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Introduction
1. The Climate of Massachusetts
To begin, it is important to note that the word "climate" is not used as rgiorously as one might believe. Climate may refer to seasons, or it may be used to describe so-called "microclimates" within a season. Climate is a well defined concept and, at the same time, a loosly applied word. Several systems that describe one or more aspects of "climate" are in use, even though this may not have been their intended application.
The US EPA has defined "Ecoregions" based on combinations of factors including geology, geography, vegetation, soil types, land use, wildlife, scale of human development, hydrology, and climate (temperature and precipitation) such that no one parameter dominates the classification. This is deemed useful in helping to determine geographical changes in biota not explainable by temperature or precipitation alone.
The USDA defines "Hardiness Zones" based on average low temperatures in a region to help farmers select appropriate plant varieties for their location. However, other environmental factors, such as prevailing wind, soil fertility, moisture regeims, heat, humidity, fertility, drainage, and day length have a great effect on crops. Consequently, one or more of these limiting factors may not be evident when matching a USDA "climate" with a given crop.
Köppen-like systems are based on rainfall and temperature, and other classification systems incorporate these same parameters as well. The remainder of this introduction will present the three most often used applications of the term "climate", and select one for the purposes of framing further discussion.
The Köppen System & Climate Divisions
FIGURE 1: Massachusetts Climate Divisions
From: NOAA Web Site
As previously described, Massachusetts is typically classified under the Köppen Climate System as Dfa, humid continental climate characterized by hot summers and cold winters, with consitent precipitation throughout the year. An argument can be made for a classification of Cfb (Maritime Temperate) for Cape Cod and the Islands if the lower-boundary for the coldest average temperature is set to -3°C, rather than the commonly used 0°C in the US, as the boundary between Köppen "D" and "C" classifications. The National Oceanographic and Atmospheric Administration uses a classification system known as Climate Divisions based on temperature and precipitation models like that of Köppen, but provides a higher resolution (FIGURE 1 above) than a single Dfa classification:
1 - West 2 - Central 3 - Coastal
Each division has similar precipitation and temperature regimes with the Coastal Division (3) being more moderate than that of the West Division (1).
The EPA Ecoregion System
FIGURE 2: US EPA Level III Ecoregions
From: Purdue University Department of Horticulture and Landscape Architecture Web Site
The Ecoregion classification for the State (FIGURE 2 above), offers a slightly more granular identification of areas with different climate. Under this (quasi-climate) model, the state is broken into three regions defined by the US EPA as follows:
58 Northeastern Highlands - The Northeastern Highlands comprise a relatively sparsely populated region characterized by nutrient poor soils blanketed by northern hardwood and spruce fir forests. Land-surface form in the region grades from low mountains in the southwest and central portions to open high hills in the northeast. Many of the numerous glacial lakes in this region have been acidified by sulfur depositions originating in industrialized areas upwind from the ecoregion to the west.
59 Northeastern Coastal Zone - Like the Northeastern Highlands, the Northeastern Coastal Zone contains relatively nutrient poor soils and concentrations of continental glacial lakes, some of which are sensitive to acidification; however, this ecoregion contains considerably less surface irregularity and much greater concentrations of human population. Although attempts were made to farm much of the Northeastern Coastal Zone after the region was settled by Europeans, land use now mainly consists of forests and residential development.
84 Atlantic Coastal Pine Barrens - This ecoregion is distinguished from the coastal ecoregion to the south by its coarser grained soils and Oak-pine potential natural vegetation, as compared to forests including hickory. Appalachian Oak forests and northern hardwoods were found in the coastal ecoregion to the north. The physiography of this ecoregion is not as flat as that of the Middle Atlantic Coastal Plain, but it is not as irregular as that of the Northeastern Coastal Zone.
The Northeastern Highlands correspond (roughly) to elevations of 1,000 feet AMSL or greater. The Northeastern Coastal Zone includes elevations up to 1,000 feet AMSL and covers the Connecticut Valley Lowlands, the southern half of the Eastern New England Upland where the elevations are somewhat lower than those of the northern half, and the Coastal Lowlands eastward to the approximate boundary of Plymouth County. The Atlantic Coastal Pine Barrens ecoregion includes Cape Cod and the islands of Martha’s Vineyard and Nantucket. Here we can see the influence of elevation and coastline on climate, as well as the other biotic and abiotic factors included in the Ecoregion model.
The USDA Hardiness Zone System
FIGURE 3: USDA Hardiness Zones
From: Purdue University Department of Horticulture and Landscape Architecture Web Site
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Another way to describe Massachusetts' climate is by looking at how well certain plants do in different areas - a model highly correlated with the Köppen Climate System (Köppen was a taxonomist and he developed his system when it was thought climate determined flora). FIGURE 3 (above) shows the USDA Hardiness Zones for Massachusetts, and FIGURE 3a (right) shows the corresponding color legend.
