Climate is the set of weather patterns and conditions typical to a particular area over time, including temperature, wind, rainfall, humidity, and all the other variables that make up weather. Climate is important because it determines what plants will grow and what animals will live in each area because life will adapt to the conditions it's consistently exposed to. We'll be discussing specific weather phenomena in a later lesson, but today we're going to talk about some general things about the conditions and characteristics that contribute to climate. Knowing these things will not only help you as you populate your world, but will also help you determine how the climate will change over time.

There is a climate classification system, which you can see at World Climate Zones. A flow chart for the system can be seen at the Koppen Climate Classification Flow Chart. However, you don't need all that information for your world building unless you're writing a highly technical Sci-Fi novel. You do need to have a general idea of what affects the climate and some idea of what each climate is like. The aspects that determine climate are latitude, distance from the sun, orbit around the sun, revolution and rotation of the planet, axial tilt of the planet, terrain, and wind. In addition, there are 3 climate categories and 12 climate types.

Distance from the sun is a very important factor in world development as there's a particular range where a planet could support human life. Too close and the planet would be too hot, too far and it would be too cold. If your life forms are not human, your planet will still need to be within a certain distance to support the kind of life you want. Conversely, the distance from the sun will determine what kind of life will develop on a planet. Remember, life will adapt to its surroundings or it will die out. If your world is inhabited by exotic creatures, you will need to do a little research to determine the temperature ranges that will work for them. Granted, you probably will never need to mention how far the sun is away from the planet in your story, but knowing how hot your planet is and why can only help you write a stronger setting and story overall.

The orbit of a planet, or the kind of path it takes as it circles the sun, can affect the length of a planet's year, the seasons, and temperature. At perihelion, when the planet is closest to the sun, it tends to be warmer; at aphelion, the furthest point in a planet's orbit, tends to be cooler. Generally, a planet's orbit will be elliptical (oval in shape) or eccentric (unusually shaped). Especially when combined with revolution, rotation, and axial tilt, extremes in orbit could cause extremes in the seasons or have one hemisphere always warmer than the other regardless of the season.

Revolution is how long it takes the planet to circle its sun, rotation is how long it takes the planet make one complete turn. Revolution also helps determine the length of a planet's year and can create extreme seasons. Rotation determines the length of its day, also known as the diurnal cycle. The length of a day can be long or short: Jupiter's day is only 10 hours, Venus's day is longer than its year, while our moon's year and day are the same length. This cycle influences a planet's atmosphere and a lot of a planet's weather is caused by the daily heating and cooling of the planet's surface. If you decrease the length of the daily cycle, it will decrease how much the planet heats up and temperatures will vary less between day and night. But if the day is longer, the planet will get hotter and the weather will get more severe. The more extreme the temperature differences, the worse your planet's winds will be.

The angle of the axis of a planet's rotation is known as its axial tilt and determines how much sun, if any, a particular area of a planet will get. One pole will get the most sun, the other will get the least. For an illustration of axial tilt, take a look at this axial tilt image. As you can see, the tilt -- the angle at which the planet faces its sun, determines where the sun hits its surface -- illustrated by the spots of color on each planet. The more direct the sunlight, the warmer the area. Axial tilt contributes to climate zones and seasons - the more tilt a planet has, the more extreme the seasons. Most planets have some degree of axial tilt, if even just a slight one. And recent studies suggest that a large moon is needed to keep the axial tilt stable or it tends to wobble, as the earth's does.

There are also factors on the planet itself that help determine climate and weather, starting with latitude. Latitude is the distance from the equator -- the imaginary line that circles a planet at an equal distance from the poles. Latitude determines the angle at which solar radiation from the sun strikes the planet's surface - the greater the angle, the lower the intensity. On earth, since the angle at the poles is greater, the sun's radiation is less intense, and the temperatures are lower. The closer to the equator, the more direct the sunlight, the higher the temperature. Another way to look at this effect is how high the sun climbs - the higher the climb, the more sunlight the location will get and the warmer it will be. The sun doesn't climb very high at the poles, so they remain cool. Latitude also affects the seasonal changes in solar radiation and how much variation in temperature an area will experience between summer and winter. The closer to the equator your are, the less variation you will experience.

