Chapter 15 Notes World Climates

Ancient Greek Climate Classification One of the first attempts at climate classification was made by the ancient Greeks, who divided each hemisphere into three zones: torrid, temperate and frigid. The basis of this classification scheme was Earth-Sun relationships. There are many different climate classification schemes available today. A scheme is used based on the intended use of the classification system. One of the most widely used systems is the Köppen classification system which is based on the distribution of natural vegetation.

Köppen Classification of World Climates The Köppen classification recognizes five principal climate groups, each designated by a capital letter: A Humid tropical. Winterless climates; all months having a mean temperature above 18OC B Dry. Climates where evaporation exceeds precipitation; there is a constant water deficiency. C Humid middle-latitude, mild winters. The average temperature of the coldest month is below 18OC but above -3OC. D Humid middle-latitude, severe winters. The average temperature of the coldest month is below -3OC and the warmest monthly mean exceeds 10OC. E Polar. Summerless climates; the average temperature of the warmest month is below 10OC. Each of the five groups is further subdivided using specific criteria and symbols.

Tropical Rainforest Climate Controls The world climate map appears to be a scrambled pattern with similar climates located in widely separated parts of the world. Actually, the climate pattern reflects a regular and dependable operation of the major climate controls. These climate controls include: latitude, land and water, geographic position and prevailing winds, mountains and highlands, ocean currents, and pressure and wind systems. Latitude. Fluctuations in the amount of solar radiation received at Earth’s surface represent the single greatest cause of temperature differences. Seasonal changes in sun angle and length of daylight are the most important factors controlling the global temperature distribution. Because all places situated along the same parallel of latitude have identical Sun angles and lengths of daylight, variations in the receipt of solar energy are largely a function of latitude. Land and water. The distribution of land and water must be considered second in importance as a control of temperature. Marine climates are considered relatively mild for their latitude because the moderating effect of water produces summers that are warm but not hot and winters that are cool but not cold. Continental climates tend to be much more extreme. While a marine station and a continental station along the same parallel in the middle latitudes may have similar annual mean temperature, the annual temperature range will be far greater at the continental station.

Tropical Savanna Climate controls Geographic position and prevailing winds. To understand the influence of land and water on the climate of an are, the position of that area on the continent and its relationship to the prevailing winds must be considered. The moderating influence of water is much more pronounced along the windward side of a continent, while on the lee side, where the wind blow from the land toward the ocean, are likely to have a more continental temperature regime. Mountains and highlands. Mountains and highlands play an important part in the distribution of climates. In the case of North America, the north-south trending mountain ranges are major barriers to the prevailing west-to-east winds. They prevent the moderating influences of maritime air masses from reaching far inland. Also, they impact precipitation patterns, creating orographic rainfall on the windward side of the mountains and a dry “rain shadow” on the lee side of the mountains. Ocean Currents. The effects of ocean current on the temperatures of adjacent land areas can be significant. Poleward-moving currents cause air temperatures to be warmer than would be expected for their latitudes. Conversely, current flowing from the poles toward the equator reduce the temperatures of bordering coastal zones, and tend to stabilize the air masses moving across them. Pressure and wind systems. The world distribution of precipitation shows a close relationship to the distribution of Earth’s major pressure and wind systems. In the region of the equatorial low, the convergence of warm, moist, and unstable air makes this zone one of heavy rainfall. The seasonal shifting of the pressure and wind belts significantly affect the mid-latitudes, and is largely responsible for the seasonality of precipitation in many regions.

