The following description of the characteristic space and time scales, arranged from the largest space scale to the smallest, follows traditional nomenclature. Ultimately, the types and spacing of weather instruments must be chosen for the scale of motion under study.
This space scale is the largest scale that meteorologists consider, with features having a diameter that would be at least the size of a continent, and often would encompass a hemisphere. The planetary scale involves the general atmospheric circulation features that are somewhat permanent. We can detect some aspects of the planetary scale circulation regime on the individual daily weather maps, using the current observation networks. Inspection of a sequence of weather maps over several days shows those features that remain over an area. On many of these maps, a large semi-permanent subtropical high pressure system can be detected, such as the one that resides over the Atlantic Ocean termed the Bermuda high and another over the Pacific Ocean called the Hawaiian or Pacific High. Another example of a planetary scale feature is the mid latitude prevailing westerlies, exemplified by the polar front jet streams that are found on the upper tropospheric constant pressure charts (such as the current 300 mb charts on the Online Weather Homepage). The seasonal monsoon circulations of both the Indian sub-continent and the Southwestern United States are examples of features approaching the size of the planetary scale.
Atmospheric phenomena that have continental sized dimensions typically are the ones that persist for the longest time interval. The typical time scale for such phenomena ranges from several weeks to millennia; this scale captures not only the seasonal cycle but other long term phenomena, such as the El Niño and Southern Oscillation. The subtropical high pressure systems, the trade winds, the prevailing westerlies and the polar front jets are permanent planetary scale features that can be found not only as distinct features on annual average charts, but on most weather maps of the given region. The intensity and the exact geographic location of these systems may change seasonally. The monsoon circulations over the Indian sub-continent and the Southwestern United States are examples of planetary scale circulation regimes that undergo distinct seasonal reversals in the wind flow. Some would argue that the shifts between an ice age type circulation regime and one more typical of the present interglacial epoch could represent an atmospheric phenomenon with a very long time interval.
Weather features in this scale subdivision are the high pressure and low pressure systems that appear on the daily surface weather maps. The term "macro", meaning large scale has some acceptance, however "synoptic" scale has been used quite often. The word "synoptic" (from the Greek for "seen together", or a broad overview) has a varied meaning in meteorology. On the one hand, synoptic refers to a scheme of data collection and weather analysis, but it also refers to a size scale that is on the order of 1000 to 2000 km. In other words, the scale has dimensions of several states or provinces, which is larger than the county-wide thunderstorms, but smaller than the continental sized monsoon circulation regimes. Macro- (synoptic) scale weather systems can be detected using traditional surface and upper air weather observation networks, such as those maintained by the National Weather Service.
These macroscale features typically have life spans that range from a day to at least one week. On rare occasion, a tropical cyclone (if sufficiently intense, a hurricane) may persist for over two weeks. On such a macroscale time scale, the traditional hourly sampling of surface weather conditions together with the twice daily observations of upper air conditions detect most of the synoptic scale features and allow the meteorologist to track such features.
This "medium" scale ("meso" is Greek for middle) contains features ranging from approximately 1 km to 100 km, or the space scale that would cover several counties. Weather systems, such as large thunderstorm cells -- sometimes called "Mesoscale Convective Complexes", are examples of the "organized turbulence" of mesoscale systems. The localized lake (or sea) breeze phenomenon that develops on a warm summer afternoon along the shoreline is another mesoscale example. Until recently, observations and analysis of mesoscale phenomena have been poor. However, increased research and operational efforts have meant that mesoscale weather systems are now being detected by various new, sophisticated systems such as WSR-88D (Doppler) radar and wind profilers. Some states, such as Oklahoma, have mesoscale observing networks, with at least one automatic weather station per county.
Weather features that are classified as mesoscale systems have typical time scales that range from an hour to nearly a day. Ordinary thunderstorm cells may undergo a complete life cycle on the order of an hour, while some of the clusters of thunderstorms that form "Mesoscale Convective Complexes" may persist for anywhere from 12 to 18 hours. Other mesoscale phenomena include the lake (or sea) - land breeze circulation regime that undergoes a diurnal reversal in the wind flow from an onshore breeze during daylight hours to an offshore breeze during the evening. The traditional hourly surface observations and radar summaries will detect many mesoscale phenomena. Additional intermediate updates are often needed to obtain a more complete analysis. Newer automatic weather stations and other sophisticated instruments, such as the WSR-88D radars will detect short lived mesoscale phenomena.
The term "micro" is from the Greek word for small and refers to a scale ranging from centimeters to a kilometer. One end of this scale includes the wind swirls that may develop downwind of a building, while a large tornado may be near the upper end of the scale. On this scale, motion is dominated by turbulence, which is often very chaotic. Special networks, such as those at an agricultural experimental station, are needed to observe microscale phenomena.
Weather features that are classified as microscale atmospheric phenomena are very short lived phenomena. Small little gusts of wind that lift litter off the ground may persist for less than several seconds. On the other hand, a typical tornado may persist for as long as an hour. Rapid and essentially continuous measurements are needed to capture many microscale phenomena. Special instrumentation usually is required.
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Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 1999, The American Meteorological Society.