Jarn’s Journal Year 3, Day 295

Year 3, Day 295

Well, they’re gone again.

I shouldn’t be surprised. They follow the herds, and the herds had gone on. I think now that they stayed in one place much longer than usual when Songbird led them to me, perhaps because Storm Cloud wanted to learn what she could from me. But I will be seeing them again at the Gather.

Songbird and Giraffe will be formally mated there, and I would not be at all surprised if Songbird is pregnant by then. Not that it will make the least difference to anyone; it is considered rather a good omen if the fertility of the bride is proven.

Meanwhile I might as well start deciding what I want to take with me and what is the best way to manage power. The solar panels are limited, they will not last forever, and I have no way of building more, so some other kind of power generation will be necessary if I plan to stay on this planet. As if I had any choice!

The computer needs electricity. It also has instructions on how to build a simple generator, though for that I will need to find copper and iron. Native copper is used by the People for ornaments, so it should be no real problem to form it into wires telekinetically. I may have to use some very delicate telekinesis to get iron from iron oxide, though.

The energy to run a generator? There are several possibilities. Direct heat from the sun, focused by a curved mirror, is one, and perhaps the best. A water wheel is a possibility as well, and the same spring that is supplying my drinking water runs into a stream flowing fast enough to use. I want something that will not require constant attention.

What to take with me? I am a little surprised at how much I have accumulated. Some of my first, rather crude containers can be left. The panels and insulation from the original escape capsule may still be useful, and of course clothing made by the People. My own crude attempts at tanning skins and using them for clothing can certainly be abandoned. Cooking utensils and the cooker from the capsule – yes, if I can manage electric power. I haven’t been using them here, but that’s because what power I have from the solar panels has to be used for the computer and lights.

I might as well start planning how to move the computer.

Jarn’s Journal is the fictional journal of a fictional alien stranded on Earth 125,000 years ago. His story is part of the remote background of my science fiction universe, the one in which Homecoming and Tourist Trap are set. The entire story to date is on my author site.

IRT Plastic: Using the Sun

IRT plastic in use. Note the puddled rainwater.

Some plants, like peas and lettuce, are happy enough with cool air and cold feet, but others insist that their roots be kept warm. This creates problems in areas such as interior Alaska where the ground is frozen so deeply that it may be well into fall before the soil is warm enough to satisfy corn, tomatoes, beans or squash outdoors.

It is possible, of course, to attack the problem with brute force. Build a greenhouse, or use heating cables in the soil. Mounding the soil also helps. So do raised beds. But all of these together are barely enough in the Fairbanks area.

Clear plastic allows the soil to retain the heat supplied by the sun. My own experience is that it also provides a perfect environment for weeds to grow under the plastic. Maybe they cook in warmer climates, but here in Fairbanks clear plastic can be pushed right up by rampant (and very healthy-looking) pigweed and lambs’ quarters.

Black plastic or landscape fabric? They stop weeds, and you’d think they would absorb sunlight and warm the soil. Nope. They’re not in good thermal contact with the soil, and while the black covers themselves may warm, they do not transfer that heat to the soil. Black covers have the net effect of shading the soil, lowering its temperature.

Luckily, it is possible to combine the two.

Ever seen a rainbow? Or the breaking of white light into colors by a glass prism? Then you are aware that sunlight is actually a mixture of light of different colors. What you may not know is that our eyes are sensitive to only part of those colors, and that growing plants need mostly the same colors that we can see. But only about half of the energy of sunlight is in these colors. A little bit is in the ultraviolet, the part of the solar spectrum that tans and sunburns our skin. That’s only a small fraction, and most of that is stopped by the ozone in the atmosphere. A much larger part of the invisible energy is in the near infrared.

Most of this near infrared energy passes pretty freely though most substances (such as air) that we consider clear. It is a large part of what makes sunlight feel warm. But plants cannot use it to photosynthesize, so if they get only near infrared light they cannot grow. No weeds!

IRT stand for infrared transparent, and IRT plastic allows the near infrared radiation through to warm the soil, but blocks the visible radiation that would allow weeds to grow. I’ve been using IRT ground covers to grow squash and beans for years, even though I grow them in raised beds.

It does have one problem: it’s waterproof. At one time, I could find it with microscopic holes that let rainwater drain through to the soil underneath, but all I can find now allows rainwater to puddle on the surface and the soil to dry out underneath. Until this year, I had to carefully shape the soil so that the plants (and the holes for them in the plastic) were in low spots. This year I’m trying something new. What? I’ll tell you when I know whether it works, but if you look, it’s visible in the photo.

