Vision: A Resource for Writers
Lazette Gifford, Editor
Vision@sff.net
Holly Lisle's Vision

Weather and Worldbuilding 101

By Karen Pon

© 2001, By Karen Pon

For 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.

If 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?   

The 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 climate. 

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 your planet.   

The 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.   

Seasons 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.   

Tropical 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. 

Weather 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.   

Tropical 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.   

The 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.   

Typically, 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.   

Polar 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.   

One 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 Polar Circle.

A 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. 

Changing 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.   

Climates 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.   

Mountain 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.   

A 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. 

On 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. 

A 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.   

Altitude 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.   

Mountains 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.   

Oceans 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.   

This 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. 

Ocean 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.   

When 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.   

 

1 The Dry doesnít have to be completely dry, although it can be.  In some areas it just rains ďnormallyĒ during the dry, rather than the flooding rains of the monsoon. (back)

2 Very generally speaking.  Currents donít just go around in circles, they branch off, merge, have mini-circulations...  A good atlas will show the major currents of the world. (back)

 

Bibliography

Whitaker, Richard (ed.).  1997.  An Australian Geographic Guide to Weather, Australian Geographic, Australia.  ISBN 1 86276 032 2