Previously, I
outlined method for deriving planetary parameters based on your world
map. I will now continue with this.
I. Surface gravity:
Though we have already covered escape velocity, surface gravity is
different. I have not found a calculator to do this, but it is not
difficult. The formula is g = G
* M / r2.
G
here is the gravitational constant. Multiply your planet’s mass by
6.67 * 10-11.
Note the negative integer. Take the resulting value and divide it by
your planet’s radius (in meters) squared. The result is in
meters/second2.
You can find calculators on the internet that will convert this to
feet/second2
if you wish.
II.
Axial tilt. The Earth is tilted 23.25°.
This tilt changes over the course of thousands of years, and the
Earth is currently at about the midpoint of its range. This, like
rotation, is determined at planet formation and influenced by
bombardment of comets and asteroids and other bodies. You can set it
at any value that you wish. Greater tilt means more severe seasons,
and less tilt more mild seasons. Reddit
and Stack Exchange are full of discussion of radical axial tilt. What
seems different is actually commonplace. Planetary
mean temperature is a function of the distance to the star and the
axial tilt, but there is really no need to go into this unless you
are really interested. And bored. Axial tilt does have an impact on
climate and weather, which will be discussed.
III.
Atmospheric Circulation. This
is how all of this got started. I asked a question: Given Earth-like
conditions and the geography that I have decided exists, will the
weather be what I want it to be? And what can I do to make it what I
want?
The
answers and what I learned to get them will
have a major impact on your planet’s climate. Around the equator
and stretching 300 to 600 miles in width is the Inter Tropical
Convergence Zone (ITCZ). The
ITCZ
is where the sun is overhead during the equinoxes (roughly
the equator),
and it heats the ocean resulting in evaporation. Warm air rises and
flows toward the poles, creating the Hadley cells. This
creates low pressure and surface air flows in to replace it. The
air descends at about 30°. This
air is dry, having dropped most of it’s moisture to
create the rain in the tropics.
Most of the world’s deserts occur around 30°
latitude.
Coastal areas, particularly east coasts, typically
avoid being deserts due to ocean currents.
On
Earth, there
are three cells per hemisphere, but rather than flow straight north
and south, the Coriolis Effect causes the air flow of the Hadley
Cells to drift eastward. The Temperate cell flows toward the Hadley
Cell. The Hadley Cell transports
warm air towards the poles and the Polar Cell and Temperate cell
facilitate the return trip. If
you know where the boundaries of these cells are, you have an
approximate location of the jet streams (Jet Stream, Temperate Jet,
Polar Jet) which are strong determiners of weather.
The references below include more detailed descriptions of this.
Here’s
where it gets complicated. The whole damn thing moves. The Tropic of
Cancer is the latitude where the sun appears overhead on the summer
solstice, and the Tropic of Capricorn is where it appears overhead on
the winter solstice. The ITCZ follows the sun and shifts the
atmospheric cells. The
size of the cells can also fluctuate, but that is beyond the scope
here. For our purposes, it is enough to know that depending on where
you are on your planet and
when,
winds can shift depending on season. If your axial tilt is only ten
degrees, that means that during the winter, the sun will barely rise
above the horizon.
All
of this is absurdly simplified, but giving some consideration to this
will help you establish wind direction and basic weather patterns.
See the references for more, but be prepared to do your own research.
I found the cengage.com link very clear, though it does involve some
reading (don’t
worry there are clear illustrations, too).
IV.
Orbital Resonance: Orbital
resonance refers to the regular gravitational influence that
planetary bodies exert upon each each other, typically involving low
number ratios. In other words, the influence of other planets helps
keep your planet in its orbit. This is pretty complicated stuff, but
Wikipedia has a nice animation demonstrating the idea. I have not
chosen to explore this much, even in addressing the moons of Kemen. A
handwave seems appropriate here. I have ruled that there are two
inner and at least two outer planets in the Kemen system to keep
everything on track. If you wish to include other planets in your
campaign, orbital resonance may be interesting to you.
Underground
fireballs are dangerous,
Levallon