Vision: A Resource for Writers
Holly Lisle's Vision
Weather and Worldbuilding 101
By Karen Pon
2001, By Karen Pon
stories set on our own world in times past and present, a little research is
usually all thatís needed to determine typical weather patterns for a given
time and place. In our planetís
future, a little imagination can be exercised.
There is the ever-present threat of global warming, but on the flipside,
why not global cooling? Interglacials
are the periods between ice ages, and the current interglacial is one of the
longest ever. What if, even now,
this interglacial is drawing to a close? Perhaps
global warming is the only thing holding off the Big Freeze and in a few hundred
years ice will once again dominate the landscape.
youíre writing fantasy or SF, the chances are your story will be set on
another world. Whether youíre
building just a small section for your characters to play in, or creating entire
galaxies for their adventures, itís important to have realistic climates.
But what influences climate?
atmosphere is a complex system, dependent on many factors both astronomical and
geographical. This article serves
as an introduction to the effects of these factors on the prevailing weather and
Astronomical factors include a planetís orbit, axial tilt and rotation
speed. Orbits affect the year
length and can, if extreme, affect the seasons.
Axial tilt is the main factor in determining seasons, and will define
your planetís basic climatic zones. Rotation
speed determines day length and will affect other aspects of the meteorology of
time it takes for your planet to travel once around its sun is the length of its
year. This is usually determined in
large part by the distance your planet is from its sun.
The ellipticity of the orbit can also affect the seasons.
If the difference between perihelion and aphelion (nearest and furthest
points in the orbit from the sun) is large, then your world will be generally
warmer or colder at these times. Taken
to extremes, you could have one hemisphere always warmer than the other,
regardless of season.
are not, however, due to the changing distance of a planet from its sun.
We have different climates and seasons because our planet has a tilt to
its axis of 23.5 degrees, and not all parts of our planet receive the sunís
energy equally. Relative to the
earth, the sun swings from being directly over the Tropic of Cancer (latitude
23.5N) on the June solstice to over the Tropic of Capricorn (23.5S) on the
December solstice. This means the
tropics, the area between those latitudes, receives more solar energy than the
areas on either side. Conversely,
the polar regions, those areas within the Arctic and Antarctic Circles (66.5N
and S respectively), are always the furthest from the sun.
When the sun is over one Tropic the opposite Polar Circle will not
receive any direct sunlight. This
is why the amount of daylight varies from summer to winter, with polar regions
having the extremes of the midnight sun in summer and eternal night in winter,
while the tropics may experience 10to 14 hours of daylight throughout the year.
In between the Tropics and the Polar Circles are the midlatitudes.
climates are characterised by warm temperatures all year round and high
rainfalls during certain seasons. Typically
they will have two distinct seasons: a Wet or Monsoon season in summer, during
which the bulk of their rainfall occurs; and a Dry season in winter, in which
comparatively little rain falls1.
There is also a short time towards the end of the Dry, before the monsoon
arrives, when it becomes hot and very humid.
This is known as the Buildup.
in the tropics is dominated by the trade winds, easterly winds which blow almost
all the time, especially in the Dry season. Trade winds are generally associated with fine weather.
In the Wet Season the winds can reverse and become westerly; when this
occurs it is known as the monsoon.
cyclones, also known as typhoons or hurricanes, can affect tropical regions
during the Wet season. They usually
start over the warm tropical oceans as low pressure systems which then
intensify. They can continue to
intensify while they are over the warm tropical waters, but once they make
landfall or move too far from the tropics, they start to weaken.
Over land, the friction of the earthís surface forces the winds to slow
and weaken, while the cooler waters away from the tropics do not supply the heat
that is their major source of energy.
midlatitudes have the typical four seasons which most readers are familiar with:
summer, autumn (fall), winter and spring. These are the regions most affected by synoptic scale
systems: the highs and lows; the fronts, troughs and ridges -- all are typical
features of the midlatitudes. Prevailing
winds tend to be westerly, and systems will move from west to east.
high pressure systems are associated with fine weather while low pressure
systems bring instability and undesirable weather. This is not always the case, as the stability brought by a
high pressure system can manifest as thick decks of fog, low cloud, or unceasing
drizzle. Likewise, regions under
the influence of a low pressure system may experience nothing more than a few
brief showers. Fronts represent an
air mass boundary, so a cold front will have cooler air behind and warmer air in
front and vice versa for warm fronts. Cold
fronts are usually preceded by thick low cloud, with perhaps some drizzle or
steady rain. As the front passes,
the sky will clear, only to be dotted with convective cloud: puffy, bubbly cloud
that can grow into a storm before your eyes.
The winds will also weaken and change direction as the front passes,
particularly because of the sometimes very strong winds that precede a
cold front. Warm fronts are
followed by thick low cloud that can produce considerable rainfall.
regions also experience the four seasons, and in many ways the weather is
similar to that of the midlatitudes. Polar
weather is dominated by a series of fronts, with associated fierce winds and
severe storms. As in the tropics,
the prevailing winds will be easterly while weather systems move to the east.
of the most fascinating aspects of the polar regions is the daylight extremes.
During summer, daylight hours increase dramatically, culminating in 24
hours of daylight at solstice. At
this time the sun, instead of rising and setting, will appear to move in a
circle. Conversely, there is the
endless night at the winter solstice, when the sun is diametrically opposite the
planetís speed of rotation also affects the day length, although it influences
the length of the entire diurnal cycle rather than the ratio of daylight hours
to night. A lot of weather is
caused by daytime heating and nighttime cooling.
