The Global Scope of Climate Chapter 8 The Global Scope of Climate In its most general sense, climate is the average weather of a region, but except where conditions change very little during the course of the year, such a broad definition can be misleading. Thus, it is preferable to define climate in terms of both average conditions and the variability of those conditions based on long records of daily and seasonal weather patterns. Latitude: Keys to Climate Near the equator, temperatures are warm and the annual temperature range is low. Toward the poles, temperatures are colder and the annual range is greater. These effects are produced by the latitudinal variation in the annual cycle of insolation. 1
Coastal-continental location: Keys to Climate Coastal stations show a smaller annual range in temperature, while the range is larger at stations in continental interiors. This effect occurs because of different rates of heating and cooling on land surfaces and oceans. Keys to Climate Air temperature also has an important effect on precipitation - Warm air can hold more moisture than cold air: This means that colder regions generally have lower precipitation than warmer regions. Also, precipitation tends to be greater during the warmer months of the temperature cycle. Keys to Climate Temperature Regimes Temperature regimes distinctive types of annual temperature cycles related to latitude and location. Each regime is labelled according to its latitude zone: equatorial, tropical, midlatitude, and subarctic. Some labels also describe the location of the weather station in terms of its position on a landmass continental or west coast. 2
Global Precipitation Keys to Climate Global precipitation patterns are largely determined by air masses and their movements, which in turn are produced by global l air circulation patterns. Five classes of annual precipitation wet, humid, subhumid, semi-arid, and arid. 3
Global Precipitation Keys to Climate Isohyets - lines joining places with the same precipitation. Seven global precipitation regions can be discerned: Wet equatorial belt, Trade-wind coasts, Tropical deserts, Midlatitude deserts and steppes, Moist subtropical regions, Midlatitude west coasts, Arctic and polar deserts. 4
Keys to Climate Seasonality of Precipitation Total annual precipitation is useful in establishing the general character of a climate, but variation from month to month through the annual cycle is also very important. Monthly precipitation patterns can be described by three general types: 1) uniformly distributed precipitation; 2) a precipitation maximum during the high-sun season (summer) when insolation is at its peak; and 3) a precipitation maximum during the low-sun or cooler season (winter), when insolation is least. 5
Climate Classification Mean monthly values of air temperature and precipitation are the two variables that are most commonly used to accurately describe the climate at a weather station and its nearby region. Various methods have been devised to group climate information into distinctive climate types. The Koppen climate classification system uses a code of letters to group climates into empirical classes according to predefined values of annual and seasonal temperature and precipitation. A second approach, proposed by C.W. Thornthwaite in 1946, is based on the concept of the water budget. In this scheme, precipitation inputs are balanced against water losses through evaporation, transpiration, runoff, and infiltration into the soil. Climate Classification A more practical approach to climate classification applied classification defines climates according to some specified purpose. The Plant Hardiness Zone map is another example of an applied climate classification. Plant hardiness refers to a plant s ability to withstand and survive temperature extremes in this case, winter conditions. 6
Climate Classification Genetic climate classifications are based on the factors that cause the climate of a region. A comprehensive genetic classification system must incorporate all the factors that affect the common properties of the atmosphere, particularly temperature and precipitation. In this method, climates are distinguished according to the air masses and frontal zones that affect them. Climate Classification Three broad climate groups are recognized: low-latitude (Group I), midlatitude (Group II), and high-latitude (Group III). Within each of these three climate groups are a number of distinctive Climates four low-latitude climates (Group I), six midlatitude climates (Group II), and three high-latitude climates (Group III) giving a total of 13 climate types. The name of each climate describes its general nature and also suggests its global location. 7
Climate Classification A climograph can readily portray climate information (the annual range in temperature and total annual precipitation are provided for a location). Some climographs also display the declimation of the sun and dominant weather features using gpicture symbols. A more comprehensive method of combining temperature and precipitation into a single diagram was devised by Walter and Leith. 8
Climate Classification The climate data and station information, including altitude and length of the temperature and precipitation records, are plotted in a standardized format for each location. For northern hemisphere stations, the diagram runs from January to December; for southern hemisphere stations, the convention is to start with July and end with June so that the warm season is always in the centre of the diagram. Climate Classification Overview of the Climates 13 climate types: Low-Latitude Climates (Group I) Wet equatorial, Monsoon and trade-wind coastal, Wet-dry tropical, Dry tropical 9
Climate Classification Overview of the Climates 13 climate types: Mid-Latitude Climates (Group II) Dry Subtropical, Moist Subtropical, Mediterranean, Marine west-coast, Dry midlatitude, Moist continental Climate Classification Overview of the Climates 13 climate types: High-Latitude Climates (Group III) Boreal forest, Tundra, Ice sheet 10
Climate Classification Dry and Moist Climates Dry climates are those in which total annual evaporation of moisture from the soil and transpiration from plants exceeds the annual precipitation. Dry climates generally do not support permanently flowing streams. The soil is dry much of the year, and the land surface is clothed either with sparse plant cover scattered grasses, shrubs, or cacti or is simply devoid of any plants. Climate Classification Dry and Moist Climates Moist climates are those with sufficient precipitation to maintain the soil in a moist condition through much of the year and to sustain the year-round flow of rivers. Moist climates support forests and prairies. Climate Classification Dry and Moist Climates The wet-dry tropical and Mediterranean climate types alternate between a very wet season and a very dry season. The striking seasonal contrast gives both of them a special character, hence they are designated wet-dry climates. 11
Climate Classification Synoptic Weather Classification Synoptic weather type classification is similar to genetic classification in that it uses information about the speed and ddirection of movement of air masses. Daily weather data for a given station is classified into one of several different predefined weather types. Climate Classification Synoptic Weather Classification Synoptic weather type classification is similar to genetic classification in that it uses information about the speed and ddirection of movement of air masses. Daily weather data for a given station is classified into one of several different predefined weather types. Examples: dry polar (DP), moist polar (MP), dry tropical (DT), moist tropical (MT) 12
The Köppen Climate Classification Under the Koppen system, each climate is defined according to assigned values of temperature and precipitation derived from annual or monthly climate data. The Koppen system features a shorthand code of letters designating major climate groups, subgroups within the major groups, and further subdivisions to distinguish particular seasonal characteristics of temperature and precipitation. The Köppen Climate Classification Five major climate groups are designated by capital letters as follows: A Tropical rainy climates BD Dry climates C Mild, humid (mesothermal) climates D Snowy-forest (microthermal) climates E Polar climates The Köppen Climate Classification Subgroups of the A, C, and D climates are designated by a lowercase second letter according to the following code: f - Moist, adequate precipitation in all months, no dry season (modifier applied to A, C, and D groups). w - Dry season in the winter of the respective hemisphere h (low sun season). s - Dry season in the summer of the respective hemisphere (high-sun season). m - Rainforest climate, despite short, dry season in monsoon type of precipitation cycle (m applies only to A climates). 13
The Köppen Climate Classification Highland (H) climates are distinguished from the arctic and polar climates because they are influenced by high elevation rather than high latitude. 14
A Look Ahead Temperature and precipitation cycles are produced by the annual cycle of insolation and by the global patterns of atmospheric circulation and air mass movements. The next two chapters examine the climates of the world in more detail. 15