NORTH CAROLINA'S WEATHER AND CLIMATE: 

REGIONAL AND SEASONAL SUMMARIES

 

    So far we have looked, more or less, at each element of the weather in turn, and treated them in state-wide context.  This is the section for synthesis.  We shall do this in two ways:

 

    - Discuss regional climate - we will divide the state into it's major geographical regions and look at the general climatology, involving all weather elements, of each region (including consideration of the variability with the seasons).  In these regional sections we shall emphasize the long-term average climate conditions;

    - Describe seasonal weather - for the state as a whole (but not neglecting variations from region to region) we will provide an explanatory description of the daily weather sequences you would expect for each month and season.  In this section the focus will be on the short term sequences of likely weather events.

 

    North Carolina has traditionally been divided into three regions: Mountains, Piedmont, and Coastal Plain (Fig 7.0.1).  The visible differences between them are most obvious in the shape of the land surface itself.  But there are differences in virtually all aspects of the environment, physical and human - not least in the climate. So we shall look at each of these three in separate sections.

 

    Even within particular regions, however there are variations in climate which are confined to, or well developed in, that region.  So we shall look at the influence of topography in the mountains, the role of buildings and cities in the Piedmont, and the contrast between land and water on the Coastal Plain.   For the first two the effects are scattered throughout the region, so there are no separate sub-regions to consider - at least on the scale of the map of Fig 1.  For the coast, however, it is convenient to divide it into three; the very dry Sandhills area, the swampy Tidewater region, and the intermediate Coastal Plain proper (Fig 7.0.1).  We shall look at the climatological causes and consequences of these differences.

 

 

 

 

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Fig 1: The major geographic regions of North Carolina (from NC Atlas)

 

   

 

 

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Fig 2: The synoptic climatology of North Carolina.  The basis for this is Fig 4.1.?, but the common storm tracks have been added.

 

 

 

    it is useful to review the synoptic climatology of our state here, since it has a major influence on the regional climate and forms the basis for the seasonal weather patterns.  Further details will be introduced in the seasonal section.

 

    The weather of North Carolina results from the interaction of cold air masses from the north, warm moist air masses from the south and cyclonic storms coming mainly from the west Fig 2).  The storms are largely guided by the air flow in the general westerly current of the mid-latitudes, which commonly has a wavelike motion - the Rossby waves - in the middle atmosphere.  Above this is the polar front jet stream, a ribbon of fast moving air.  This is often responsible for causing changes in the Rossby wave pattern and is a major player in the development of our frontal depressions. 

 

    As the position of the Rossby waves change, the location of the air masses adjust.  These changes can occur at any time, but there is always a distinct seasonal sequence.  In summer the westerlies are usually to the north of North Carolina and we get warm moist air mass weather for most of the time - always with the occasional frontal depression passing and sometimes interrupted by hurricanes.  As winter approaches the jet stream and westerlies, on average, are located farther south, more or less overhead for us.  As a consequence we tend to get a steady stream of frontal depressions, separated by periods when cold dry and warm moist air masses alternate.

 

    Both the regional climate and seasonal weather patterns are, in most part, the result of refinements or modifications of the statewide patterns we have considered previously.  Differences from place to place depend on the local operation of the processes - the energy and water balances, the orographic effects and so on - that we looked at earlier.  Here we cannot give a description of every spot in the state, nor one of every possible weather sequence. Rather, the chapter provides hints about what to expect - and why - in your own area. 

 

 

MOUNTAIN WEATHER: PEAK AND VALLEY CONTRASTS

 

    Mountains have orographic effects, so that there are very rapid spatial variations in precipitation - both on a day-to-day and on a climatological average basis.  Temperatures also vary.  So topography plays a vital role in mountain weather and climate. We will look at the temperature effects.

 

 

        The worst climate in the state?

 

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   There is a decrease in average temperature with altitude.  As a consequence, for example, the length of the growing season decreases with altitude.  We can often note the effect, particularly in spring, by going from the Piedmont into the Mountains - or vice versa - and noting that the Mountain vegetation is one or two weeks 'behind' the Piedmont.