Others influence on climate and weather are terrain, water, and wind. We'll cover the first two in more detail in a later lecture, but here's some general information. Air cools approximately 3.5F for every 1000 feet, so the higher the altitude, the cooler an area will be. This means mountain climates tend to be cooler. This cooler air also has an influence on rainfall, which means mountains will affect an area's rainfall. Because water heats up and cools slower than the land, the temperature of a nearby ocean influences the temperature of the air above it and the climate of the area downwind of it. As they tend to have less variation in temperature, oceans also reduce the variation in the seasons for nearby land.

Wind distributes heat and moisture, is often the cause of some of the storms that develop, and the reason for the dry and rainy seasons of some climates. While wind may seem like a random occurrence, it actually travels in bands around the planet. There are 6 belts of wind that circle the earth: a northern trade wind, a southern trade wind, a northern westerly, a southern westerly, and two polar winds. These belts are the cause, as well as the carriers, of storms, can create deserts, and are the origin of one of the most familiar words weathermen use: fronts. A front is the boundary of an air mass. Cold fronts are led by warm air with cool air behind it,. Depending on altitude, cold fronts bring low thick clouds and bring drizzle, steady rain, or snow. Warm fronts are followed by thick clouds that can bring quite a bit of rain. As a front passes, the winds weaken and change, so the conditions aren't constant.

Trade winds blow between the equator and either 30 degrees north or 30 degrees south. The north winds blow from the northeast while the southern winds blow from the southeast. Where these winds meet at the equator, they cause air to rise and cool and create a band of cloudy, rainy weather known as the doldrums. Above and below the trade winds are the westerlies which blow between 30 and 60 degrees latitude north and south. In the northern hemisphere they blow from the southwest, and in the southern hemisphere they blow from the northwest. The westerlies steer storms from west to east across the midlatitudes and warm dry, air descends to replace the air that's been displaced, often creating tropical deserts like the Sahara in Africa and the Sonora of Mexico. Polar winds cover the poles from the 60 degree line, blowing southeast in the Artic (north pole) and northeast in the Antarctic (south pole). The boundary between the polar winds and the westerlies is what is known as the polar front. Where they overlap the westerlies, storms can develop, and these will move along the polar front bringing cloudy weather and rain or snow.

As mentioned earlier, there are 3 basic climate categories and 12 specific climate types. As we look at these climate categories and types, keep in mind that what follows is EARTH climate. If your world is not earth-like, these will change according to the conditions on your planet. Let's start with the 3 categories: tropics, midlatitudes, and polar.

Tropics have warm temperatures year round, generally have 2 distinct seasons, and experience most of their rain during the wet or monsoon season. They are dominated by trade winds, but during a monsoon the wind turns west. Tropical cyclones, typhoons, and hurricanes also occur during the wet season. Winters in the tropics are dry and relatively little rain falls comparatively. Towards the end of the dry season, there's a period of hot, humid weather known as the buildup. Midlatitude and Polar climates both have 4 distinct seasons - summer, autumn, winter, and spring. Midlatitude climates are most affected by the highs and lows, fronts, troughs, and ridges you hear so much about in the weather report on the news. Low pressure is unstable and bring unpleasant weather, although that may be nothing more than a few brief showers, and winds tend to be westerly, moving storm systems from east to west. Polar climates has weather patterns similar the to midlatitudes, but are dominated by a series of fronts, fierce winds, and brutal storms. The winds tend to blow eastward, moving systems from west to east. Daylight hours tend go to extremes, with 24 hours of sun during the summer solstice winding down to an endless night at winter solstice.

Within these 3 categories are 12 types of climates that are based on more specific weather conditions, including whether the location is tropical, arid, humid, arctic, wet, or dry and how extreme these conditions are. Most fall within specific latitudes, but there is some crossover, as you will see.

