LYR'S CLIMATE ZONES
by Chris Wayan, 2006
dedicated to Poul Anderson for his remarkable world-building
Lyr's homepage - map - creatures - evolution - climate zones - geology and geography - gazetteer - nomenclature - definitions - more worlds? Planetocopia!
Sections: Heat, Spin, Map of Climate Belts, Orbital vs Axial Seasons, Unfrozen Poles, Life Zones, Altitude, Gravity, Summary of Factors, Map
On our bright little thin-aired Earth, we think sunlight powers weather systems. That's naive. Venus gets more solar energy, but its weather's much blander. Its dense atmosphere doesn't just trap heat--it also distributes it better, so the equator and poles are equally hot.
So does a heat-gradient from equator to poles really drive weather systems? That hardly explains Saturn's winds or Jupiter's stormy belts. Their poles aren't much colder than their equators. But there's a second gradient hidden to surface dwellers like us (avian readers would think of it instantly): the gradient between warm depths and cold upper air. Hot air rises, cold air falls... convection loops develop even without the sun. Sunlight's more a trigger for an inherently unstable system. This explains why Saturn, say, with its feeble sunlight, still has seasons. The energy's internal--the sun doesn't drive the weather, just steers it.
But why does Venus have simple planet-wide winds, while Earth has three belts (tropical, temperate, polar), and Jupiter, many belts? You might guess that small worlds are uniform while big worlds are stripy... but that won't hold up. Venus and Earth are near-twins after all. Nor is it a heat gradient from equator to pole, or Earth would have thousand-mph hurricanes and Jovian stripes, while Jupiter and Venus would be equally bland. No, the missing factor here is...
The faster a planet spins, the more its weather forms lateral belts. This makes sense if you recall that there's a big energy gradient (as big as the heat gradient?) between the poles and the equator--1000 miles an hour! And that's just Earth! Lyr's huge, with a 12-hour day; stand on Equatoria and you're moving 7500 kph faster than you would atop Mt Cerberus near the south pole. 4800 mph is a steep energy slope--a slope that winds must climb, or descend. Earth's modest spin twists north-south winds into spiral weather patterns--storms. But what of planets like Jupiter where the convection patterns are less sun-driven? If Jupiter had no sun at all, it'd still be hot beneath and cold on top, and still develop convection cells. But they wouldn't become belts, as on Earth--local fountains and sinks like the Great Red Spot could be everywhere. Except for spin! It sweeps even random convection cells into belts.
LYR'S CLIMATE BELTS
Lyr's bigger than Earth, with a thicker atmosphere; but it gets less insolation. These two differences largely cancel out in terms of average temperature: less energy input, but more of it trapped by the greenhouse effect. The thermal gradient pole-to-equator is weaker than Earth, though far stronger than Jupiter. Lyr's spin is slower than Jupiter's, too--but not much. Put all these together, and you get neither Earth's three-cell system nor Jovian stripes, but...
Just because you're in a dry, high-pressure zone doesn't mean it won't rain! Yes, falling air starts out dry, but after traveling a few thousand km over Lyr's endless oceans, the air re-hydrates. This is similar to the formation of waves--the longer the "fetch", the stretch of water that the wind can work on, the higher the wave. A long fetch of sea, even in a drybelt, creates clouds--and rain. Florida and Hong Kong are in Earth's drybelt! It's only where drybelt winds sweep over a wide continent, or mountains high enough to catch most storms, that you get deserts downwind--in the interior or the west, where the dry descending air finds no place to rehydrate. Lyr's continents are small and the sea's everywhere. So most deserts are mere strips along west coasts with tall mountains; other drybelt lands are wooded or grassy, not desert at all.
ORBITAL VS. AXIAL SEASONS, or, THE PULSING SUN
Lyr's eccentric orbit means that the sun's apparent size changes during the year; light and warmth vary substantially. These seasons are called orbital summer and winter, and by their nature, they're planet-wide. Mars suffers strong orbital seasons, making temperature-swings harsher in the south. Earth, with a relatively circular orbit, has weaker orbital seasons--still, they're one reason Antarctica's far colder than the North Pole.