  This simple relationship commonly holds for the day-to-day temperatures.  The exact difference between sites at two different altitudes will depend on their topographic setting and on the synoptic situation (Fig 3).  For some wind directions one might be sheltered, whereas it is exposed when the wind is from a different direction. We would expect that, in general Charlotte would be warmer than Asheville, which would, in turn, be warmer than the summit of Mt. Mitchell.  On most days this is true, although the daily variation is apparent (Fig 3).  In all seasons, however, there are a few days when Asheville is warmer than Charlotte, although this is rare in July.  In winter there are even the occasional days when Mt. Mitchell has a higher temperature than Charlotte!

Fig 3  Frequency distribution of differences in temperature between Charlotte and Mount Mitchell ((CLT-MIT) and Asheville and Mt. Mitchell (AVL-MIT), for four mid-season months.  Note that occasionally Mt. Mitchell is warmer than Charlotte.

 

 

        The Thermal Belts: A history        

 

 

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    In the mid-nineteenth century, Silas McDowell a mountain farmer, noted that his crops seemed to do well on the valley sides, but not in the bottoms or on the ridge crests.  He sought advice and information from various professors, and Dr. LeConte (after whom the mountain is named) responded that this was an area of 'thermal belts'.

    The name stuck, and now in the area are places such as "Thermal City" and "Isothermal Community College"

 

 
Road sign in Polk County indicating the presence of the Thermal Belts State roadside marker honoring Silas McDowell (to come)

 

    The valley side 'thermal belts' became a favored site for apple orchards, and the region was highly productive and prosperous. Early in the twentieth century the North Carolina Department of Agriculture, in cooperation with the US Department of Agriculture, undertook a measurement program to find out exactly what the temperatures were.  Sets of observing stations were installed up about a score different hillsides and a unique dataset was developed (Fig 4)..
Fig 4: Map of locations of stations and area generally regarded now as the thermal belts
One of the Hendersonville stations for the Thermal Belt research.  The area then, 1916, was already involved with apple growing. (Photograph courtesy of NWS)

 

 

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Fig 5: The variation of the length of the growing season with altitude for selected stations of the Thermal Belt study.

    The results shown in Fig 5  indicate the influence of landscape on temperature, using the length of the growing season as a measure of relative warmth.  The lines join the observation stations on the same hillside.  Like most environmental data, some lines clearly indicate cold tops and bottoms, warm slopes, while others seem to work the other way.

  Unfortunately the original data were lost, and only the summary data are now available. We cannot explore this phenomenon further using this data set.

 
    Later investigations in other areas, along with increased understanding of the factors causing temperature changes, identified the night-time long-wave energy loss from the mountain tops as creating the cold summits. The cold, dense air then drained down the slopes  via the katabatic wind, to collect as a pool of cold air at the base (Fig 6).  Meanwhile, the slopes remained relatively warm.   

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Fig 6:  Katabatic and anabatic winds, former leading to the creation of a frost hollow.

 

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    The thermal belts are still there, but agriculture has changed and the region no longer grows the apples for which originally became famous.

Remains of an abandoned orchard on a gentle slope in the Thermal Belt southeast of Henderson 

 

 

 

     The pool of cold air collecting at the bottom of the valley - most likely to be seen on calm, clear nights - constitutes a 'frost hollow'.  This is, obviously, a place susceptible to frost.  Indeed, if we look at  growing season lengths across the area we have been considering generally as the thermal belts, there are several mountain relationships.  In the relatively low area of the Piedmont west of Charlotte there is a gentle decrease in growing season with altitude.  Once we are into the mountains and in the valley of the French Broad, there seems to be little difference in length with altitude.  The stations are all at more or less the same altitude, but have very different growing seasons.  Indeed, this is a response to the local topography around the various stations.  In the mountains themselves there is a return to the simple growing season - altitude relationship.  There is a great exception, however.  There are two stations with a growing season less than 140 days.  One is the Mt. Mitchell summit.  The other is the station at Celo, close to the mountain, but over 1250m (almost 4000 ft) lower.   This is a true frost hollow.  The frost dates for mountain stations (Table 1) bear this out.

 

 

      

 

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Fig 7 Growing season length as a function of altitude in western North Carolina.

 

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    Frost hollows are by no means restricted to the mountains.  Many Piedmont valleys are cold spots on calm clear nights.  They occur even on the Coastal Plain.  