Weight Gain

“Calories in – calories spent = weight gain.” Sounds simple and rather obvious – conservation of energy, right? But as applied, it makes some rather bad assumptions. And as many will testify, it doesn’t seem to work.

To start with, caloric input is NOT the same as the calories you eat. To some extent this is recognized. Cardboard has calories, but there is no way a human body can use them. While fiber (cellulose or soluble fiber) is often excluded from calorie counts, even digestible calories may not always be digested. The true caloric input is the calories your body is able to turn into glucose and lipids in your blood stream. I suspect that people vary quite a lot in how efficient their digestive systems are, and that may even vary with time for the same person. Certainly variation with time could help explain the “set point” for body weight.

Inefficiency in our digestive system? There are digestible calories in what comes out the other end, and not just in diabetics who lose sugar in the urine. Pigs and dogs scavenge human feces, among other things, if given a chance. It is the difference in calories between what we eat and what comes out that is the important energy input, and there has been very little study of how much that form of energy out might vary.

Then there is energy usage. Certainly exercise, even walking, burns more calories than simply sitting. But it takes energy to keep our body temperature up, our heart beating, our lungs expanding and contracting, and especially to keep those big brains operating. Sitting as quietly as you can in a cool room may burn a good many calories, though I wouldn’t recommend it as a way to lose weight. (It is, however, recognized as one of the reasons people working in the cold may need more calories. If your body is very efficient at all these “housekeeping” tasks (low basal metabolism) you may need fewer calories to maintain constant weight than someone whose basal metabolism is higher.

For that matter, some people may use their bodies in exercising more efficiently than others.

I strongly suspect this is an oversimplification of what seems to be a near-epidemic of excess weight. I certainly wouldn’t argue with the idea that something in our environment (including our food environment) is tinkering with the efficiency of our digestive processes, though I suspect serving size has a lot to do with it. But why don’t we ever consider calories out? It would be simple enough in test animals, if not in humans.

Jarn’s Journal: Year 2, Day 122

Year 2, day 122: Day 736 since my arrival

Last night I dreamed of flying.

It’s not something I’m very good at. I’m afraid once I decided to become an engineer and design starships I didn’t pay much attention to my esper lessons. But I’ve been forced to do a lot of esper over the last two years. Teleporting, perceiving, and telekinesis, mostly, but I’m dong all three much better than I ever did at home. So why not try levitation?

Not flying, exactly. But one of the things I’ve found I can do is teleport to a distinctive landmark. The higher I am, the better my chances of spotting a distant landmark I can use as a destination. So why not levitate to gain that height?

It does take just as much energy as I would need to climb to the same height. There is a way of getting around that, by using the energy of falling water or a landslide, but I’m going to have to learn how all over again. Even using my own energy, though, I managed to rise far enough into the air to see a distinctive tree and teleport to a spot above it. With practice, I could explore in much larger steps. And it wouldn’t wear out my sandals.

I think I will see what the computer library holds on levitation.

Much later in the day

Why didn’t my esper instructor tell me that all of that counterweighting and similar jargon simply referred to the conservation laws of physics? No wonder teleporting to a place at a higher altitude exhausts me; I’m using my own energy instead of swapping energy and momentum with my surroundings! I tried teleporting to the top of a butte while moving a similar mass of dirt and rock down, and it took almost no energy. The same with levitating to butte height. Water would work even better as an exchange medium, but for that I’ll need to find a waterfall.

So, my first priority is to practice exchanging energy and momentum with my surroundings, which should make teleporting much easier, and the second is to find a convenient waterfall. I wonder if I could locate that gather?

Author’s note: Jarn has finally worked out a calendar. He’s decided to start each year with the northward equinox, and to count the year he arrived as year 0. His Journal to date is on my Author Site.

The Four Horemen Revisited

I don’t often repeat posts, but with the projection of the  world population passing 7 billion this week, I thought it was time to bring this one out again.

Domestication is a mutual process—the plants and animals domesticated historically have met us halfway.

We and our domesticates have entered a kind of symbiosis—both we and they benefit, at least in numbers.

Plant and animal domestication was the first step toward civilization.

There are only two ways of increasing agricultural yield: Increase the amount of food produced per acre, or increase the amount of land farmed.

Once domestication occurred, we were locked into a positive feedback loop between food production and population. But a positive feedback loop is inherently limited and unstable. Are we approaching a crash?