Lyr's orbit is quite oval, but in this era its aphelion (furthest point from the sun) happens to be at right angles to Lyr's axial tilt, so neither hemisphere is much harsher in climate. The northern hemisphere has a warm, early spring (short days but a big sun), a shortened fall (small sun), and an early winter--in essence, all the seasons are pushed forward. In the south, all the seasons come later. The overall ecological effect is modest. Orbital seasons are most visible, surprisingly, at the equator. On Earth, tropical rainforests have stable climates--warm, rainy, timeless. On Lyr they have distinct orbital winters (warm and rainy) and summers (hot and torrential). In drier near-equatorial lands, this can become true wet and dry seasons like Earth's monsoons. The cause differs, of course: winds aren't reversing, the sun's changing apparent size, like a throbbing heart! From mild warmth, to heat that drives hurricanes...
THE POLES: DARK YET UNFROZEN
Lyr's more tilted than Earth, with Arctic and Antarctic circles (beyond which the sun disappears in winter for at least a few days, and the midnight sun appears in summer) at only 54 degrees north and south. By our harsh, nearly-Ice-Age standards, Lyr's poles are merely cool-temperate, with icepacks melting completely in summer, so it doesn't mean so much, climatically, to cross the Arctic Circle. On Earth, these circles happen to be near the edges of the polar desert--cold, darkness and drought combine to form our polar barrens. But on Lyr, crossing the Arctic Circle doesn't mean going from wet to dry--quite the opposite in fact. Lyr's poles suffer long winter darkness, yet aren't that cold--and parts of them aren't dry at all. As in ancient Antarctica, forests on Lyr can flourish far above the Arctic Circle.
Even the winter dark's not where you'd expect. Lyr's higher gravity holds its dense atmosphere close to the ground. Air pressure drops fast as you rise. Such narrow, dense layers tend to bend light. It happens on Earth too--when you see the setting sun on the horizon, it's an illusion. It's really a degree or so below the horizon already! On Lyr, this lensing is stronger--from five to ten degrees. This greatly stretches out dawn and dusk. Absurd though it sounds, day and night are unequal on Ly--the year-round average, planet-wide, is nearly 60% daylight, 40% night! The poles benefit greatly from this lensing: the midnight sun appears as far south as 45 degrees, yet the midwinter darkness usually occurs only beyond 63-64 degrees. It sounds impossible, but big worlds are... peculiar.
Equatorial rainforest is hard to write about because it's impoverished in what interests me most--intelligent species. For example, Lannach in Diomedes is huge and lush, but isolated and ecologically monotonous; it has fewer intelligent species than the Oronesian islet of Eyath, a hundredth its size. My comparison's unfair--Eyath is a cultural crossroads, an informal capital with many quasi-diplomats and tourists who loved it and settled down. But many small islands with varied climates outdo all Lannach.
It may sound like I'm equating Lannach with the Amazon or Congo--lush but thinly inhabited, actually less friendly to people than, say, savanna or Mediterranean zones. I don't mean that at all--Lannach has koreens, lebbirds and tauraffes, plus occasional icari and sphinxes on the coasts, and even the odd, adventurous griffet; the Congo basin has at least four brainy species (gorillas, chimps, bonobos and a freshwater porpoise) and two huge-brained species (elephants, humans). The Amazon has humans, freshwater porpoises, giant otters, and many species of parrots, all highly intelligent. Not bad, really. Still, if we measure by people per ton of biomass, or intelligent species per million total species, such rainforests are surprisingly thin.
Of all Lyr's zones, the most sapiogenic (fostering intelligence) is the drybelt. For dry on Lyr isn't all that dry--mountains and east coasts still snag storms and water forests, while on the lee sides, savannas and deserts add to the diversity. The equatorial belt has greater biomass AND number of species, but is it barren in a subtler way? Are intelligent species indicator species, indicators of a subtler kind of diversity? Intelligence, deals in change, borders, diversity, novelty. Maybe the richest ecologies aren't the friendliest for consciousness! Just as music can grow too complex and blur into a wall of sound, can life flourish so much it forms a wall of green--a new kind of monotony?
