26: Weather Patterns and Weather Types IAN G MCKENDRY Department of Geography, The University of British Columbia, Vancouver, BC, Canada In popular usage, the terms “weather pattern” and “weather type” are used variously and often imprecisely to describe states of the atmosphere. An understanding of weather processes and patterns has important applications in such diverse areas as air-quality management, hydrology, water management, human health, forestry, agriculture, energy demand, transportation safety, the insurance industry, economics, and tourism. These terms are formally defined and the various classification techniques that form the basis of synoptic climatology are described. In summarizing the various methods used to identify weather patterns and types, it is abundantly clear that underlying such approaches are significant assumptions about the state of the atmosphere together with varying degrees of subjectivity inherent in the techniques themselves. It is important that these constraints are adequately acknowledged in all applications of the techniques outlined. Finally, the impacts of computing advances, new techniques, and enhanced datasets on synoptic climatology are described. INTRODUCTION has been invested in identifying the linkages between par- ticular states of the atmosphere (as manifested by weather “Weather” is defined as the instantaneous state of the patterns and types) and the environment. This field of atmosphere at a particular location and is typically charac- investigation, characterized by a variety of classification terized by a range of meteorological variables (or weather techniques, forms a significant part of the subdiscipline of elements) such as pressure, temperature, humidity, wind, synoptic climatology (Yarnal, 1993) and will be the focus cloudiness, and precipitation (Barry and Chorley, 1995). of this entry. Weather at any particular location is a consequence of the In popular usage, the terms “weather pattern” and movement and evolution of “weather systems”. These may “weather type” are used variously and often imprecisely be considered to operate at several time and space scales, to describe states of the atmosphere. For example, in the ranging from the microscale (e.g. small-scale eddies rec- popular media a tornado might be described as an “unusual ognizable as gustiness in winds) to the mesoscale (e.g. weather pattern” that is associated with a particular type of sea breezes), through the synoptic scale (e.g. midlatitude weather (severe). Alternatively, an El Nino event in the cen- cyclones and tropical storms), and finally to the planetary tral Pacific may be referred to as part of a global “weather scale (e.g. jet streams and Rossby waves). Variations in pattern” that results in particular weather “types” (e.g. wet weather occur fundamentally because the atmosphere is a winters in California or drought in Australia). Although highly dynamic fluid that moves both horizontally and ver- such usage does implicitly recognize the inherent scales, tically in response to the differential heating of the earth regularity, and interconnectedness of weather processes and by solar radiation. Such variability has significant impacts systems, use of these terms in the atmospheric and hydro- on human activities. Consequently, an understanding of logical sciences tends to be somewhat more specific, most weather processes and patterns has important applications commonly referring to the association of a characteristic in such diverse areas as air-quality management, hydrology, combination of weather elements (e.g. marked by spe- water management, human health, forestry, agriculture, cific states of the atmosphere with respect to temperature, energy demand, transportation safety, the insurance indus- pressure, cloudiness, wind, and temperature) with particu- try, economics, and tourism. Given such important applica- lar weather systems. Furthermore, “weather patterns” and tions, it is not surprising that considerable scientific effort “weather types” tend to be most commonly linked to the Encyclopedia of Hydrological Sciences. Edited by M G Anderson. 2005 John Wiley & Sons, Ltd. 2 METEOROLOGY AND CLIMATOLOGY synoptic scale of atmospheric phenomena. At this scale, on its axis. Furthermore, at the global scale, broad latitudi- the day-to-day movement and evolution of weather systems nal bands can be identified in which particular types of modulates and influences a wide range of human activities weather predominate and which arise as a result of the to a considerable degree. Implicit in both terms are notions differential heating of the planet and the application of of classification. In this context, the terms “pattern” and the particular forces influencing atmospheric motion. This “type” are often used interchangeably and with sometimes average condition is known as the general circulation of subtle nuances in meaning. For example, “pattern” can be the atmosphere and in essence represents a “gigantic heat used in both its spatial and temporal senses. A repeatable engine” (Barry and Chorley, 1995, P97). Although inter- sequence of weather may be regarded as a weather pattern, rupted and modified by the global distribution of land and or more commonly, “pattern” refers to a particular arrange- water, the dominant features are persistent winds from an ment of isobars on a weather map (sometimes referred to easterly quarter in the tropics (the trade winds) and the as a map pattern, synoptic type, or circulation type). broad zone of midlatitude westerly winds in which midlat- itude depressions are constantly forming and dissipating. These features are represented for the idealized case of THE ORIGINS OF PATTERN the Northern Hemisphere in Figure 1. Essentially the same Throughout human history there has been recognition of patterns are evident in the Southern Hemisphere (i.e. south- pattern within atmospheric behavior. At the simplest level, easterly trade winds and midlatitude westerlies). However, this can be seen in the regularity of the seasons or in the significant differences between the two hemispheres with association of particular wind directions with temperature respect to the distribution of land (continents) and water (e.g. cool winds from the north in the Northern Hemi- results in significant differences in the nature and variability sphere). At larger scales, sailors have noted and exploited of the major circulation features. For example, the absence the remarkable persistence of the easterly “trade” winds in of significant landmasses in the Southern Hemisphere mid- the tropics, in marked contrast to the constant and somewhat and high latitudes, results in much stronger and persistent chaotic succession of cyclonic systems forming and dissi- midlatitude westerlies than observed in the Northern Hemi- pating as they move eastward through the midlatitudes. In sphere (hence, the southern latitudes renown for their strong the twentieth century, development of a conceptual model westerly winds: the “roaring forties”, “furious fifties”, and of the midlatitude cyclone formalized patterns in weather so on). elements associated with the passage of such low-pressure The particular scales of motion (both time and space) systems (or depressions). For example, passage of a cold mean that the atmosphere assumes patterns that are geo- front is typically associated with a particular sequence of graphically constrained and quasi-repetitive. Superimposed cloud types, winds, pressure changes, precipitation, and on these broad geographical patterns is variability in a range temperature changes. Typically, in the midlatitudes, cold of scales. For example, west coast midlatitude continental front passage is marked by the arrival from the west of locales (e.g. Washington State and British Columbia, south- a band of cloud (often convective in form) accompanied ern Chile, The British Isles, and Norway) are exposed to by rainfall and sharply decreasing temperatures, a change a succession of eastward moving cyclonic systems during in wind direction (often from the south-west to the north- winter. Then, during summer, the subtropical high-pressure west in the Northern Hemisphere). Behind such a front systems expand poleward providing some relief from the there is often rapid clearing, rising pressures, and occa- characteristic cool, moist, and highly variable weather pat- sional showers. Although cold fronts exhibit considerable variability, the weather pattern described above is recogniz- terns that tend to dominate in these regions due to their con- able to most inhabitants of the midlatitudes whether in the tinental location and latitudinal position. In other areas (e.g. Northern or Southern Hemispheres, and forms the basis of southeast Asia and Africa), monsoon patterns may dominate simple predictions that can be made on the basis of cloud the annual weather cycle (such patterns are associated with and/or pressure observations. a continental-scale seasonal wind reversal accompanied by Given the constancy of Earth’s orbital characteristics, distinct changes in precipitation). Furthermore, at large solar output, boundary conditions (earths distribution of scales, natural modes of oscillation in the atmosphere/ocean land and water and atmospheric constituents), and axial circulation, as manifested in the El-Nino-Southern Oscilla- tilt, and the universal application of forces governing atmo- tion (ENSO), the North Atlantic Oscillation (NAO), the spheric motion (i.e. linked to gradients in atmospheric Pacific Decadal Oscillation (PDO),