TEMPERATURES IN NORTH CAROLINA

 

    With the great variety of energy flows which influence temperature, it is not surprise that the pattern of temperatures across the state is complex and variable.  Here we consider mainly the state-wide situation, and describe the general or typical averages, extremes, sequences and trends, with a concluding section reviewing methods of measuring temperature. 

 

Average temperatures around the state

 

     Letís start our consideration of the observed temperatures with the annual average conditions (Fig 1).  This map tells us many things, and some may not be very surprising.  That temperatures decrease from east to west - that it is colder in the mountains - is probably no surprise.  Similarly, we would probably expect the north to be somewhat colder than the south.  But the pattern is not even. The Sandhills area west of Fayetteville, for example, seems to continue the coastal warmth some distance inland, possibly because of the dryness of the region.  The coastline itself seems to be distinct, probably reflecting our earlier finding that this is the area away from the continentality effect.  Further, although it is very difficult to be too definite with a map of the whole state, temperatures appear to change rapidly with location in the west, but only slowly in the east.  

 

    To provide a very general summary, mean annual temperature ranges from the low 50's in the northern mountains to the low 60's along the southern coastal plain.

NCTemp-Annual_mean_map.gif (10996 bytes)

 

 

Fig 1: Long-term average annual mean temperature for North Carolina. This map, along with most of the others in this section, is based on 30 years of daily temperature observations at upwards of 200 stations around the state.

wpe4.gif (7724 bytes)

    The monthly and seasonal patterns across the state look similar to the annual one, so instead of a series of similar maps we can show a graph of the monthly values for the 8 climate divisions (Fig 2).   The two mountain divisions stand out as the cold areas throughout the year.  In winter the Piedmont is generally cooler than the Coastal Plain, but in summer the differences are small.  In each of the three physiographic regions the north is cooler than the south, commonly by a couple of degrees in winter and nearer one degree in summer.  This seasonal difference reflects the general equator-to-pole temperature gradient, which for most areas is much steeper in the winter than summer.  This difference is a major cause of the much more vigorous weather of winter.

Fig 2: Long-term average monthly mean temperature for selected individual stations. 

   

   Further, the curves also show that all divisions have their maximum temperatures in July, their minima in January.  This follows from the previous section's discussion of energy flows as the cause of temperatures.

 

NCTemp-Jan_min_map.gif (11562 bytes)

 

The average minimum temperatures in January (Fig 3) range from the low 20's in the mountains to the low 30's near the coast, and approaching the mid-30's as the Outer Banks are reached..  

    At the opposite extreme, the average July maximum temperature (Fig 4) shows the familiar pattern, with values from 70 to 90 from west to east.

NCTemp-July_max_map.gif (10802 bytes)

Fig 3: Average January minimum temperature across North Carolina.

Fig 4: Average July maximum temperature across North Carolina.

 

 

  

 

 

NCTemp-Jan_sta_max_min.gif (6125 bytes)

   

  NCTemp-July_sta_max_min.gif (6066 bytes)

    Comparison of the maximum and minimum temperatures for selected stations at these same months, again emphasizes that the mountains are the coldest region in each season (Fig 5).  Wilmington has the highest winter maximum temperatures, but slightly lower winter minima than does Cape Hatteras. In summer, however, Fayetteville has the highest daytime temperatures, while Cape Hatteras again has the highest nighttime ones.  In general, therefore, the actual temperature distributions reflect the action of the various energy streams considered in the last section.  The maritime influence on the coastal stations and the effects of sandy soils on the Fayetteville temperatures indicate this.

 

Fig 5: Long-term daily maximum and minimum temperatures for selected North Carolina Stations for (a) January and (b) July.

 

1-AVL; 2-CLT;3-FAY;4-RDU;5-ILM;6-HAT

 

 

 

 

 

 

Day-to-day temperature fluctuations 

 

    The averages we have discussed so far say little about day-to-day variability.  A major seasonal contrast involving temperatures is the difference between the rather small day-to-day temperature changes of summer compared to those of winter (Fig 6).  Rarely do these changes involve temperatures alone.  Summer may be characterized, at least outside the mountains, as a continuous daily stream of hot and humid weather, the only concern being whether it will rain or not.  Winter, however, may be a seemingly constant stream of changes, one day with cold, dry, and cloudless conditions, to be followed by a day or so with warm, damp, cloudy conditions which in turn yields to another spell of cold clear air. 