I’ve been taking a Teaching Company course on DVD for the last couple of weeks, and I have to say it’s one of the best I’ve taken so far. I’ve always been interested in the process of domestication, especially since it became clear that the early agriculturists were generally less healthy than their hunter-gatherer ancestors. How did wolves become dogs? Who first thought of riding a horse? Did riding come before or after driving? And are cats really domesticated, or did they domesticate us?

The course is “Understanding the Human Factor: Life and Its Impact” by Professor Gary A. Sojka, but it’s really about human impact. I can’t say it answered all of my questions, or even asked them, but it did a good job of summarizing our current state of understanding, and of steering a middle course between “domestication is a sin and all domesticated animals should be returned to the wild” (most would not survive, and we probably wouldn’t, either) and “animals have no feelings and were put on this world solely for our use.” There are fewer moral problems with domesticated plants and microbes, though even here there are quandaries. How dangerous are monocultures, for instance? Or reliance on a small number of closely related varieties? (Think the Irish potato famine.)

If I have an argument with Professor Sojka, it is that he is too optimistic about the future. This may be appropriate for a college course, but I don’t feel enough sense of urgency. Yes, some people—a small minority even in the West—are beginning to think about long-term sustainability. (The politicians aren’t, by and large.) But the major problem—a population that is rapidly outstripping the carrying capacity of our planet (if it hasn’t done so already)—has become a taboo subject for serious discussion.  “The demographic transition will take care of it.” But will that happen soon enough?

Historically, our population has been kept in check by the Four Horsemen of the Apocalypse. Famine. War. Disease. Death by wild beasts—today, accidental death of all kinds. All of these are premature deaths—death by old age simply is not mentioned.

Today, we tend to regard such deaths—those of the young—as particularly tragic. We fight them in every way we can—and in many ways, we’ve succeeded. What we’ve forgotten is that every person born dies eventually, and to reach sustainability we have to reduce the number of people being born until it balances the number who die. Otherwise the four horsemen will eventually increase the death rate to match the birth rate—or more.

Food and energy both rely on sunlight—the sunlight that falls on the earth today and the sunlight that fell hundreds of million years ago, and is now stored in fossil fuels. I group food and energy for several reasons. Fertilizer. Biofuels. Pesticides. Transportation. Pumping water to where it is needed for crops, in some cases pumping down water that has been in storage since the ice age. All of the advances that have allowed us to hold back that horseman, Famine, ultimately rely on those fossil fuels and fossil water, or plan to replace them with agricultural products. And fossil fuels are becoming increasingly risky to exploit—look at the BP oil spill.

But an increase in agricultural output to match the increase in population means more efficiency—which we are obtaining today largely through fossil fuels—or more land in agricultural production. There is only so much land suitable for agriculture, especially if we want to keep the ecosystem services we depend on going. And one of the oldest causes for war is the desire for more land. Desire for more energy, often perceived as a need, is a rising cause of wars today.

Disease? In part that ties back to our methods of food production, as well. Certainly much antibiotic resistance can be linked to the widespread use of antibiotics in animals, and many diseases that started out in animals have crossed over to human beings. I find it interesting that all of the great world religions, many of them very pro-natalist, trace their origins to early city dwellers. Disease can spread rapidly among city-dwellers. In fact until the last century or two, urban areas were dependent on immigration from the countryside to maintain their populations. Having many children was important to these early city-dwellers—most of their children would die before having children themselves. That’s not true today, thanks largely to public health improvements—but the mindset and the religious imperative remain.

All living things—plants, animals, and human beings—are driven to reproduce. In our case, that deep-seated drive is reinforced by religious and social pressures. We claim we have a right, even a duty, to reproduce. But do we? Not in nature. Nature says the “right” to reproduce must be earned. It’s a lesson I hope we can learn before it is enforced by the Four Horsemen.

This is Post 486. Comment to join the drawing.

Lights, Batteries, Temperature?

I need to replace the bulbs in my outdoor lights—the porch light, the old dog run light, the lights over the garage door, and the light on the Arctic entry off the bedroom. And I find myself in a quandary.

Ordinary incandescent bulbs work at the outdoor temperatures we have up here in Alaska — below -40°F most winters, and not uncommonly below -50 or even -60°F. Their lives are probably shortened when they’re turned on at these temperatures, but they do turn on.

