neil Armstrong walking around during his moon walk, the first thing you will notice is the dust. It appears that the moon is covered with very fine dust. It results from the impact of millions of micrometeorites, which pulverize the lunar surface rocks. The rocks get so hot on impact that they vaporize, and then condense as tiny glassy spheres, the dust particles we know so well from images of rover tracks and footprints on the moon. Those footprints will be there a while, because the only thing that can make them go away is more impacts from these micrometeorites. There’s no wind or rain on the moon, so there are really no forces of erosion, at least like we have here on Earth. That’s why the moon is still covered with craters. There’s nothing to make those craters disappear. The only factor you can think of as erosive is the ongoing micrometeorite impact issue. It transforms the surface of the moon very slowly,  and not especially dramatically.

And why is there no wind on the moon?  Because for all practical purposes there is no atmosphere on the moon. Wind as we know it is a function of pressure differentials in the atmosphere, air moves from areas of high density (and pressure) to areas if low density (and pressure), at its root it is simple diffusion. If there is no air, or gaseous atmosphere of some other mix, there can be no wind, because there is nothing to move around. To be fair, the moon does have an incredibly diffuse amount of gas at its surface, sometimes called an exosphere. But these gaseous molecules are so few and far between, they don’t behave in ways remotely like an atmosphere.

Another impact the lack of an atmosphere has on the moon is temperature. By now we are all familiar with the concept of the green house effect here on Earth. The Earth’s atmosphere allows in light energy from the sun, and then traps that energy as heat, keeping the planet warm at night. Incidently, the other thing our atmosphere does is NOT let in all the light energy from the sun. A large proportion is reflected back into space, which prevents us from getting too hot! The atmosphere moderates Earth’s temperature in two ways, preventing excess energy from entering, and preventing heat from escaping back into space. You may see where this is going. The moon, lacking an atmosphere, also lacks temperature moderation. So during the lunar day, temperatures can reach 100 C the boiling point of water, or higher.  During the lunar night, temps can then plunge 250 degrees C, dropping to about 150 degrees below zero. 

And why doesn’t the moon have an atmosphere? The short answer is that it isn’t big enough. It is gravity that keeps the Earth’s atmosphere on the Earth, and the moon simply isn’t big enough to have enough gravity to keep gaseous elements around. It’s also no longer tectonically active, so there are no volcanoes to out gas and supply the surface of the moon with gaseous elements.

And lastly, I mentioned the lunar day. You might be surprised but a lunar day is quite a bit longer than an Earth day. An Earth day is 24 hours long, the time it takes for the Earth to complete one full rotation. For the moon to complete a full rotation it takes about 27 Earth days, which is also the same amount of time it takes for the moon for to complete its orbit of the Earth. You might have to let this sit in your brain for a little bit, but the net result of this is that the same side of the moon is always facing the Earth.

So there you have it; if we were to go to the moon right now, we’d take a walk around a dusty, very still, crater pocked vacuum. We would either be incredibly hot and well lit or incredibly cold and in the dark for nearly two weeks, and we would weigh about 1/6 of what we do here on Earth. These conditions may sound appealing to some and uncomfortable to others, but I think we would all benefit from the perspective the view of Earth rise over the moons horizon would bring.


Info about the 12 people to have walked on the moon:

New York Times article about the Chinese lunar mission:

Images of from the Chinese lander and rover:

Science Daily on lunar dust:

Great list serve on all things astronomical

Cool site with lots of interesting lunar info, widgets and apps:

La Luna: Moonrise

Note: This program first aired December 14, 2013.

A few years ago, I started to make my own little Stonehenge. From a single spot on my land, I watched the sun as it changed its position on the horizon, with the seasons. I wanted to mark the spots on the horizon where the sun rose on the summer and winter solistices, the days that are the end points of the sun’s journey back and forth across the sky. I think I understand this pattern, and by marking it on the landscape, I could check my reckoning on a yearly basis, and test my hypothesis that this pattern is unchanging, at least on the time scale of my life. I never did move any large rocks around , but the idea still stands.

A celestial pattern I find more difficult to follow is that of the location of moonrise. One might think it would be easier to follow, being a shorter cycle and changing every night. But when I looked into it I realized just why I’ve never been able to “get” the pattern of the location on the horizon of moonrise. Its because its fantastically complicated. This may be one of those examples where a picture really is worth more than a thousand words, but I will attempt to explain it anyway.

