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Hollow Moon Planetology: Blue Moon - Climate & Weather

by Sharon Dornhoff

Lunar Climate Zones

Mean air temperatures in the Hollow Moon setting are pretty cold, hovering around 40 degrees F; however, thanks to the interacting, cumulative effects of vulcanism, crystalbarrens, angles of incoming sunlight and the variable thickness of the atmosphere, particular regions within the Nearside may be much colder or far warmer than this. The moon has no axial tilt to speak of, so temperatures don't rise and fall on a seasonal basis, but the equatorial regions naturally receive more sunlight than polar regions as with any spherical world. The lunar 18-month "year" is defined by its tidal cycles -- including Matera's unique Storm Cycle -- rather than by an annual orbit around the sun.

Over the course of a month, light levels warm things up just a little during the sunniest hours, and chills set in during the darkest. There's not nearly as great a difference between "day" and "night" inside Matera as there is on the surface of Mystara, though -- it's never a shift from light to dark, just from dark to very dark. Hence, temperatures in any given region of land will vary by only +/- 5 degrees, respectively in the warmest (skybright) and coldest (fulldark) parts of a month. One month is very like any other in its temperatures; it's the timing and type of precipitation -- rain, storm, sleet, or snow -- which sets the monthly weather-patterns apart, not heat or cold.

The warmest region in the HM setting, and the most well-watered, is the midlands. The Great Spindrift Sea and its southeast coastline, from Promontory Archerusia to the Straight Wall, are kept at a balmy (for Matera!) 68 degrees F by the extra-thick atmosphere which is held in place by Mystara's perpetual tide-pull. Although this is slightly below room-temperature on Mystara, the lack of winters allows both temperate and subtropical shade-tolerant plants and animals to live here in fair comfort. True tropical habitats are unknown in the Hollow Moon, so species requiring greater warmth than this are usually set down in the lee of a semi-active, midland volcano by the Immortals. Rainfall in the midlands is quite high, with light showers occurring during non-storm periods, as well as the inevitable Storm Times that arise in response to Pateran tidal forces. The only sizeable marshlands on Matera -- the Rilles and Palus Putredinis -- are kept boggy by these frequent rains. The subterranean geysers of the Stofler-Walter-Purbach-Arzachel Range, and the steam-plumes on the crowns of the Apennines, also have their aquifers filled by such precipitation.

To the east, the region of Maria Foecunditatis and Tranquillitatis are cooler (~55 F average) and less soggy than the midlands, with temperate conditions on the nearward shores of Foecunditatis, and the nippy, rarefied air of Mystaran mountain heights on the farward shoreline. This chill seldom drops below the freezing point, so crops can be grown here year-round; the fact that this eastern equatorial zone is the first part of the Nearside touched by direct sunlight each month, and the last region to go completely dark, makes this the only place in the Hollow Moon where grain crops -- as opposed to root crops or edible shrubs -- can be grown with any success. (Most grasses sown elsewhere can't handle the days-long interludes of darkness, between lunar dusk and dawn.) Rainfall comes at Storm Times and occasionally during an anticlockwise "circle tide". The isthmus which lies between the two saltwater seas and Mare Nectaris is covered by the Hollow Moon's only (temperate) rain forest, receiving moist air from over the surrounding bodies of water and getting drenched thoroughly from every month's rains.

North of these maria, the Taurus Mounts and points farward from there are much drier, with first scrubland and then desert lying behind the mountains. Storm times in the driest parts of the northeast Nearside are more likely to raise duststorms than to let down precipitation; barring unusual circumstances, true rains fall on the arid Sohktar lands only four times in the 18-month weather cycle. Like most deserts, Sohktar gets very cold (36 degrees F) during the hours of darkness -- i.e. all the time, in the HM! -- and it occasionally frosts over during fulldark. Still farther north, the glacial sheathing of Mare Frigoris demonstrates how the North polar region of Matera remains below-freezing (roughly 15 F) year in and year out; layers upon layers of snow and frozen runoff have encased this isolated mare in yards-deep, unmelting ice. The Alps and Juras are kept above-freezing by vulcanism, much like IRL Kamchatka, while the Alpine Valley's warmth has a secret origin all its own. Conditions out on the open crystalbarrens, themselves, have already been described. Blizzards and/or sleet are the norm here in Storm Time, and there's too little wind on Mare Frigoris to prevent its accumulation, the same way that unceasing winds clear the drifting snow off the non-glaciated, crystal flatlands.

Proceeding clockwise around the rimlands, Stygia and Mare Humorum aren't extraordinarily cold (~50 F) or dry, but they ARE very, very dark ... even by the standards of the Hollow Moon! The same peninsula of land which juts out to Mare Orientale, and encompasses the Rooks and Cordilleras, also shades this region from the light of the lunar "dawn"; this keeps the Stygian landscape extraordinarily gloomy and eerie-looking, even to visitors who know nothing of its inhabitants. East of Stygia, the shading-effect peters out, but the cold, dry climate continues well into the hsiao forests. Not until you reach Deslandres off Mare Nubium does the light become bright enough, and the climate, warm enough (60 F) for human farming techniques. In the extreme south, near the pole, an orbital quirk of Matera prevents the southernmost tip of the moon from ever receiving overhead sunlight: Whenever the Farside is turned to face the sun, the lunar orbit's slight skew also takes it south of the ecliptic, so the south pole remains in shadow. Combined with the lack of Nearside maria in the south and the usual effects of a polar latitude on weather, the south pole's "Shadow Zone" is generally between 25 and 30 degrees F, and almost totally devoid of water or vegetation.

