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Overview

Rapid swings in temperature and precipitation are normal for Orange County, but create stressful conditions for plants. In addition to our routinely erratic weather, unusually severe events like a 100-year drought, a 100-year ice storm, and record summer heat have occurred recently. Resilient plants, careful siting, and good soil preparation are critical for long-term success.

USDA hardiness zone 7b on the 2012 map
average annual minimum temperature of 5–10°F,
a half-zone warmer than the 1990 map
AHS heat zone 7 (61–90 days where temp. exceeds 86°F)
Ecoregion 45c (Carolina Slate Belt)
NC climate division Northern Piedmont
Average first frost
Average last frost
October 21–31
April 11–20
*It is best to wait until 2 weeks after Apr 20 to plant tender transplants.*
Growing season 190 ± 14 days (reference period 1950–1980)
Avg. sunny days per year 230

The Chapel Hill weather and climate website lets you change to a different location by clicking on the STATE MAP button at the top of the page. Data on this site may differ from the above because the site uses the entire period for which records are available, usually beginning 1/1/1891.

Climate divisions

climate division map of NC

NOAA created the climate divisions to monitor drought, temperature, precipitation, and heating/cooling degree days at a regional level. Regional data can provide a more accurate picture of our weather trends than looking at data from any one individual station. Currently, for years 1931–present daily station values from all of the Cooperative Observer Network (COOP) stations in a division are averaged to compute divisional monthly averages/totals. For the 1895–1930 period where data are less complete and of lower quality, values are derived using a regression technique. NOAA is transitioning to a 5 kilometer grid system with more stations, better quality control, adjustments to eliminate bias, and improved computational methodology to address topographic and network variability.

Plant suitability

Climate numbers provide a limited picture of whether a particular plant is adapted to our area. Plant Hardiness Zone Maps and Wikipedia discuss the shortcomings of traditional climate data in greater detail. Ornamentals provides online lists of locally-adapted plants and has a detailed discussion of factors that determine adaptation. We also have handouts on woody plants, perennials, and annuals that are adapted for our area. Adaption depends on all of the factors below:

  • winter hardiness
  • tolerance to heat, drought, humidity & tree root competition
  • disease resistance
  • sunlight & daylength requirements
  • dormancy/chilling requirements
  • soil preferences
Unique climate features

It is natural to think of the Triangle as a single climate region, but the topography of North Carolina creates distinct weather patterns that are not always shared between adjacent counties. Ice storms are a good example — Orange County lies near the center of an ice storm alley that affects NC unevenly. Severely damaging ice storms (more than ¾″ freezing rain) like the ones in 2002 and 2014 occur approximately every 10 years in Orange County, but only every 15 years in Durham County.

likelihood of damaging ice storm State Climate Office of NC A Winter Weather Climatology for the Southeastern United States

The sections below were adapted from various articles by the State Climate Office of NC.

Jet streams

Two jet streams directly affect our weather: the polar jet and the subtropical jet. Both move north–south seasonally, shifting with global horizontal temperature fields that follow the areas of strongest sunlight.

globe with jet streams NC CES Jet streams

The polar jet stream is more important for NC. For NC, the strongest effect of the subtropical jet is in winters during El Niño years when it is positioned directly over southern Georgia and Florida, giving us cooler and wetter weather than normal.

polar jet stream NC CES Jet streams

Weather systems

The Bermuda High and the El Niño - Southern Oscillation (ENSO) cycle influence the positions of the jet streams that control our weather.

  • In summer the Bermuda High is usually the major factor, creating hot, humid conditions.
  • El Niño years are usually cooler and wetter, especially in winter, with a relatively mild hurricane season.
  • La Niña years are usually hotter and drier, especially in winter, with a stronger hurricane season. The effect of La Niña on summer drought is complex and relatively small.
Bermuda high

The Bermuda High is a semi-permanent high pressure system in the Atlantic Ocean that influences the position of the jet streams and the El Niño-Southern Oscillation.

Bermuda High USA TODAY research by Chad Palmer

In summer, the high sits just off the east coast. Wind around the high blows warm, moist air from the tropics inland, creating relatively wet, humid conditions. The moisture permits late-afternoon convective storms, but the high blocks most storm systems from the west, keeping our summers dry. When the high is strong, it can bring summer droughts and heat waves.

In winter and early spring, the High moves eastward, permitting the jet stream to dip farther south and bring wintry weather and spring showers into our area.

El Niño - Southern Oscillation (ENSO)

The ENSO cycle consists of coupled variations in the surface temperature of the tropical eastern Pacific Ocean (El Niño — La Niña) and air surface pressure in the tropical western Pacific (Southern Oscillation). The warm ocean phase, El Niño, coincides with high air surface pressure in the western Pacific, while the cold phase, La Niña, coincides with low air surface pressure in the western Pacific.

Oscillations vary in strength and are not evenly distributed over time.

ENSO over time modified from NOAA Multivariate ENSO Index

The influence of the oscillaton is greatest in winter.

Weather El Niño La Niña
Winter precipitation wetter drier
Winter temperature cooler warmer
Hurricane season milder stronger

El Niño (warm phase)
Effects are strongest during winter because ocean temperatures worldwide are at their warmest. The increased ocean warmth enhances convection, which then alters the Pacific jet stream so that it becomes more active over the South.

NC CES El Nino

La Niña (cool phase)
Decreased convection across the entire equatorial Pacific results in a more suppressed southern jet stream.