The first thing to know is that the plane formed by the orbit of the Earth around the sun is called the ecliptic. Star gazers know this term because it is the path that the sun takes through the sky. At night it is the line across the sky where you will find the planets, as all of the planets orbit the sun on this same plane. If the Earth were spinning on an axis that was perfectly perpendicular to the plane of the ecliptic, our observations would be oh so much easier to explain. The path of the sun would be in the same place in the sky year round, rising and setting at the same spots on the horizon each day. The height of the sun in the sky would correspond with your latitude, at the equator the sun would always be directly overhead at noon, at the poles it would be on the horizon. If the Earth were rotating perpendicular to the plane of the ecliptic, it would be an equinox every day. And there is something appealing and egalitarian about that, but its not how it works. The Earth’s axis of rotation is inclined 23.5 degrees off the plane of the ecliptic, which is why we have seasons and different day lengths and all that good stuff. Its also why the sun’s point of emergence on the horizon migrates north and south over the course of a year. For example, in the summer, the northern hemisphere is inclined towards the sun, leaning into the plane of the ecliptic. That means the arc of the sun across the sky is higher in the sky than it would be if the Earth were straight up and down. The arc of the sun is higher, and the points of the sun’s emergence and stetteing on the horizon are further north (going along with that higher arc). I go through all of this for the sun and the Earth because it’s the easy example. In this case, the slowness of the year long cycle makes it easier to keep track of.

For the moon it all happens much faster. Just like the Earth orbits the sun, the moon orbits the Earth. The plane of the moon’s orbit around the Earth is not the same as the ecliptic. The moon’s orbit is inclined off the plane of the ecliptic by about 5 degrees. So over the course of a month, the moon travels around the Earth, and is at times as much as 5 degrees above the ecliptic, and at times 5 degrees below it. Put another way, half of the month the moon is above the plane of the ecliptic, and the other half it is below. This also means that there are times when it is exactly on the plane of the ecliptic, these are called the nodes, and incidentally, eclipses can only occur when the moon is at the nodes of its orbit.

Image of moon’s orbital plane vs. Earth/sun (from the Hyperphysics instructional website):

 So we know that the latitude of sun rise changes as the Earth’s angle relative to the sun changes over the course of a year. The same holds true for the moon. The apparent latitude of the moon rise on any given night is determined by the combination of the Earth sun relative position and the position of the moon in its orbit. It’s a cycle within a cycle within a cycle, which is why it is so wildly difficult to follow. It has a regularity to be sure, but one that changes constantly. I think that only the most dedicated sky watchers, and mathematicians have sorted it out.

Perhaps that is the basis of the idea that the moon makes us crazy. Not only does it change shape every night, but it moves about the sky in a different pattern each month, in sharp contrast to the plodding regularity of the sun and its seasons. At this point, I think I will stick to my solar Stonehenge, and simply keep watching the moon, delighting and finding humility in the surprise of where I find it on the horizon and in the sky each night. Maybe some day I will have watched long enough to unlock the pattern, but I am many nights way from that as of yet.


Find the time of moon rise, and set and lots more data here:

Neat stuff from Cornell University, complete with podcasts:

Most of the stuff out there about the moon’s orbital plane stems from questions about eclipses. This website is an example, it has a nice visual graphic that shows the moon’s offset orbital plane, in the context of explaining why and when eclipses happen

La Luna: Phases of the Moon

Note: This program first aired December 7, 2013.

If you have been doing your homework and keeping a moon journal, by this time you will have noticed many things about the moon, the first of which is that it doesn’t look the same every night.

The only reason we can see the moon at all is because it is getting hit with light from the sun, that light reflects off of it and that is what we see. Recognizing that relationship with the sun is the first key to understanding why the moon looks different on different nights. No matter where the moon is in the sky, fully half of its sphere is getting hit with sunlight. The same is true for Earth by the way, hold a tennis ball up to a lamp and you will see what I mean. Light illuminates half of the surface area of the sphere, and the other half of the sphere remains in shadow. The moon regularly changes its position in the sky, relative to the Earth and the sun, and what this means in practice is that sometimes we see the fully illuminated face of the moon, other times we see only part of that illuminated face, and still other times we only see the unlit face of the moon.

When the moon is oriented so that we see its entire illuminated face, we call this a full moon. This occurs when the moon is lined up with the Earth and the sun (in that order) in a straight line. The sun’s rays pass by Earth and hit the moon.  The fully illuminated face of the moon is what is facing the Earth. If you are drawing this picture right now, you might say “Hey, isn’t the moon in the Earth’s shadow?” This does happen, but only very occasionally, and when it does, we call it a lunar eclipse.  For the most part, the Earth is quite small relatively speaking, and the shadow it casts does not line up with the exact position of the moon.

When the moon is lined up with the Earth and the sun, but on the other side of the Earth, the one between the Earth and the sun, the only side of the moon that is facing us is the non illuminated side. We see precisely 0% of the lit side of the moon. This we all know as the new moon. To say we see it is really not accurate, we don’t see it at all, but there are a few reasons for that, we’ll get into in another program.

