HONOURS GEOGRAPHY
Wind, Weather, and Climate

*Today we are going to talk about climate and weather, and there’s a difference.  Weather is the condition of the atmosphere at a given point in space and time—e.g. ‘it’s raining’ or ‘it’s sunny.’  Climate is the typical weather pattern for an area over a long period of time:  Seattle is in a Marine West Coast climatic region, so it rains there often, while Arizona has a mixture of steppe and desert climates, and is typically sunny and dry.

*Climate patterns result primarily from the Earth’s relationship with the Sun.

*The axis on which the Earth rotates is tilted at an angle of about 23½ degrees compared to the plane of Earth’s revolution around the Sun.  Therefore, not all parts of Earth receive equal amounts of sunlight all the time. 

*The Earth’s tilt is always in the same direction, so as the Earth revolves around the sun, different parts of the Earth are tilted towards or away from the Sun at different times of year, and the angle at which they face the Sun also varies.  The amount and intensity of light reaching a certain place from the sun is known as insolation.

*The equator, being at the mid-point between North and South, receives the most sunlight and the most direct sunlight.  It is also the dividing point between the Northern and Southern Hemispheres, in which the seasons are always reversed.

*The sun is directly over the Equator on the equinoxes (the first days of spring and fall); at this time, all around the world, every place receives 12 hours of daylight and 12 hours of darkness—which is why equinox means ‘equal night.’  After that, as the Earth continues to revolve around the sun, the sun will appear directly overhead at other spots, North or South of the Equator, depending on which end of the world is experiencing summer.

*When one pole (or the other) is as close as it gets to the Sun, that is a solstice—the summer solstice for that hemisphere and the first day of winter for the other hemisphere.  At this point, the Sun will be directly over one of the tropic lines.  The northern tropic is the Tropic of Cancer, at 23½° North latitude.  The southern one is the Tropic of Capricorn, at 23½° South latitude.  Because that is as far north as the sun gets, from the point of view of anyone north of the tropics, the Sun always seems to be in the South (which is why moss grows on the south side of trees).

*Because the tropics are close to the Equator, and have low numbers of longitude, they are called the Low Latitudes.

*Because of the earth’s tilt, the farther one is away from the tropics, the less direct the sun’s light is—and the less heat it can provide.  If one gets far enough away, the sun is actually invisible for part of the year.  At the North and South poles, the sun is hidden by the rest of the Earth for half the year, during the fall and winter of that hemisphere’s year—but in the spring and summer, day lasts 24 hours, too.

*The Arctic circle (and the Antarctic) are the opposite of the tropics in many ways.  They are the lines above which it is possible for there to be no sun at some point in the year.  The are 66½° North and South, and on the lines, there is no sun on the winter solstice for the hemisphere; the farther north or south beyond the line one gets, the more nights last all day.  On the other hand, on the summer solstice, it is day for 24 hours.

*Because the Artic and Antarctic are far from the Equator, and close to 90° North or South, they have high numbers in their co-ordinates, and are known as the High Latitudes.

*The areas between the Tropics and the Artic/Antarctic are known as the Mid-Latitudes.

*Because different parts of the Earth receive different amounts of sun, a location’s latitude has a lot to do with its climate and weather.

*There are three major wind systems in both the Northern and Southern Hemispheres.  The tropical system (also called the Hadley Cell, after an 18th Century scientist who described it) consists of air that is warmed at the equator rising, slowly circulating north as it is displaced by more rising air and the rotation of the Earth, and eventually descending around 30° North (or South) as it cools off.  Thus, winds in this area generally blow towards the equator, where pressure is lowest as the hot air rises.

*The Polar Cell works much the same way:  around 60° North or South, air is warmed by the Mid-latitude sun, it rises, and eventually circulates up to the poles, where it cools off and descends and flows away from the poles.