    The valley of the Meherrin River near Murfreesboro,  Hertford County, is a well developed one. The 'gorge' is sheltered from wind in virtually all directions, so that the climate tends to be considerably cooler than the surrounding areas. This provides the right environment for a vegetation community we would normally expect to see in the mountains, not the Coast.

Meherrin River near Murfreesboro, Hertford County.  The river has eroded a 'gorge' deep enough to act as a frost hollow.   

 

 

Table 1:The probability of occurrence of the last (32oF) frost in spring and the first frost in fall on or after the indicated date, for several selected NC stations. Frost is defined here as an overnight air temperature at or below 32oF. The length of the growing season is included.
Station Altitude Last in Spring First in Fall Growing Season Length
    Probability Level (%) Probability Level (%) Probability Level (%)
    90 50 10 90 50 10 90 50 10
Andrews   4/19 5/04 5/19 09/27 10/10 10/22 180 158 136
Asheville   3/28 4/10 4/24 10/11 10/24 11/06 213 196 179
Celo   5/02 5/18 6/04 09/16 09/30 10/13 154 134 113
Transou   4/28 5/16 6/03 09/16 09/30 10/13 155 136 117

 

 

Mountain weather through the year

 

 

   

THE PIEDMONT: TOWN AND COUNTRY CONTRASTS

 

    The Piedmont is a region of rolling hills, with effects such as frost hollows and orographic effects akin to those of the mountains.  The effects are less marked.  At the same time it is an agricultural area with a mixture of crop land and forest, wet areas and dry patches.  However, the differences are less marked than those of the Coastal Plain. Probably what makes the Piedmont distinct is the present of the major cities of the state.  So the emphasis here is on the interaction of humans and weather and climate.

 

Our role in altering local weather

    Much of the discussion throughout the book so far has been bee on ways in which the weather influences us, or - occasionally, ways in which we can use the weather.  Section 5.6 also indicated that human activity can modify the atmosphere itself.  We can also modify the weather and climate of an area.  Both deliberately and inadvertently.  So our discussion of the Piedmont pays particular attention to this.

 

 

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      One of the most common ways to modify the weather deliberately - or at least provide shelter from it - is to build a house.  Over the centuries housing styles in an area have reflected the building materials and technologies available and the weather.  Houses in snowy climates tend to have steep pitched roofs to shed the snow, desert dwellings tended to have thick walls and small openings to combat the heat of the day and the cold of the night.

    One of the main problems for us is the heat and humidity of summer.  Porches provide shade, while the stoop - whether or not it has a swing - maximizes the chance of catching and passing breeze.

Photo 7.2.1:  An old style house with overhang, generally well adapted to the weather of the southeast USA.

 

    A second way of deliberately modifying, which we have already considered briefly, is irrigation.  This is a major user of our water - and thus ultimately dependent on our rainfall in all three regions of the state.

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Irrigation equipment on the coastal plain - Bladen County

     

   

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   For the Piedmont, probably the greatest strain on water is the demand of the ever-growing population.  New reservoirs are needed, or old ones must keep up with demand.  Although reservoirs smooth out the variations in rainfall over time, they still cannot always compensate for particularly dry periods.  So, although the climate may not be changing much, it can seem that the frequency of 'droughts' is increasing.

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New developments, whether residential or commercial, and anywhere in the state, increase the demand for water. A reservoir cannot always meet the demand, particularly in dry periods, and drawdown of reservoir levels becomes apparent..

 

 

The climate of cities

 

    The building of a city leads to changes in virtually all aspects of the weather.

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Table 7.2.1:  Contrasts between urban and rural environments

 

  

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    The most characteristic change is the development of an 'urban heat island', with temperatures within the city being a few degrees warmer than the surrounding rural areas.  The detection of this effect requires special observation programs.  Only in Chapel Hill has this been explored in any great detail.  This involved a series of automobile traverses. A recording thermometer was attached to the front wing of the car, shielded to make sure that engine heat did not affect the sensor.  The temperature was recorded as the car proceeded across the city. The traverses were made at night, ensuring that any temperature changes were due to urban effect, not to differential solar heating. Fig 7.2.1 shows the results of one such traverse.

 

    Although it is difficult for us to detect this effect without this kind of special instrumentation, we often see one result - blossoms or blooms frequently occur in the central parts of cities a week of so before they do in the suburbs.