Lyr's air pressure at sea level is high, around six atmospheres. But because gravity is also high, the pressure drops off faster as you climb; 3 km (2 mi) up, the air thins by half, to a mere three atmospheres. Around 6 km / 4 mi up, pressure drops to a mere 1.5 atmospheres--though there's nowhere you can climb that high, for Lyr's mountains are modest--the highest is 5100 m or 16,800', and most major peaks are just 3-4 km high--Alpine, not Himalayan.
But numbers deceive! The faster pressure-drop means that even modest ranges can sharply affect local climates. Highlands are climatologically about 30% "higher" than similar peaks on Earth. Not only are they cooler; Lyran mountains also wring more rain out of storms, since clouds cool faster as they rise. At some latitudes on Lyr, mountains can wring so much water from prevailing winds that you'll find deserts downwind, even along coasts.
Earth mountains have alpine zones and cast rainshadows, of course, but they're exaggerated on Lyr. For all practical purposes, a peak 3 km (2 mi) high has much the effect of a 4-km (13,000') peak on Earth; peaks 4.5 or 5 km high (and there are quite a few) are ecologically more like 6-7 km (20-23,000')--as formidable as the Andes, and almost Himalayan! Remember that, as you tour. You can freeze on the equator, if you're stupid.
Since the air pressure on even the highest peaks is still more than Earth's at sea level, life doesn't fail entirely. On most mountains, even though life-zones band more tightly and treelines are lower than they'd be on Earth, alpine life extends higher; while soil temperature and wind chill determine treelines, the cold but still-dense air of the heights holds more moisture and shields against radiation. There are few alpine deserts. If there's any summer thaw at all, life creeps up!
Weather and plant communities aren't the only things affected. A high proportion of Lyran animals are winged, including intelligent species. Since Lyran fliers evolved in such a dense soup, they're quite muscular but have smallish wings by Earth standards. I know my sketches look cartoony, but they aren't THAT inaccurate! Stubby-winged critters. So most sea-level species won't fly more than 4-5 km up or settle above 3 km (10,000'). On mountainous islands, high-altitude subspecies tend to evolve, with more Terran proportions. Interesting compromises arise between the need for greater thermal efficiency, and larger wings and lungs. We'll meet some of them.
Try not to gawk. You look funnier than they do.
On Lyr, your weight varies by latitude. The planet spins fast, and not only does centrifugal force lighten you as you stand on the equator, it lightens Lyr's hot, plastic mantle so much that the planet bulges visibly at the waist. That puts you even further from Lyr's dense core, and your weight drops even more. You're about 14% lighter on the equator than near the pole. On Earth it's less than 1%.
This gradient has some of the same effects on Lyran life as altitude: wing-to-body proportions shift. The effect's not spectacular, because (as with altitude) cold counters gravity--the need for thermal efficiency limits life's tendency to evolve larger wings and smaller bodies. Thus, gravity zones have little visible effect on life.
But travel, and you'll sure feel it. Flight is distinctly harder near the poles. Hold other factors constant--say, a warm sheltered coast in the north--and most creatures do have proportionately large wings and light bodies.
Am I stretching the definition of "climate"? Well, it's a consistent regional variation in the environment, and it sculpts life. What else can I call it? Thinking ecologically, and I'm always thinking ecologically, gravity has to be considered a variable much like insolation or altitude--clear climatic variables. As long as it is variable! On Earth, the gravity gradient's unnoticeable and biologically trivial. Not so on Lyr!
LYR'S CLIMATE FACTORS
TOUR LYR! Climb volcanoes, swim seas, meet weird creatures. First: survival tips! Then, pick a region:
Ythri -- Polesotechnic Chain -- Troisleons -- Roland -- Oronesia -- Gaiila -- Flandry -- Diomedes -- Ak'hai'i -- Averorn
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