*The space between these two Cells is known as the Ferrel Cell, after a 19th Century scientist.  This Cell is largely dependent on the other two, particularly the Hadley Cell, as warm air descending from that cell actually keeps the warm air in the Mid-Latitudes close to the Earth’s surface.  This air flows away from the tropics.

*All these cells shift north or south a bit during the year, based on where the sun is actually overhead, so that at the solstices, the two Hadley Cells actually meet closer to one of the tropics.

*The air flowing on the Earth’s surface is also affected by the rotation of the Earth.  As the Earth turns, air above the surface and water on the surface try to stay where they are, and so seem to be moving in the opposite direction from the spinning earth.  This is known as the Coriolis effect, and results in wind and water currents tending clockwise in the North and counter-clockwise in the South, although many things can effect this on a small scale.  Still, this means that the tropical and polar winds tend to blow west and the mid-latitude winds tend to blow east. 

*However, because we name winds after the direction FROM which they blow, the mid-latitude winds are called Westerlies and the polar winds are called Easterlies.  The Easterlies are famous for being especially fierce.

*The winds in the tropics were most useful for moving trading ships in the 18th and 19th centuries, and are consequently known as the Trade Winds.

*At the equator and about 30° North or South, the fact that the prevailing air currents are moving either straight up or straight down often means that there is not much wind blowing across the surface.  This was disastrous in sailing days. 

*The zone of little or no wind near the equator was called the doldrums—a name that came from the low spirits sailors found themselves in when there was no wind or progress.

*A similar zone between about 30-35° North or South is known as the Horse Latitudes, because when ships were stranded there, they supposedly threw their horses overboard to save food and water, or killed the horses for meat.

*Students should get out the maps they made.

*Ocean currents work much the same way as the winds, as the sun heats the ocean water, causing it to rise or sink, and the Coriolis effect causes it to slowly turn.

*Ocean currents also used to help ships move.  They also affect the temperature of land near them. 

*The Gulf Stream, for example, carries warm water from the Gulf of Mexico to Britain and Northern Europe, so that although England is farther north than New York, it gets far less snow. 

*In a few cases, currents keep shorelines unnaturally cool, as is the case along the coast of Chile.  In the rare instances when the cool Peru Current does not go by Chile and Peru, the warm water than comes in kills fish, and causes strange and violent shifts in the weather, mostly in the form of greatly increased precipitation in South America (and elsewhere), which causes floods and other problems, and much drier weather in Australia and parts of Asia.  Because this phenomenon often occurs around Christmastime (in the years when it does occur, between 2-7 years apart), it is called El Niño (the Boy Child or the Christ Child).

*The currents of the oceans and winds do a great deal to create weather.  Obviously, the wind patterns create wind, and when a polar jet slips down from the north, it cools things off.  However, warm and cool air also tend to move in masses, marked on weather maps by their fronts, or edges.  Warm air creates areas of low pressure (and usually good weather), while cool air exists in areas of high pressure (often with wet weather).  When two fronts meet, the cool air causes moisture in the warm air to condense, so precipitation occurs most often where fronts collide, as does more exciting weather like tornadoes and hurricanes.

*Another consideration is the fact that land heats up more quickly than water does, but it also cools down more quickly.  This means that weather changes more rapidly and at greater extremes over land than over or near water.  Being near water moderates the temperature, which is known as the maritime effect.

*Variation in weather is known as seasonality (so land has greater seasonality than water). 

*Elevation also effects climate, as higher places are generally cooler than lowlands, and in mountainous areas, the orographic effect can create rain forests and rain shadows.

*The most generally used climate classification system is that devised by Vladimir Köppen around 1900.  It divides climates into five major groups:  tropical, arid, temperate, continental (cool, but not cold), and polar, with an additional classification for highland areas that can have wide variation in local climates.  The Glencoe textbook largely follows this scheme, although they group the temperate and continental climates together.