NCTemp-GSO_jan_1995.gif (19658 bytes)

NCTemp-GSO_jul_1995.gif (17438 bytes)

 

Fig 6:  Typical day-to-day maximum and minimum temperature fluctuations in January and July.  In this case the station is Greensboro - Winston Salem - High Point Airport,1995.

 

    The seasonal difference arise because summer is dominated by one type of 'air mass', the hot humid one, while winter has two, a cooler version of the summer one and a contrasting cold clear one.  We shall  consider these in detail, and continue our exploration of the factors creating temperature changes on a day-to-day basis as a major part of our weather, later.  

 

Temperature extremes

 

    No discussion of temperatures is complete without consideration of temperature extremes.

 

    The record for the North Carolina all time high is held by Fayetteville, with 110oF on 21st August 1983.  Several other stations observed their record high on or near this date (Table 1).  There was a similar widespread hot spell in the middle of August 1988.  Both these periods led to major heat waves. Records at other stations were established on a wide variety of dates.

 

    The actual values show that the extreme maximum temperatures which are likely to occur range from the mid-to-high 90's on the shore and in the mountains to  around 105oF on the coastal plain and Piedmont.

Station

Temperature

Date

 

Asheville

 

100   

 

8-21-83

Cape Hatteras

96   

7-10-92

Charlotte

103   

5 times - incl

8-21-83

Elizabeth City

107   

7-18-42

Grandfather Mtn

91   

8-27-68

Greensboro

103   

8-18-88

Laurinburg

107   

8-18-88

Murphy

99   

8-23-83

Raleigh-Durham

105   

7-23-52

8-18-88

Wilmington

104   

6-27-52

Table 1: High temperature (oF) records for selected stations in North Carolina

 

 

 

Dupl_504.jpg (41555 bytes)

    I, along with many other people, have seen electronic time/temperature displays indicating that the temperature has exceeded the state record - and we have probably been ready to believe it. However all 'official' weather observations have to be made using standard methods, instruments and exposures. We will expand on this a little later, it is enough to say here that the official observations must be shaded from the direct glare of the sun, and they must be made over a grassy surface. Grass is almost always cooler than tarmac, simply because it can loose energy, and therefore cool, by evaporation.. 

A typical bank time/temperature sign, in an area surrounded by concrete or other hard surfaces.  Such surfaces, unlike grass, provide little opportunity for cooling by evaporation.

 

   

wpe9.gif (5409 bytes)

 

 

    Using the official observations to determine less extreme, but still hot conditions - the number of days each month with temperatures over 90F - the pattern across the state appears somewhat surprising at first glance (Fig 7). The concentration in the summer is probably not surprising, and neither is the fact that high-elevation Asheville has fewer days than Raleigh, which in turn has fewer days than coastal Wilmington.  However, Cape Hatteras right on the coast has very few days over 90 even in the height of summer. The low values at Hatteras are a direct result of its shoreline position.  Hatteras is a rather windy place, which helps to keep things cool.  In addition, much of the time that wind is blowing from the sea and, because of the continentality effect, is cooler than wind from land. The Wilmington station, at the airport, is a few miles inland, enough to cut down wind speed and remove the cooling effect of the oceanic air.

Fig 7: The average number of days each month with average temperatures above 90oF for selected stations.

 

    When we turn to the low temperatures and need to establish records, we have a somewhat different problem from that at the high end.  Temperatures rather rapidly decrease with altitude, so that in most cases the thermometer on the highest mountain-top sets the low temperature record.  Currently that is held by Mount Mitchell, with -34oF recorded on January 21, 1985.  On that day low temperature records were set over virtually the whole of the state (Table 2) 

 

    The exception was the southeast coast from Southport to Morehead City, which had record cold - just around 0oF - on December 25, 1989.  This was the time of a major snowstorm.  The presence of the snow surface undoubtedly influenced the energy balance at this time and fostered the unusually low temperatures.

Station

Temperature

Asheville

-16       

Cape Hatteras

6       

Charlotte

-5       

Elizabeth City

-2       

Grandfather Mtn

-32       

Greensboro

-8       

Laurinburg

-3       

Murphy

-16       

Raleigh-Durham

-9       

Table 2:  Selected stations where all-time low temperature records were established January 21, 1985.

 

 

NCTemp-MtMitchell_Kress.jpg (78194 bytes)

 

 

 

    Although Mt. Mitchell holds the low temperature record, it is difficult to construct a long term climatology for the mountain.  There have been numerous station moves over the years.  Originally at the summit (Fig 8), it has had to be relocated several times, both to ensure that the weather did not destroy the instruments and to allow observers to read those instruments whatever the weather. The relocations thus increased the amount of shelter and reduced the altitude (by over 300 feet).   Both tend to increase the temperature which is recorded, so that long-term averages or trends are difficult to compute.