The USDA Zones are defined by 5°C increments and show greater differentiation across the state. There is general agreement, however, among the three classification systems with the level of detail increasing from Köppen through Ecoregions to USDA Hardienss Zones. It should be noted that the more detailed Level IV Ecoregions are available for Massachusetts, but have not been included in this introduction so as to limit the variations presented. It should come as no surprise that Level IV Ecoregions are different from those of the USDA Hardiness Zones.
Classification systems used to describe climate, as you have seen, may not actually be climate models per say and are a result of the underlying variables used in their definition. The classification systems described (and there are still others) have an implicit reliance on varying combinations of biotic and abiotic factors. Geology, elevations, soil type and fertility, flora, temperature, etc., are all interdependent and any system based on one is also influenced by the others.
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FIGURE 3a - USDA Zones, °C
From: Purdue University Department of Horticulture and Landscape Architecture Web Site
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Asserting that the saying of "climate is what you expect, weather is what you get" is a pragmatic tool for evaluating climate systems, Level III Ecoregions will be used as the reference model for climate in this discussion. Having spent a decade or two travelling around the state, my personal experiences suggest that the Level III Ecoregion model is both a sufficient and accurate representation of the local climates despite its not being a climate model per say. This system does a better job of taking geography into account, both in the higher elevations to the west and for the Cape and Islands to the east. Köppen is not specific enough and misses the more temperate eastern portion of the state alltogether, and USDA Haridness Zones are too numerous to be of use in describing generalities. Climate Divisions are sufficient but for the fact that the noticeable difference between the Connecticut River Valley and the Highlands to the northeast is not evident and shows more of a maritime influence on Bristol and Plymouth Counties than would normally be experienced.(compare FIGURE 1 with FIGURE 2).
2. Weather
Massachusetts experiences all types of weather, including tornadoes. Not many natives of the state realize that Massachusetts experiences, on average, 2 tornadoes per year and is ranked 35th among the states. In May-June of this year (2006), among the coldest periods on record, northeast portions of the state experienced severe flooding from a stalled front that dropped over 18" of rain in several waves of storms that crossed the state. Subsequent flooding washed out roads and bridges leaving debris transported by rivers to wash-up on area beaches north of Boston. Video footage taken from local news helicopters was startling, but also familiar to anyone exposed to news footage of flooded areas from years past. While the state will experience flooding, hail, tornadoes, hurricanes, and Nor' Easters, the most often observed severe weather during the season of this field trip (summer) is the Thunderstorm.
FIGURE 4 - Life Cycle of a Multicell Cluster
From NOAA NWS Skywarn Spotter Training Materials
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Multicell Cluster storms are the most common storm in Massachusetts, and consist of several storm cells moving along with the trailing cell becomming energized by the leading cell. This can be visualized as a series of thunderstorms that grow and dissipate as the front moves forward.
Referring to FIGURE 4 (left), as Cell 1 enters the Dissipating Stage, the downdraft that cuts off the inflow of energy to that cell and weakening it, also feeds energy to the updraft of Cell 2 directly behind it. This inflow of energy to Cell 2 helps the updraft of the cell develop and enter the Mature Stage, the strongest and potentially the most damaging stage of a thunderstorm's life cycle. Approximately 10 minutes will pass before Cell 2 can no longer support the increasing amount of precipitation aloft and the downdraft will dominate as Cell 2 enters the Dissipating Stage as Cell 1 had done previously. Meanwhile, Cell 3 has begun to move from the Cumulus Stage, dominated by updrafts and upward growth of "cauliflower" clouds to the tropopause, to the Mature Stage previosuly described.
Occasionally, though not infrequently, a Multicell Line can form increasing the potential for damaging "straight-line" winds. This type of configuration is commonly referred to as a "squall line", and as the squall line moves out of the Highlands in the western part of the state into the Coastal Zone, it often begins to dissipate. Hail and downbursts are not uncommon, however hail size tends to be small (less than ¾ of an inch) and downbursts are typically small (microburst - 2.5 mi in width, or less) with wind speeds on the order of 50 mph.
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FIGURE 5 shows the portion of the Multicell Cluster where precipitation is heaviest, as well as showing the cross section of a typical Multicell Cluster.
The direction of movement of the system is towards the east (prevailing westerly wind) with the "front" trending NE. As is shown in FIGURE 5, The most dangerous section of this system is to the south of the currently active cell, the cell currently in the Mature Stage.
Since we will be outside for much of this Geoscience Tour, it is important that you are able to recognize these storms and your position relative to their direction of travel. Having a "Plan B" ready (nearby shelter in a car or building, knowing which way to move to avoid the most dangerous part of the cluster, etc) is the best way to prepare for severe weather. More detail on severe weather will be discussed during the field trip.
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FIGURE 5 - Multicell Cluster Cross Section
From: NOAA NWS Skywarn Spotter Training Materials
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Forecasting Tools
We will make heavy use of the National Weather Service' forecast tools available from the Boston Office's (Taunton, MA) web site. A portion of the NWS Boston web site is shown in IMAGE 1 (below, right) for reference. This screen was imaged on June, 24, 2006 - layout and/or content may have changed since this page was last updated.