Incandescent bulbs, however, are being phased out. The idea is to replace them with fluorescents, and I’ve done that wherever possible indoors. I even replaced the hanging fixture over the kitchen table with a ceiling-mounted fluorescent.

Outdoors, however, is another story. Fluorescents (or rather their ballasts) simply will not work at the winter temperatures found in interior Alaska – or the northern tier of states, for that matter. Even low temperature ballasts only start working when it warms up to -20°F – and warms up is the way we think of it up here.

LED’s do work, and I’ve had outdoor LED Christmas lights for several years now. Over the last year, I’ve begun to see a few screw-in LED bulbs. But they are either very low light output (useful for replacing the bulbs in night lights) or highly directional – useful in some, but not all, of my outdoor fixtures. Yes, there are self-contained outdoor LED lights. They use batteries. See my earlier post on indoor-outdoor thermometers, and the problem with the outdoor sensor being battery-powered – even lithium batteries are questionable at temperatures below -40°F. And a size “C” lithium battery? Just try to find one! They’re available on line, but they are obviously a very expensive specialty item, and I’m not at all sure they’ll work at temperatures colder than -40°F.

It’s not the first time national policy has failed to take Alaskan temperatures into account.

I am reminded of my first new car – bought the year Congress mandated seat belt interlocks, which required that you have the seat belt buckled before the car would start, and which activated a blaring alarm if the seat belt was not buckled. 1973, I think. Fine, I thought. I put on my seat belt as a reflex. My father drilled holes in the frame of our old Woody so he could install seat belts. I’d never be bothered by failure to do something as automatic as that.

Turned out the car I got had two switches to implement the Federal requirement. One was in the seat, and turned on the seat belt safety mechanism if there was weight in the seat. The other was in the buckle, and told the car whether the seat belt was buckled.

The switch in the buckle did not work if the temperature of the buckle was below about 0°F.

I did not have a heated garage then.

I finally figured out that I could start the car at low temperatures by bracing myself between the back of the seat and the floor, so no weight was on the seat. Once the interior warmed up, the alarm would quit.

That worked until the temperatures got below -40°F, and the rather poor heater was unable to bring the interior temperature of the car above 0°F. At those temperatures, the alarm screamed constantly – a serious distraction while trying to drive in ice fog with frosted windows. I would never have heard a siren, for instance.

The dealer said sorry, federal law prohibited them from touching the interlock system, never mind that it wasn’t working properly and was a safety hazard rather than a safety feature.

Cars are not my thing. I lived with that alarm for the next couple of months, until the ban on interfering with the system was removed January 1.

It got disconnected January 2.

North Pole Weather

It’s weather Monday again. Sunrise this morning was at 6:09 am and sunset will be at 9:34 pm, with civil twilight extending nearly an hour before and after the sunrise and sunset. The day is now 15 hours 25 minutes long, and we’re gaining 6 minutes 53 seconds a day. Driving light is actually almost 17 1/2 hours.

The snow is still melting pretty slowly, at least where it’s pristine enough to reflect the solar radiation back to space. It’s evaporating and melting internally, but the rate of disappearance is very dependent on the surface and the surroundings. The snow stake, in the middle of my south yard, shows 14″ of snow. The area just next to the south wall of my house is clear of snow. Snow on the north side of the house appears untouched in the shadow of the house, but is gone where sunshine can reach and the birch seeds and rose bushes have darkened it. Official snow depth at the airport is 17″. As I said last Friday, vegetation and exposure have an enormous impact on snow melt. It’s interesting also to note that low spots, where windborne seeds and dirt collect, melt faster. And I’m hoping my black compost barrel will get warm enough in the sun to get the composing process started even with snow still on the ground.

It’s slightly cool for this time of year, and the forecast for the next few days is basically more of the same. Highs around 40, lows in the low 20’s, and no precipitation. April is our driest — and muddiest — month, with precipitation slowly increasing until we reach a peak in August. This April so far we’ve had 1.7 inches of dry, fluffy snow — .07″ of water equivalent. It’s still too early to tell what the effects of the rain last November will be, but at least most of the ice is gone from the roads. Mud, however, is starting to appear wherever the snow is gone.

Mass into Energy

“If you can Heal, you may eventually be able to exchange mass and energy if you conserve hadrons,” Derik told Roi. (from Homecoming.)

Exchange mass and energy? And what are hadrons?