When the moon is not lined up with the sun and the Earth, we see something between 100% and 0% of the illuminated face of the moon. For example, when the moon is oriented exactly perpendicular to the line created by the sun and the Earth, we see half of the moon’s illuminated face, the half circle hanging in the sky. It is called a quarter moon, because technically, it is only a quarter of the moon’s face we are seeing, and because it is one quarter of the way through the sidereal cycle of the moon’s path around the Earth.
The moon changes position because it is a satellite, and it is orbit around the Earth. Each day during its orbital path, it is in a different position relative to the Earth and the sun, so each night, it looks a little different because a little more or less of its illuminated face is visible to us.

We will talk more about this pattern in coming weeks, but for now realize that it is indeed a pattern. How dull it would be if the moon rose at the same time in the same place and was the same shape every night. We would forget about it. The moon’s actual recurrent pattern keeps us on our toes, our very bodies are calibrated to it. When we have a week of clouds and I lose track of the moon, the first clear night brings with it surprise—Can it really have changed that much in just a few days?

The moon’s daily shape shifting waits for no clouds, no over scheduled lifestyles, no busy evenings. It carries on relentlessly, without us. It needs neither our attention nor our devotion to continue its trip through space and time. The responsibility is entirely on us, to pay attention to that celestial continuity and the tidal rhythms in our own bodies. We ignore these at our peril.

La Luna: Lunar Geology

Note: This program first aired on November 23, 2013.

--> One of the questions that is inevitable when we contemplate the moon is how did the moon get there? Was it always there? And what does “always” mean anyway? The moon’s origins date back to almost the origin of the Earth 4.6 billion years ago. The Earth formed from material that slowly coalesced, gas and dust that were part of a nebula that collapsed and became our sun. All the planets were formed from the dregs of the remaining left over material.  Early Earth would have been a difficult place to visit, as it was covered by a magma ocean. The whole planet was liquid rock, heated by the radio active decay of elements in the core, and heat transferred to the surface by near constant asteroid impacts. The early days of the solar system were chaotic, and asteroid strikes were very common.

Because the Earth was at that time, entirely molten, fluids of different densities were able to sort themselves out, following the laws of physics. Denser materials were able to sink towards the center of the Earth, less dense materials floated outwards in response. By the time of the moon’s creation, 100 million years or so after the creation of the Earth, the Earth was already stratified, the layers we all learned in grade school, the core, the mantle and the crust were differentiated by their differing densities. This fact is im portant because it relates to one of the key clues we have about the origins of the moon.

The current most widely accepted theory of the origin of the moon is the impact theory, which states that the moon formed when a very, very large asteroid, or small planet collided with the Earth, and the force of that impact caused a huge mass of material to be ejected from the opposite side of Earth. That ejected material coalesced and became the moon. So the first point to remember is that the moon is made of Earth material. At the time of this collision, the earth was still covered by the magma ocean, meaning, it was still so hot at the surface that the rocks were still molten. The material that would make up the crust, when it cooled down enough to form, was what was at the surface, but in fluid form. When the asteroid hit, it was this material, and some of the underlying liquid mantle rocks, that were thrown up into space to become the moon.

We know this because the moon has differentiated layers, a crust, a mantle and a core, just like the Earth. This tells us that also just like the Earth, the moon was at one time, entirely molten. The only way the materials of a planet can differentiate by physical properties is if those materials are fluid enough to move. The moon is a great deal smaller than the Earth though, so those layers have cooled much faster than Earth’s layers. The moons layers also differ from Earth’s in their composition. The Earth’s core is made of metal, iron and nickel primarily, the heaviest of commonly occurring elements present when Earth formed. The moon’s core lacks large quantities those heavy metal elements which tells us that though the moon came from Earth material, most of that Earth material came from what would end up as Earth crust and upper mantle. The heavy material on Earth had already sunk down to the center of the Earth, forming the core, and was thus not available to be ejected from Earth when the asteroid hit. On a side note, the lack of iron in the moon’s core means that the moon as an incredibly weak magnetic field, as it is the spinning of the iron in a planet’s core that generates that field.

So when I said last week that the moon is made of us, I meant it. The stuff of our bodies, the elements we take in from our food, we breath in with the air came from no where else but the surface of the Earth itself. We are literally made of Earth, and Earth is made of star dust and elements formed inside long dead stars. And the moon is also made of Earth, so when you look up on these winter nights, see her for what she really, on some level, is: your sister.


Age of the solar system They also have many other terrific short articles on the age of the moon, and the Earth, and all kinds of other things astronomical.

From the Planetary Science Research group, info about lunar geology:

Many scientists are very devoted to the moon, and fund their own institutes pursuing lunar science:

Here’s an entire e-course on Astronomy from the University of Northern Iowa:

They like to chat it up at NASA, here’s the transcript of a NASA chat about the moon: