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Pressure and Altitude
The atmospheric envelope surrounding any planet
is thickest at the world's surface, and decreases in density the
further from the surface one gets. The irregularities of a
world's surface cause mountains and valleys to form; where these
features are of significant size, air pressure will be noticeably
different.
A base line for the measurement of a planet's
atmospheric pressure is always set. This is the Surface Level
pressure. On planet's without water, surface level is
calculated as being the level, within +/- 100 meters, at which the
greatest fraction of the planet's surface area lies — this being
determined by survey. In effect, the referee may set this
baseline as desired. Planets with water traditionally
calculate Surface Level pressure at the top of the planet's oceans —
at "sea level."
Surface Level is always set at 0. Altitudes
are then calculated above (plus) or below (minus) surface level.
Pressure increases below surface level, and decreases above it.
The curve of pressure change is influenced by a number of factors,
including the overall air pressure, planetary gravity, and
temperature. The curve is not a smooth one, but pressure tends
to build more rapidly the nearer the point in question is to the
center of the planet. Thus pressure below surface level
increases at a faster rate than does pressure above surface level.
Atmospheric pressure is measured in atmospheres
(abbreviated "atm"), an absolute measurement not dependent upon
factors that vary from planet to planet. The surface level
pressure of Earth is 1 atm, but other worlds may have quite
different surface level pressures.
Three surface-level pressures are of interest for
the purposes of these rules. These bands represented the three
broad categories of atmospheric pressure where breathing is possible
for humans and similar races.
Thin or Thin, Tainted atmospheres have a
surface-level pressure of between 0.425 atm and 0.75 atm. The
average, roughly 0.5 atm, is most commonly used for identifying a
basic "thin" atmospheric pressure. On Earth, this corresponds
to the pressure encountered at an altitude of roughly 5400 meters.
Standard or Standard, Tainted atmospheres have a
surface-level pressure in the range between 0.75 atm and 1.5 atm.
Earth's seal-level pressure of 1 atm is used as an average figure
throughout these rules.
Dense or Dense, Tainted atmospheres represent a
surface-level of more than 1.5 atm. Although in theory
breathable to fairly high pressures, a dense atmosphere generally
will be less than 2.5 atm; the average and common practice for these
rules is 2 atm pressure.
The
Atmospheric
Pressure table shows how to calculate the pressure at various
altitudes, depending upon the surface-level pressure of the planet's
atmosphere. Thus, on Earth, we read down the center column,
and discover that 0.5 atm pressure is found at 5500 meters altitude.
In similar fashion, we can find an altitude — say 2500 meters — and
read across, noting that, in a Thin atmosphere, the pressure at that
altitude is 0.4 atm; in a Standard atmosphere, it is 0.8 atm, and in
a dense atmosphere, 1.6 atm.
The colored ranges show the limits of
breathability, marked off by Thin, Standard and Dense atmosphere
ranges. Outside these safe zones, artificial assistance will
be needed to avoid serious damage, and ultimately, death.
Although exact pressures are given on the table,
it is rarely all that essential to know what the specific pressure
is — either you can breathe or you can't. If you're in a Very
Thin atmosphere, it matters little if the pressure is 0.2 atm or 0.4
atm — you're in trouble without oxygen, and that's that.
The precision in measuring pressure, however, is
useful for some purposes. For example, it is very easy to
supply local "color" through varying the surface-level pressure of a
world within given limits. It will make a difference if the
"standard" atmosphere world is at the upper end, the lower end, or
comfortably in the middle of the pressure scale. Referees can
set pressures arbitrarily (or have them furnished for various
planets), or can randomly establish the pressure at surface level
for a new world by following the simple procedures outlined in the
paragraph below.
Find the lowest pressure falling within a given
category on the table. Roll 2D6-2; move that number of bands
up the chart (for Thin atmosphere worlds, the modifier should be
2D6-4, instead) — a negative number requires a downward move.
The result of this throw give the surface-level
pressure to be used for the world. Where the atmosphere thus
determined has moved out of the proper range (too low in Thin
atmospheres, or too high in Dense ones), an unusual situation has
occurred. The planet is, in fact, a mild version of the Type
D, E or F atmospheres. Surface-level pressure is not capable
of sustaining human life, but settlements at a lower or higher point
will enjoy the proper atmospheric type.
Once the surface-level pressure is established, a
table specific to the planet can be established to correlate
pressure and altitude based on the specific surface figure.
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