World Climates World Climates are organized as follows: Wet tropics – found along the equator, characterized by rain forests Tropical wet and dry – north and south of the wet tropics, include the monsoon areas Dry – north and south of the tropical wet and dry, extends into continental interiors Humid middle-latitude – poleward from dry subtropical regions, prevail of the eastern side of continents. Marine west coast – on the windward coast of continents Dry summer subtropical – also called Mediterranean climates Humid continental – primarily in the Northern hemisphere, these are the “breadbasket” areas Subarctic – vast zones of coniferous forests in Canada and Siberia, known for long and bitter winters Polar – summerless areas of tundra and permafrost or permanent ice sheets Highland – occur on mountaintops regardless of global position

The Wet Tropics (Af, Am) The most obvious feature that characterize these areas: Temperatures usually average 25OC or more each month. Note only is the annual mean temperature high, but the annual range is also very small. The total precipitation for the year is high, often exceeding 200 centimeters Rainfall is not evenly distributed throughout the year, tropical rain forest stations are generally wet in all months. If a dry season exists, it is very short

Tropical Wet and Dry (Aw) The tropical wet and dry is a transitional climatic region between the wet tropic and the subtropical deserts. Temperature data show only modest differences between the wet tropics and the tropical wet and dry. Because of the somewhat higher latitude of most Aw stations, annual mean temperatures are slightly lower, and the annual temperature range is a bit greater. This climate receive between 100 to 150 centimeters of precipitation per year, but the defining characteristic is the seasonal nature of the rainfall. Wet summers are followed by dry winters.

Arid and Semi-Arid Climates The Dry Climates (BWh, BSh, BWk, BSk) A dry climate is define as a climate in which the yearly precipitation is less than the potential evaporation. The classification uses formulas that involve three variables: (1) average annual precipitation, (2) average annual temperature, and (3) seasonal distribution of precipitation Dry regions of the world cover about 30% of Earth’s land area. The two climates defined by a general water deficiency are (1) arid or desert, and (2) semiarid or steppe. Semiarid climates are a marginal and more humid variant of arid climates, and are transitional zones between deserts and the bordering humid climates.

Fog Common along Pacific Coast Line Humid Middle-Latitude Climates with Mild Winters (Cfa, Cfb, Csa, Csb) These climates occur where the average temperature of the coldest month is less than 18OC but above -3OC. Included in this climate classification are several subgroups: Humid subtropical – on eastern sides of the continents between the 25- to 45-degree latitude range. Summers are hot and sultry and winters are mild. Marine west coast – extends from near the United States- Canada border as a narrow belt into southern Alaska. Maritime air masses cause mild winters, cool summers, and ample rainfall throughout the year. Dry-summer subtropical (Mediterranean)– typically found along the west sides of continents between latitudes 30O and 45O. In the summer the regions are dominated by the stable eastern sides of the oceanic subtropical highs. In the winter they are within range of the cyclonic storms of the polar front.

Northern Coniferous Forest-Taiga Humid Continental Climates with Severe Winters (Dfa, Dfc, Dfd) Located only in the Northern Hemisphere, between the latitudes of 40O and 50O. Average temperature of the coldest month is -3OC or below, and the average temperature of the warmest month exceeds 10OC. Precipitation is generally greater in the summer and generally decreases toward the continental interior and from south to north. Wintertime precipitation is chiefly associated with the passage of fronts connected with traveling middle-latitude cyclones. Subarctic climates, also called taiga climates, are situated north of the humid continental climates and south of the polar climates. The characterizing feature of these climates is the dominance of winter, although the summers, while short, are remarkably warm.

Permafrost in the Northern Hemisphere Polar Climates (ET,EF) Polar climates are those in which the mean temperature of the warmest month is below 10OC. Annual temperature ranges are extreme, with the lowest annual means on the planet. Although classifies as humid, precipitation in the polar climates is sparse, around 25-cm per year. Two types of polar climates are recognized: (1) the tundra climate, characterized by permanently frozen subsoil called permafrost; and, (2) the ice-cap climate, which does not have a single monthly mean above 0OC. Permafrost in the Northern Hemisphere

High Altitude Highland Climates These climates are characterized by a great diversity of climatic conditions over a small area. Areas include the Rockies, Sierra Nevada, Cascades, and the mountains and plateaus of Mexico in North America. The best known climatic effect of increased altitude is lower temperatures, but greater precipitation due to orographic lifting is also common. Because atmospheric conditions fluctuate with altitude and exposure to the sun’s rays, a nearly limitless variety of local climates occur in mountainous regions.