Indeed, the whole region east of the Stofler-Walter-Purbach-Arzachels is quite dry, with the exception of areas on the maria coastlines. The southeastern Nearside is more savannah than desert, and the stormclouds which form in this area tend to drift away without shedding a drop. Only storms that fall at the "true" lunar dawn or dusk are truly reliable; less powerful tidal forces, as when Patera doesn't pass that close to its neighbour, can't always overcome the natural aridity of the region and get a real stormshower going. (When it DOES manage it, of course, there's usually floods....) Temperatures out on the Rheita savannahs fluctuate around 45 degrees F, give or take 5.

In addition to these large-scale climatic trends, there are also literally thousands of microclimates in the HM: volcanic rifts, swampy sinkholes, islands, copses, small plateaux, and (of course!) loads of "lost valley"-style hidden dells and dales, within which Immortals can stash their favourite endangered species while tweaking the local temperature and weather conditions to suit them.

The Storm Cycle

Because true weather-changes on Matera generally result from Patera's tides, it's possible to predict the occurrence of severe storms and other climatic events, usually to within the nearest seven-hour period. As a rule, Matera suffers weather-changes whenever Patera passes within a 15-degree geometric arc of its sister-moon: the closer the pass, the more drastic the weather tends to be. A pass at 15 degrees distance will generally only induce showers or a few hours of gusting wind; a pass at only 5 degrees can kick off sandstorms in Sohktar, trigger tornadoes in Rheita, or overturn small watercraft on the Great Spindrift Sea. (Passes which are closer than 5 degrees affect the Hollow Moon's geological activity, instead of its atmosphere; although technically both geology AND meteorology should be affected by such near passes, Seshay-Selene manipulates precisely where the invisible moon's gravity will have an effect, ensuring that Materans don't have to cope with restless volcanic ground, PLUS an unsettled atmosphere, at the same time.) Storms forming during a lunar night, when the upper atmosphere is frigid, tend to unleash snow, while those taking place by day -- the vast majority of "Storm Times" occur in the lunar daytime, as it turns out -- fall as rain.

There are 18 Storm Times in a lunar 18-month year, 12 of which take place in the daytime. The remaining 6 Storm Times -- along with two "Fire Times" in which natural disasters are common -- occur on the dawn and dusk preceding, and the dawn and dusk immediately following, the two yearly circle tides. Dusk storms tend to release snow, while dawn weather-events usually consist of sleet and/or ice-storms. The midlands seldom, if ever, experience snow or ice during a Storm Time, while Mare Frigoris' region receives nothing but snow, snow, and more snow. Any kind of dangerous weather that's appropriate to the geography of an area can erupt on Matera, during Storm Times ... expect for hurricanes, which may ONLY occur in a Storm Time that takes place immediately after a circle tide.

A full run-down on the Storm Times that take place during an 18-month lunar "year" will appear in a future post, along with Fire Times and the Hollow Moon calendar.*

[* - Anyone got any ideas for what I should name the months? It's a LOT tougher coming up with 18 names that sound good, than 12 ... especially when there's no seasonal differences to refer to, between one month and the next!]

Circle Tides and the Atmosphere

When Patera's orbit moves perpendicularly to Matera, the resulting "spiral" weather pattern makes winds blow counterclockwise the first time such a circle tide takes place in a year, and clockwise on the second occasion. Only under these circumstances can a hurricane ever form in the Hollow Moon, always over the Midlands Ocean (spiralling in the same direction as the circle tide, itself). And only by taking advantage of this remarkable phenomenon can a Voidship easily leave Matera's inner atmosphere to sail through the moon's interior void. For spacefarers, circle tides in the Hollow Moon serve much the same purpose as tubular breaches or Vortigern's Vortices on Mystara.

During the 3 1/2 days that a circle tide lasts, the Skyshield of Matera's inner atmosphere becomes stressed and elongated by the air's own rotation, above the "hub" of the midlands and lunar coordinates (0,0). Stretched out of shape -- drawn downward into a funnel and spun around and around, like the ocean's surface when it's distorted by a whirlpool -- the inner moon's Skyshield becomes so remarkably thin and attenuated, during a circle tide, that a ship or flying creature may pass through it unharmed and continue freely onward into the Void. On the positive side, this is much easier on one's ship than taking a ride on a tubular breach or risking Vortigern's route. On the negative side, the "funnel" remains open only during the three-and-a-half Mystaran days in which Patera orbits at a 90-degree angle to Matera, and wind patterns during that period will be totally unlike anything the ship's pilot might've previously learned about the Hollow Moon's atmosphere. Circle tides only happen once every nine months, so parties who miss this "launch window" and can't summon the magical firepower to open a Skyshield-breach artificially will be stuck with a long wait, before they get to try again.