There are two tropical climates: tropical wet and tropical wet and dry. Tropical wet climates are hot and humid year round, rainfall is heavy with frequent showers and thunderstorms all year resulting in an average rainfall of 70 - 100". The high temperatures don't vary much through the year and rarely fall below 64F. This is where you find dense tropical rainforests as plants grow year round. Tropical wet and dry climates are found near tropical wet areas and have a similar pattern. The biggest difference is the amount and timing of rainfall. Winters are dry, summers are wet. The length of this wet, rainy season and the amount of rain that falls decreases as latitude increases. These areas don't have enough rain to support forests, so you will find primarily savannahs - grasslands with scattered trees. In addition to tropical climates, there are areas with a subtropical climate. Subtropical dry summer climates --sometimes called the Mediterranean climates -- have warm to hot, dry summers and mild, rainy winters. They tend to be found on the west side of continents between 30 and 45 degrees latitude. Temperatures become more moderate and there's less contrast between summer and winter closer to the coast in this climate.

Semiarid and desert climates develop in areas with dry air and little to no rain - less rain for desert climates than for semiarid. Semiarid climates border deserts and are also known as steppe. And cover more area than any other climate type on earth. These areas tend to have clear skies, so the days are really hot and the nights really cool. When clouds block the sun, they slow how fast the planet heats up during the day; when they cover the skies at night, they help prevent heat from escaping into the atmosphere and slow how fast it cools at night. They are found in both tropical and midlatitude latitudes, with the midlatitude semiarid/deserts the rain shadows of mountains. A rain shadow is an area on the leeward (the side where wind does not blow up against the mountain) side of mountains and can cover 100s of miles. Winter also tends to be colder in the tropical semiarid/desert climates.

There are three types of humid climates. Humid subtropical climates, generally found on the southwest side of land masses between 25 and 40 degrees latitude, have warm to hot summer and cool winters. They get rain year round with winter rain and snow coming from larger storm systems and summer rain coming from thunderstorms and the occasional storm or hurricane. Humid oceanic climates are found only on the western side of continents where the wind blows generally from sea to land. There is less temperature variation between seasons and they tend to have cool to mild winters and warm summers. Moderate rainfall is experienced throughout the year, and low clouds, fog, and drizzle are common, while thunderstorms, cold/heat waves, and droughts are rare. The third humid climate, humid continental, has mild to warm summers and cold winters, with the differences in temperatures between these two increasing as you go further inland - ranges can be as high as 45-63F. Rains falls year round, though many inland areas have more rain in the summer. Snow is common and can be heavy and continual, particularly in the mountains, reflecting the sun's rays back into space which lowers the day temperature and sending out infrared radiation at night to bring down night temperatures.

The three coldest climates on earth are subarctic, tundra, and icecaps. Subarctic and tundra climates have short summers and long, extremely cold winters. In subarctic climates, snow falls early in autumn and lasts through early summer. While most of the rain falls in summer, freezes are not uncommon.. Tundra climates are dry with extremely cold winters and chilly summers. Permafrost - permanently frozen ground - lies under much of the area and the regions tend to be treeless. Even colder than the subarctic and tundra climates, icecaps are the coldest places on earth. Summer temperatures rarely rise above freezing, winters are long and dark, and what little precipitation there is always comes as snow.

The last climate, the highlands, is found in mountain areas and tend to be made up of a patchwork of climates. For example: you could start at the base of a mountain in a humid subtropical climate and end up in a tundra climate at the peak. As noted earlier, temperature is modified by altitude, making these areas cooler than their flatland counterparts, and rainfall and snow typically increase with elevation.

Because weather and climate have an influence on every aspect of life on a planet, from what kinds of animals and plants are found there to the kinds of culture that develop, they should have an affect on our stories. Rain, heat, humidity all change our behavior and should influence the behavior of our characters. Weather and climate aren't just add ons to a story, but as integral as character and plot.