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.