Einstein’s famous equation—E=mc²—says that a very small amount of mass can be turned into a very large amount of energy or vice versa. If you could turn a gram of mass (that’s about the mass of a small gourmet jellybean) into energy, you’d get roughly 25 million kilowatt-hours. In practice, you have to conserve hadrons—essentially the total number of neutrons and protons in the cores of atoms, which make up most of their weight.

Thus it is possible, with enough heat and pressure, to force four hydrogen atoms (each with one proton and one electron) together to form one helium atom (with two protons, two neutrons and two electrons.) The four hydrogen atoms turn out to have about 1% more mass than the helium atom, and this extra mass, the binding energy, reappears as gamma rays and particles. This is the energy of the hydrogen bomb, and also the energy that powers the sun and most of the stars. The process is called fusion (coming together), but as of yet we cannot control it. (I don’t think you can call a hydrogen bomb controlled.)

A second way in which mass can be transformed into energy involves very heavy elements, such as uranium. It turns out that all of the protons and neutrons packed together in these elements aren’t really happy. They may split apart on their own, or because they are hit by some other particle, but the net effect is that the mass of the parts they split into is less than that of the original atom, so the split produces energy. This is nuclear fission (splitting.) This is the energy of the original atomic bomb, but it is also the energy that drives plate tectonics and volcanoes, and is occasionally tapped for geothermal energy. This one we can control to some extent, and it is the energy source for nuclear power plants—but those large, unstable atoms are rare in nature.

The middle-weight elements—such as iron—are stable. There is no way to extract binding energy from iron—it is sometimes called the nuclear ash of stars. (Note that many of the elements of all masses have isotopes in which an imbalance between neutrons and protons leads to instability, but these are very rare in nature. They are very important in man-made nuclear waste, however.)

Most of the energy we use today comes from sunlight past or present—fusion energy. Solar energy intercepted by the earth today fuels not only what is called solar energy but also wind power, hydropower, and biofuels. These are all “renewable” sources—we can use them at the rate present-day sunlight generates them. But they are limited by the amount of sunlight available.

Fossil fuels are storage for the sunlight of the past—and we are using them up far faster than they can be replenished. Only a tiny fraction of the sunlight falling on the surface of the earth is actually captured as biomass, and only a tiny fraction of that biomass is buried and eventually becomes coal, oil or natural gas. But ultimately, it is all nuclear energy.

So yes, mass can be turned into energy, and in fact almost all of the energy we use actually comes from such conversion. But there are limits.

Planet Building 8: Weather

We’ve talked about the need to transfer energy from the surface of a planet to high in the atmosphere, where greenhouse gasses can radiate it away. But solar energy is not absorbed uniformly over the face of a round planet, or uniformly in time if that planet is rotating. This means energy must also be transferred horizontally, and water plays an important role here, as well.

Since we’re close to the equinox, let’s look first at how incoming energy is distributed then. The sun at this time of year is almost directly over the equator, and on the horizon at the poles. This means that the incoming energy is at a maximum—in both time and space—at the equator, and zero at the poles.  Unless the poles are at absolute zero (- 460 degrees F) energy must be transferred from the equator to the poles.

Remember that liquid water can store an enormous amount of energy as heat, and about half of that transfer is actually carried out by the ocean currents. When you add the rotation of the planet, we have currents like the Gulf Stream and winds in temperate latitudes generally blowing from west to east. The result is that west coasts are generally warmer than east coasts—southern France, for instance, is at the same latitude as Maine. This will carry over to any planet with oceans, and because oceans are so good at storing energy, it is true at any time of year.

The atmosphere also carries energy from the equator to the poles. In fact, this transfer of energy is what drives the whole weather system. The eddies we call cyclones and anticyclones, the lows and highs on the weather map, carry energy from the regions of surplus near the equator to the regions that radiate energy away near the poles.

Some of this energy is carried by warm air moving poleward while cold air moves equatorward.  But a large part is carried by latent heat, just as in pure vertical transport. As the warm, moist air moves poleward, it tends to ride over colder air coming equatorward, and as it is lifted and cools, water condenses. Thus the latent heat added by evaporation over the tropical oceans is released to the air at much higher latitudes.

What about other times of year, such as the solstices?

The solstices are defined as the dates when one or the other of the planet’s poles are pointed most nearly directly at the sun. This affects day length as well as the angle at which the sun falls on the surface, with the surprising result that for the earth’s axial tilt (23 degrees 27 minutes) the maximum incoming energy at the top of the atmosphere is at the summer pole, and there is very little variation of incoming energy with latitude in the summer hemisphere.