 

    As a result, we tend to use the Grandfather Mountain station for such purposes.  This was started in 1955 and since then has had only one minor relation, in 1986, which decreased the altitude by about 6 feet.  Consequently this has a much more consistent record, and we tend to use this in considering North Carolina's mountaintop climate.

Fig 8: The summit of Mt. Mitchell in 1954, showing the thermometer screen and the rain gauge just below the mountain peak (Postcard published by S. H. Kress & Co.).

 

 

Long-term temperature changes

 

 

wpeB.gif (7967 bytes)

    During the last 100 years the annual average temperatures for the state as a whole (Fig 9) have risen, dipped, and risen again.  The state is now somewhat warmer than it was 100 years ago.     

    When we look at long-term trends in individual places, or for individual months, the results are not as simple as Fig 9 would imply.  Some places seem to have an overall warming, some a small cooling (Fig 10). 

NCTemp-trend_map.jpg (95285 bytes)

Fig 10: Annual temperature trends across North Carolina - a positive value indicates a warming trend, a negative one cooling.

Fig 9:  Long-term trend in annual average  temperature for North Carolina and the globe

 

    Trying to summarize, or even make sense of, the temperature changes over time, therefore is very difficult.  Since there is no clear, marked trend in North Carolina temperatures, there is likely to be little benefit in considering changes when we make practical use of climate information.  For example, the applications in the next section do not consider it.  Some applications, very sensitive to small temperature changes over long times, may need precise and detailed analyses. These would be for much more specific purposes than the general ones considered here. 

 

    This is not to suggest that possible global warming as a result of the greenhouse effect is not important. Clearly the planet as a whole has been warming over the past century, very rapidly over the last couple of decades.  So global trends suggest that during the next few decades there will be marked and significant temperature changes.  These in turn will change our weather patterns.  We shall look at all of these possible changes in section  6.5.

 

Measuring temperatures

 

          Almost all of the information given in this chapter comes from official National Weather Service observations.  The NWS runs a 'network' of stations, where all the stations observe in the same standardized way.  This means that the same types of instruments are used, they have the same exposure to the atmosphere, the surrounds are all of a uniform type, and the observation times and te chniques are all the same.  Holding to the standards means that we are able to compare observations from place to place and from time to time.

 

Oran_107.jpg (38448 bytes)   

 

            For much of the past 100 years the standard NWS temperature measuring instrument has been the mercury-in-glass thermometer mounted in a meteorological screen (Fig 11).  As was indicated when considering 'bank temperatures' above, such shelter is vital.  It must protect the thermometer from direct sunlight, or else we would have little information other than whether the sun hit the bulb or not. It must also provide ventilation, otherwise it would be measuring the temperature of a hot box, not the free air. The shelter itself should be away from other obstacles which could radiate energy onto the shelter and the thermometer (Fig 12).  In our part of the world it should also be over a grass surface, to ensure that the solar energy reflected to the bottom of the shelter is uniform for all sites.

Oran_106.jpg (51929 bytes)

 

Fig 11: A thermometer shelter with mercury-in-glass thermometers installed. This is the interior of the stations shown in Photo2.3.3 Fig 12: The instrument shelter at the Chapel Hill 2W station in 1985

 

 

 

          Standardization is needed if we are to truly determine the climate and changes in the climate.  But it is very difficult to maintain over a long period of time and a large number of stations.  For example, one problem throughout our nation is the problem of urbanization.  When stations were started a hundred years or more ago, they were often in very isolated sites.  Now, without any move on their part, they are on the edge, or even near the middle, of towns.  And towns create their own climates such that they are commonly warmer than the surrounding rural areas

Gran_101.jpg (15941 bytes)

Fig 13: A modern electronic shelter, although rather near a building and so likely to give readings which are rather "warm" compared to the surroundings.

 

 

dare_107m.jpg (35500 bytes)

    Social and technological changes make it difficult to use the old-style thermometers and shelters shown in Figs 11 and 12.  New instruments are now being introduced (Fig 13 and 14). This new instrumentation also has the potential of creating temperature changes having nothing to do with the climate.  So, while these long-term records are very important, especially with the current concern with global warming, they must be used with care because local temperature changes may be completely mixed in with global ones.

 

Fig 14:  A modern suite of observing instruments at Cape Hatteras.

 

 

 

Return to Course Schedule