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There are many forecast products available from this page. Primarily, we will be using the following products:
Current Surface Map
Forecast Surface Map For Today
Computer Forecast Weather Maps
- Local surface and upper-level charts
Computer Forecast Weather Maps
- North America Maps For 4 Models (NCEP).
We will also compare our observations with those from area stations available in the left-hand navigation area under Recent Conditions (Observations).
IMAGE 2 (below) is an excerpt from the 500mb chart (roughly 18,000 feet) and is useful for identifying areas of relative and shear vorticity. Positive vorticity can enhance uplift, or help get uplift started, while negative vorticity is associated with more stable conditions.
IMAGE 2 - 500 mb Chart (ETA)
From: NOAA NWS Boston Web Site
IMAGE 2 is showing negative curvature vorticity (anticyclonic rotation) over the New England area. Winds at this level are moving in a curved path from left to right, at speeds around 35 knots (~40 mph). Because the path is curved, the "top" part of the air current is moving faster than the "bottom", and would cause in a parcel of air at this level to sping clockwise in much the same way as a stick would rotate in similar stream currents. shear vorticity is also apparent as wind speed is decreasing from the upper left of the image to the lower right. The combination of the two would be described as "Strong Negative Relative Vorticity" as the two anticyclonic motions are additive.
IMAGE 4 (right) is an excerpt from the Stability forecast, one of the forecast products we will be checking each morning. Areas shaded in reds and yellows are less stable, while areas in black are netural. Yellow areas are marginally unstable and the brighest red areas are very unstable. Unstable areas represent greater potential for thunderstorms providing sufficient moisture is available. LI values greater than 0 are very stable, values equal to 0 are neutral, and values less than 0 are unstable. LI values of -4 or less (more negative) indicate possible severe storms while values of -10 or less indicate possible tornadoes.
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[Click Image to Open Site in New Window]
IMAGE 1 - NWS Boston Forecast Office Site
From: Boston NWS Web Site (screen capture)
IMAGE 3 - 250 mb Chart (ETA)
From: NOAA NWS Boston Web Site
IMAGE 3 is an excerpt from the 250 mb Chart (roughly 35,000 feet) and is showing the jet stream. Color contours represent wind speeds with the purple area denoting the highest speeds (100 knots, or ~115 mph) referred to as "jets" or "jet streaks." These high-speed areas can be thought of as stretching the air resulting in divergence (spreading out) aloft. Divergence aloft is associated with convergence (coming together) below and, together with either positive or negative relative vorticity in the middle atmosphere (500 mb), can be used to forecast the potential for unstable conditions favorable for precipitation.
IMAGE 4 - Stability (Lifted Index)
From: NOAA NWS Boston Web Site
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By studying these and other charts from the NWS Boston site, you can get a better feel for what weather to expect for a given day. IMAGE 5 (below) is an excerpt from the computer forecast of precipitation (in liquid form) for the same 12-hour period as the other charts presented above. The legend at the left of the image shows the color-coded contours representing precipitation amounts in inches.
The low pressure area is to the left and the high pressure (Bermuda High) area is to the right. The maximum vorticity is exactly where we would expect, just ahead of the trough in the jet stream corresponding to the highest instability.
IMAGE 5 - Precipitation
From: NOAA NWS Boston Web Site
By interpreting the various charts on this site, taking into account divergence in the upper levels (above 500 mb), convergence in the lower levels (below 500 mb), low-level warm air and moisture (700 mb & 850 mb plots), upper-level jet stream and streaks (250 mb), and mid-level (500 mb) vorticity, especially strong positive absolute vorticity, We can see how the precipitation forcast incorporates activity from all levels of the troposphere.
Summary
The background information presented here covers a fair amount of information. As noted in the introduction, climate is not so easy a thing to plot on a map as say a region of forest would be, and different classification systems can be used to convey a sense of "climate" even though the were created to model a system that depends on temperature and precipitation data. In covering weather basics, the thunderstorm was detailed Owing to its being the most common type of storm during the summer season that coincides with this field trip. Finally, a brief orientation of the forecasting tools available on the NWS Boston web site was provided using actual charts as a means of demonstrating their use in forecasting.
What Next?
The information presented above is meant to provide a context with which to understand the weather and climate of the region. During the field trip, additional information detailing data collection and interpretation will be presented.
Continue on to the
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Chaston, Peter R., Balsama, Joseph J., 1997,
Weather Basics
Chaston Scientific, Inc.
Chaston, Peter R., 2002,
Weather maps: How to read and interpret all the basic weather charts, 3rd Edition
Chaston Scientific, Inc.
Ludlum, David 1991,
National Audubon Society Field Guide to North American Weather
Knopf
Williams, Jack, 1997
The Weather Book: An Easy-to-Understand Guide to the USA's Weather, 2nd Revised Edition
Vintage
Lutgens, Frederick K., & Tarbuck, Edward J., 2004
The Atmosphere, An Introduction to Meteorology, 9th Edition
Prentice Hall
Oliver, John E., & Hidore, John J., 2002,
Climatology, An Atmospheric Science, 2nd Edition
Prentice Hall
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