Ice at the Earth’s poles keeps surface temperature low and the low angle of the sun means more energy is absorbed in the atmosphere, but high midsummer temperatures at the pole would be quite possible on a world without water. If the tilt were increased even farther, to around 70 degrees, the annual average incoming radiation at the poles would actually exceed that at the equator.

Another important way in which water affects temperature is its large capacity for storing heat, which means that in summer water is generally cooler than land. This leads to coastal climates being cooler in summer than those inland. It also leads to the summer monsoons, when hot air rising over the continents sucks in the cooler, moister air from the nearby oceans.

This is reversed in the winter hemisphere, where the water is usually warmer than land. Cold air flows out from the continents, often triggering storms when it moves over the warm, moist oceans.

Because the energy input near the equator is only a little less than during the equinoxes, while that of latitudes above the polar circles is zero, a great deal of energy must be transferred from the equator to the pole of the winter hemisphere. Thus the violence of winter storms, as energy is transferred both upward and toward the winter pole.

Finally, the daily cycle is hardly noticeable in water temperature, but it has a large effect on land temperature. The result is local winds from colder to warmer temperature near the surface—the sea breeze in the daytime and the land breeze at night. In some cases these moist, cool winds from the sea trigger thunderstorms when they move over the warm land.

All of these types of weather will be found on any rotating planet with oceans of liquid water. Don’t think they are “earth-only” effects in building a planet.

Planet Building 7: Tropical Cyclones

What time of year? Marna frowned a little. She had noticed the days were getting shorter when—over a month ago?–and since she wasn’t living by clocks and calendars, the actual solstice had probably been considerably before that. Late summer, then. She looked again at the sky to the south, and felt a sudden chill that had nothing to do with the wind. Tropical storm season. She’d forgotten about that, too, living on the satellite. She reached southward with her mind, and gulped at the intensity of the swirling winds she felt. She was only on the outermost fringe of the storm, and already her little boat was close to its limits. If the storm moved north …. From Homecoming, by Sue Ann Bowling

Is it going too far, to have hurricanes on another planet?

No, not if the conditions for tropical cyclones are present.

Three conditions are critical. First, the planet must have liquid water in fairly large quantities—oceans, in fact. Second, the planet must be rotating. Third, the temperature of the water must be high enough. The actual paths and frequencies of tropical cyclones are strongly influenced by details of the atmospheric circulation, but these three are critical for a planet to have tropical cyclones–also known as hurricanes and typhoons–at all.

The need for water is fairly obvious—you can’t have clouds without water.

The need for the planet to be rotating may not be as obvious, but on a rotating planet things moving on the surface of the planet do not (except at the equator) move in straight lines. If the north pole of the planet is defined so that sunrise is on the right hand when facing north, anything moving in the northern hemisphere is deflected to the right. In the southern hemisphere, deflection is to the left. Because of this deflection, called the Coriolis effect, air moving toward low pressure spirals in rather than flowing directly toward low pressure.

Among other things, the rotation of our planet causes the spiral structure of a hurricane as seen from space. Remember this does not happen right at the equator, and in fact tropical cyclones cannot form at the equator. On Earth, the earliest development of a tropical cyclone takes place at least 300 miles from the equator.  On a planet that rotated faster than earth they could form closer to the equator; on a planet rotating more slowly, they would need to form closer to the poles.

This is where temperature comes in. It turns out that when the water temperature is 80 degrees F or warmer, and winds are accelerating evaporation by whipping up the water’s surface, the rate of evaporation, and thus the transfer of latent heat to the air, provides enough energy to speed up the wind and increase the evaporation. In order to keep this process going, the depth of the warm water must be over 150 feet.

This is pretty warm water. In fact, warm, deep water far enough from the equator to allow hurricanes to form is found only after the summer heating period—summer and fall.

There are other conditions. The instability of the air is important, as is mid-level humidity, the lack of vertical wind shear so the top of the developing storm is not separated from the base, and an initial area of disturbance. But the temperature of the water is critical, and is the reason hurricanes lose force so rapidly when they move over land.

Does this mean that global warming will increase hurricanes and related storms such as typhoons? Possibly. It will almost certainly increase the area over which tropical storms can form—the area with water warm enough and deep enough more than 300 miles from the equator. But other changes, more difficult to predict, may also affect hurricane formation and steering.

At any rate, rotating planets with oceans and with temperatures similar to or warmer than the Earth would be quite likely to have hurricane-like storms.