 

Fig 7.2.1:  The Chapel Hill, Orange County, urban heat island. 

 

    The NWS Cooperative observing stations that we have been using for most of our numeric information so far rarely have pairs of stations, one in a city center, one in the suburbs.  Nevertheless, it should be possible to detect differences in temperature trends between a rapidly expanding city, such as Charlotte and the predominantly rural stations surrounding it. Using stations shown in Fig 7.1.2, the major factor appears to be the rapid year-to-year changes (Fig 7.2.2).  Most display a cooling in the 1950s and 1960s and then general warming. Shelby displays a major cool period in the early 1960s and then returns to values similar to other stations.  Towards the end of the period Charlotte appears to be warming faster than the other stations.  This may indeed represent an increasing heat island effect. (Data for the rest of the 1990's to be added).

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Fig 7.2.2:  Annual average temperature departures from the long-term mean for selected cities around Charlotte.

 

 

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  A similar analysis, using July minimum temperatures at Greensboro (Triad International Airport) and Mt. Airy ( a station in a rural area), shows two trends (Fig 7.2.3).  First, both stations are getting warmer. This might be a sign of a local expression of global warming, discussed in section 6.5.  Second, the warming is more marked in Greensboro that in Mt. Airy.  An urban heat island effect?  (check with more stations/months...)
Fig 7.2.3: July average minimum temperature trends from Greensboro and Mt. Airy

 

    The buildings of a city create a rough surface - whether the high rise buildings and high building density downtown, or the suburban mix of detached houses and plenty of trees - which leads to an overall decrease in the wind speed in an urban area.  The buildings themselves, especially those downtown, act as obstacles to the wind (Fig 7.2.4).  In some cases this leads to the funneling of the wind, and an increase in wind speed in local areas.  In other cases the building may provide shelter, or it may set up cross currents.  The overall result is that both wind speed and direction can vary greatly over short distances within a city.  

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Fig 7.2.4:  Examples of buildings providing obstacles to the wind.

 

Will development change our weather?

 

  Putting together most of the previous considerations, the changes made by humans - with the exception of the enhanced greenhouse effect and possible global warming - are all rather small scale.  The result is that the local weather or climate may be changed, sometimes dramatically, but usually in small ways.  At present, however, they individual small changes in isolated patches have not come together to create major changes in the atmosphere.

    It is not at all clear, not just for North Carolina, but for anywhere in the world, how big an area, and how big a change, is needed before they begin to affect regional and eventually global, conditions.

 

 A Piedmont meteorological year

 

 

 

 

 

THE COASTAL PLAIN: LAND AND WATER CONTRASTS

 

 

   Weather and climate difference, apart from them generated by the immediate coastline, are not as apparent on the Coastal Plain as they are in the mountains.  Nevertheless, there are several important variations across this large region.  Most of them are to do with the type of surface, particularly the amount of water in the soil, in an area.  Further, junctions between different areas may play a role in the generation of storms.

  

 

Sea and land breezes

 

    Sea breezes are generated because land and sea respond differently to the daily input of solar radiation.  Early on a cloudless morning in calm or near calm conditions the coastal land will warm rapidly in response to the increasing solar intensity.  The nearby water, in contrast, will warm very slowly.  As the day progresses the temperature of the land, and the depth of the warmed column of air above it, will become much greater than that over the water.  This will set up a pressure different, with higher pressure over the water, low pressure over land. Eventually air will start to move towards the lower pressure and a sea breeze will form.  

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Fig 7.3.1: Schematic diagram of the formation of sea- and land-breezes.

 

 

 

 

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    The sea-breeze air will flow inland, encouraging the already unstable air over the land to rise as a series of cumulus clouds (Photo 7.3.1). In complete calm the clouds will then move sea-ward in the counter-current aloft.  Rarely in North Carolina is there complete calm, and inland drift - sometimes for 40 or 50 miles or so - is most common.

    The coastline from Morehead City to the South Carolina border is the main area of likely sea breeze formation.  North of this the complex intermixture of land and water, barrier island, sound, field, and swamp offers little opportunity for formation of the required temperature contrasts.

    

 

 

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Photo 7.3.1a: Lines of cumulus clouds formed along the coast as a result of sea breeze action.  The view looks north across the Brunswick County mainland from the Sunset Beach causeway. 