*Distribute grids for students to complete.  They will learn where different climate types tend to be located in the world (because most climate regions are determined by latitude), what kind of weather they have in the winter and summer (summers tend to be wetter than winters, because warm air can hold more moisture, but there are exceptions!), what vegetation is natural to such areas, what kinds of crops tend to be grown there, and some examples of places where such climates can be found. 

*Students should also look at pages 66-67.

*Tropical rain forests are found in the low latitudes (near the equator).  They have frequent rain and high temperatures, and it rains in all seasons of the year.  Their natural vegetation is mostly tropical forest.  Common crops include rice, spices, rubber, and certain fruits.

*Tropical savanna is mostly found in the low latitudes.  These climatic regions are also hot, but have rainy summers along with dry winters.  They mostly have scrub, grass, and low trees.  They are mostly used for grazing, but can grow some corn and other grain, as well as cotton and sugarcane.

*Deserts are very dry:  they receive no more than 10 inches of precipitation a year.  They can be found in any part of the world, although the largest ones tend to be near the tropic lines.  They have little natural vegetation (mostly cacti and scrub vegetation) and few crops are grown in them.  Oases in deserts sometimes grow dates, and irrigated desert sometimes supports cotton.

*Steppes have more rainfall than deserts, typically 10-20 inches a year, but they are still dry (and usually hot in the summer and cold in the winter).  They are mostly grasslands, with some small brush and very few trees.  Steppe climates can grow some cotton and wheat, and grazing is also very important.  They are mostly found in the interiors of continents, and usually next to deserts.

*Marine west coast climates tend to be found in the mid-latitudes along the west coast of continents (and sometimes on the east coast of Southern Hemispheric continents).  They tend to have cool summers and damp winters.  With so much rainfall, heavy forests are common in these areas as natural vegetation, and humans practise lumbering as well as growing fruit and grains, and raising livestock.

*Mediterranean climates are named for the Mediterranean Sea, around which they are found, although they also appear on the coast of Southern California and South Africa (lower mid-latitudes).  Mediterranean climates have mild, rainy winters and hot, dry summers.  The natural vegetation consists of chaparral, or dry, woody bushes and shrubs, along with low, hardy trees (especially evergreens).  Humans tend to grow grapes, olives, and citrus fruits in this climate.

*Humid sub-tropical climates are typical of the lower mid-latitudes, and have short, mild winters and rain in both the summer and winter.  These areas have both forests and grasslands, and are used for a wide range of crops, including corn, cotton, and tobacco.

*Humid continental climates are found in the higher mid-latitudes.  They also tend to be inland.  They have long, snowy winters and short, cool summers.  The natural vegetation tends to be forest, especially evergreen forest.  In these regions, people grow corn and wheat and raise cattle.

*The high latitudes have cold climates.  The border between the mid and high latitudes is usually characterised by subarctic climates.  These have cold winters and short (but often hot) summers:  summer and winter temperatures can vary as much as 120 degrees Fahrenheit.  In some subarctic regions, there is only a thin layer of soil that thaws each year; the frozen ground beneath it is known as permafrost.  These regions support some evergreen trees, and low, tough plants.  Very few crops are grown in these regions.

*Tundra climates are closer to the poles than subarctic climates.  These have a very thin layer of thawed soil, and experience long nights in the winter, and even the long summer days are cold.  Trees do not grow in these regions, although mosses, lichens, and even small bushes can be found.  Crops are not grown here, either, although some people herd reindeer or hunt seals.

*The extreme polar regions are covered with ice caps.  There is little natural vegetation, although in some places a little bit of lichen can survive.  No crops of livestock are typically raised in these regions.

*Highland climatic regions are a miscellaneous group used to cover the mountainous regions of the world.  Because of the frequent changes in elevation, mountainous regions tend to have very diverse climate types within a fairly small area.  In highlands, climate is determined more by elevation than by latitude, and all climate types can be found in highlands, depending on how high up a given point is.



This page last updated 16 January, 2006.