Decreasing the length of the diurnal cycle will decrease the amount of
heating that can take place and temperatures will not vary much between day and
night. Lengthening the amount of
sunlight will allow a much larger range of temperatures during the diurnal cycle
and also allow for more severe weather with the increased heating.
the planet's rotation speed also affects one of the major influences on the
weather - the Coriolis force. This
is the reason cyclones rotate clockwise in the Southern Hemisphere and
anti-clockwise in the Northern Hemisphere, but it is not
why the water drains out of your sink the way it does.
This is a popular myth, but in fact the geometry of your sink has more to
do with the direction of rotation. The
Coriolis force is due to the rotation of the earth; however, its magnitude is
directly proportional to the speed of the flow. On the small scale that is your sink, the Coriolis force is
miniscule compared to the effect of the shape of your sink.
Itís on the much larger scale of synoptic weather systems that the
Coriolis force has a major influence. Even
smaller weather systems can be too small. Tornadoes
can spin in either direction, since the major rotational force is the
centripetal force. The Coriolis
force increases with the speed of rotation of your planet, and varies according
to latitude, strongest near the Poles and weakest at the equator.
are not determined by the planetís place and orientation in the universe
alone. The geography of an area also has an enormous influence, especially such
features as mountains and oceans.
ranges are a staple of fantasy worldbuilding.
They are a useful way to separate groups of people, but they come along
with an extra set of rules for the weather.
One of the most common effects is a rain shadow, where the windward side
of a mountain range receives much more rain than the lee side.
This is most effective with mountain ranges perpendicular to the
prevailing winds, and receiving onshore winds.
Basically, the warm, moist air goes up the mountain slope, and as it
rises it cools. As it cools, the
humidity of the air increases until there is condensation and clouds form.
The cloud may then rain, depositing its moisture on the windward side of
the mountain and leaving the other dry. Even
if it doesnít rain, there is often more cloud on one side of a mountain due to
this same effect.
similar process can produce fog overnight, as cool moist air is pushed up the
mountain slope and condenses. Fog
can also form as the cool air descends into the valley and condenses as the
overnight land temperature drops.
the lee side of a mountain range, there can often be quite strong downslope
winds. As the dry air descends it
warms, and this can result in large temperature and humidity differences, which
in turn can produce strong, hot winds. A
typical example of this is the chinook, which flows down the eastern side of the
Rocky Mountains in the US.
similar effect, but with a different cause, is the katabatic wind of Antarctica.
Cold, dense air flows down the slopes of the Antarctic plateau, and with
the combination of gravity and the near frictionless surface of the ice, the
cold air accelerates rapidly as it is channelled through valleys, reaching
speeds of up to 320 km/h.
will also affect the climate of an area. In
the atmospheric layer in which we live, the air temperature decreases with
altitude, making some parts of the world colder than they would otherwise be
according to latitude. Much of
southern Africa is on a plateau, and the climates there are generally colder
than the sea level climates of Australia, which are at similar latitudes.
are not the only geographical feature to affect the climate of a region.
Oceans and other waterbodies have their own local effects, although
oceans generally have a larger influence than smaller water bodies such as
rivers and lakes.
have a moderating effect on temperatures, with coastal areas having extreme
temperatures than those inland. This
is due to the difference in the thermal coefficients of land and water.
The land heats up and cools down much more quickly than water, which goes
through a much smaller temperature range.
is also the source of the seabreeze, with the temperature difference between
land and water inducing an air circulation with cool onshore winds at the
surface. This is another reason for
coastal areas to remain cooler during the day, since the seabreeze helps to cool
the land. Sometimes the seabreeze
is so important it is given a name. In
Perth, Western Australia, there is a regular afternoon seabreeze in summer, and
itís known locally as the Fremantle Doctor, for the port district of Fremantle
and the relief from the heat that the seabreeze brings.
The opposite can also occur, with overnight landbreezes forming, although
these are usually much weaker.
currents also play a major role in local climate. Generally speaking2,
ocean currents flow clockwise in the Northern Hemisphere and anticlockwise in
the South. This means that the
western side of a continent will have the cold polar water flowing along its
coast, bringing cool seabreezes in the heat of summer but also thick fogs as the
cool, moist air interacts with the warmer land.
Eastern continental boundaries experience the warm equatorial waters,
which can result in milder winters for those regions.
There are exceptions of course, like the Leeuwin Current off the Western
Australian coast. Although the cold
West Australian Current does flow from the south along the coast, over the top
is a shallow warm current from the north, the Leeuwin Current, which is thought
to be a branch of the East Australian Current, flowing through Indonesia before
making its way down the Western Australian coast.
youíre worldbuilding, let your imagination run wild. Maybe your world spins on its side, as Uranus does, or has no
tilt at all. Perhaps the orbit is
highly elliptical, so that summers are extremely hot while the winters are
extremely cold. How many seasons do
your people recognise? Take a look
around your own region; how does local geography affect the weather?
Perhaps you are in the rain shadow of a mountain, or enjoy the cool sea
breezes off the ocean.
Whitaker, Richard (ed.).
Australian Geographic Guide to Weather, Australian Geographic, Australia.
ISBN 1 86276 032 2