 

Photo 7.3.1b: The same sea breeze system, 30 minutes later, and some 10 miles inland.  Many of the clouds drifted overland in response to a light breeze above the surface.  They then re-evaporated into the warm atmo sphere. 

 

 

    The land breeze, the night-time counterpart of the sea breeze (Fig 7.3.1) is weak.  Land-sea temperature contrasts are usually small, so that the temperature gradient, the driving force for the wind, is also small.  We would expect it most in the summer, particularly if the Bermuda high is sitting above us.  In that case, however, the high humidity tends to discourage much land-wave radiation loss, and night-time temperatures do not decrease much.  Unfortunately, we have few sensitive wind instruments along the coast to actually determine whether we get this breeze.  So the most we can say is that we believe that the land breeze is rather rare. 

 

Soil, vegetation and weather

               

 

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    Different surfaces much give different energy regimes, and as a result have very different temperatures.  Much of the coastal plain is well watered - some of it being clearly swampy.  But some is much drier.  This is particularly tru of the Sandhills.

    The soils of the Sandhills region  is predominantly sand.  It drains very quickly, and even a few minutes after rain, it appears dry.  A more 'normal' soil is much slower drying. Consequently the vegetation is adapted to this alternate wet/dry soil, and pines predominate.  Also as a direct consequence, temperatures tend to be higher.  The Sandhills region has the reputation of being North Carolina's 'tornado alley' (see Section 6.2) and many of the highest temperatures are recorded in the region.

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Fig 7.3.2:  Sandhills regional landscape.  The soil encourages drying and high temperatures and, probably, although data are sparse, low humidities. Fig 7.3.3:  A Coastal Plain landscape almost opposite to that of 7.3.2 - here the abundant standing water should ensure cooler temperatures and higher humidity.

 

    There is also increasing evidence that the contrast between wet and dry areas - crating differences in the amounts of the latent and sensible heat from the surface into the atmosphere - may have a role in the re-intensification of storms.  THis seems to be particualrly important for hurricanes after they have come onshore.

 

 

    For many years we have known that in winter the temperature contrasts between the cold land and the warm water, especially the warm offshore waters of the Gulf Stream, have helped to generate the "Hatteras lows" as major storms.  These often turn into nor'esters on our northern coasts. Camd_402.jpg (9324 bytes)  (left)  Nor'easter conditions at the South Mills locks of the Dismal Swamp Canal - clouds, persistent rain, strong wind.  The canal waters have been driven southwards and have collected here as exceptionally high canal levels. Behind the photograph the waters leading to Albemarle sound are much lower than normal.

 

The Coast:  Storms and climate change

 

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      The coast, particularly the Outer Banks, has a unique climate - and a unique response to climate.  Wind and wind-driven waves (aided and abetted by human actions) create dunes which are at the mercy of the elements. 

    Knowledge of micrometeorology may help to decrease the rate of movement and stabilize the dunes.  A sand fence (working the same way as a snow fence in higher latitudes) slows the wind, causing it to deposit its load. 

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Erosion is slowly, inexorably moving the coastline westward.  Things standing n the way of the advancing sea are, inevitably, removed.   A sand fence is rebuilding a dune after a blowout during a winter storm near Cape Hatteras.

 

 

      Vegetation also helps preserve dunes.  The roots add organic matter and structure which binds the sand so that it is less easy to displace.  In addition, it provides obstacles to the wind, increasing surface friction, and thus greatly slows the wind right near the surface.  Less sand can then be moved by this slow wind.

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   Grass planted to help (along with the sand fence) stabilize the dune.

 

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    The current westward dune movement is a response to a rising sea level.  This is largely a result of climate change. During the past few hundred thousand years there have been cold periods, and sea level has been as much as 85 m below the current level.  Equally, there have been warm periods, with the level up to 15 m above the present.  Naturally, we can see rather little evidence for the lower conditions, but for warm episodes there is evidence of old shore lines.  These can be dated and give an indication of the rate and amount of climate change.
A slight rise in the road indicates the western edge of the Great Dismal Swamp and an old shore line - US 158 in Gates County just west of the Pasquotank County line

 

March of the seasons in the Coastal Plain

 

 
 

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