NC CES La Nina

Links

Wikipedia: Bermuda High
National Weather Service Climate Prediction Center: ENSO
NC State Climate Office: ENSO
Wikipedia: Jet streams

The fraction of time spent in each of the 5 sky cover categories


fractional time for cloud cover types Weatherspark: Greensborough, NC 1974–2012 Cloud Cover Types

Daily cloud cover


median cloud cover percentages

Black line median daily cloud cover
Inner band percentile 40–60
Outer band percentile 25–75

Weatherspark: Greensborough, NC 1974–2012 Median Cloud Cover

Direction

The position of high and low pressure systems relative to our location determines wind direction. The prevailing wind blows from the southwest except in September and October, when it blows from the northeast due to prevailing high pressure north and northwest of the state and the frequent low-pressure storms off of the coast.

The graph shows the daily fraction of time spent with the wind blowing from the various directions. Stacked values do not always sum to 100% because the wind direction is undefined when the wind speed is zero.

wind directions Weatherspark: Greensborough, NC 1974–2012 Fraction of Time Spent with Various Wind Directions

Speed

The graph below shows that we have windy winters and springs. Evergreens and newly-installed plants can easily dessicate, so be sure to keep them watered. You should stake newly-planted trees and many shrubs, especially evergreens.

monthly wind speeds Weatherspark: Greensborough, NC 1974–2012 Wind Speed
average daily minimum
average daily maximum
average daily wind speed
Inner colored band: percentile 25–75
Outer colored band: percentile 10–90

Windbreaks

Water is lost from soil by evaporation and from plants by transpiration from leaf surfaces. Winds during winter and spring can be fierce and can cause significant evaporative loss from soil and transpirative loss from susceptible plants, as well as erosion from dormant garden areas. A properly designed windbreak reduces both evaporation and transpiration loss, as well as creating valuable habitat for wildlife.

Generally, a windbreak protects an area 10-15 times its average height.

windbreak design Lincoln, SD Conservation District Windbreak considerations

A windbreak actually should not be solid like dense evergreens or a board fence. A windbreak with 40–60% density in two staggered rows works well and should be comprised of a diversity of species to discourage diseases that can plague monocultures.

  • As wind is deflected up and over a windbreak, low pressure on the downwind side draws the wind back down. This low pressure is stronger in dense windbreaks, drawing the wind down quickly and reducing the protected area size.
  • Windbreaks with high density also tend to decay over time due to root competition and shading and any gap (like a path or driveway or plant decline) acts to funnel wind through at high velocity.

Temperature is only one of many factors that affect plant performance. For example, USDA Hardiness Zones are drawn solely on winter average annual minimum temperatures. While Seattle, Tucson, and Atlanta are all in the same hardiness zone, their rainfall and summer growing conditions are very different.

Average temperature


average temperatures over the year daily average low temperature
daily average high temperature
Inner colored band: percentile 25–75
Outer colored band: percentile 10–90
Weatherspark: Greensborough, NC 1974–2012 Daily High and Low Temperature

Fraction of time spent in various temperature ranges


fraction of time spent in various temperature ranges

Description Range (°F)
Freezing 15–32
Cold 32–50
Cool 50–65
Comfortable 65–75
Warm 75–85
Hot 85–100

Weatherspark: Greensborough, NC 1974–2012 Fraction of Time Spent in Various Temperature Bands

Growing season

The average growing season lasts from the second week in April until the last week in October, though it is best to wait until May 1 to plant tender transplants.

Microclimates

Within any hardiness zone there will be areas that are slightly warmer or cooler because of elevation, urbanization, wind patterns, and other factors. Examples in your yard are: a spot protected from winds and/or sun by a structure, an elevated area exposed to winds or a low-lying place where cooler air settles. These microclimates may permit you to extend your plant selections by at least a half-zone in either direction.

Seasons

Winter

The Piedmont is partially protected by the mountains from the cold air masses that move southeastward across the central states. When cold waves do cross, they are usually modified by the crossing and the descent on the eastern slopes. However, we are too far from the Atlantic Ocean to benefit from its moderating influence, which raises the average winter temperature and decreases the average day-to-night range in coastal areas. The temperature drops to 10–12°F about once during an average winter in central North Carolina, and temperatures as low as 0°F are rare. The temperature falls below freezing on more than half of the days in winter but rarely remains that low for 24 hours.

  • Winter risks to plants include freezing, dessication, and limb breakage.
  • Winter also affects the levels of insect populations during the subsequent growing season.

Summer

In most summers our weather is dominated by the Bermuda High pressure system, resulting in calm, virtually cloudless conditions. Morning temperatures in summer average about 20 degrees lower than those in the afternoon. A temperature of 90°F occurs about 50 times from late March until well into October.

  • Summer risks to plants include high night temperatures and drought stress.

Autumn

Autumn is the season of most rapidly changing temperature. The drop-off is greatest during October and by the end of November, average daily temperatures are within ~5° of the lowest point of the year.

  • Autumn is the best time to plant.
  • Early frosts in autumn can also pose challenges. See Frosts for more information.

Frosts & freezes

  • See Frosts for maps, frost dates, a discussion of types of frost, and suggestions for preventing damage.
  • AgroClimate shows freeze likelihood maps. You input the date range.
  • The SERCC graphs the likelihood that the temperature will drop below a particular value on a given day. For instance, the red line in the first graph shows that in the first week of October, there is less than a 10% chance that the temperature will fall below 36°F. By November 1, the chance has increased to ~85%. Because temperature is measured at five feet above the ground, plant surfaces can be colder than the air temperature suggests. The 36°F red line is therefore a better indicator of the likelihood of frost than the 32°F orange line.

Chill hours

Most temperate plants including fruit trees, berries, and deciduous trees enter a dormant period during late fall and winter. Dormancy enables plants to tolerate freezing temperatures and prevents growth during mid-winter warm spells. Once dormant, plants require accumulated exposure to cool temperatures for resumption of normal growth, referred to as the chilling requirement. Chilling requirement is measured in either of two ways, as accumulated:

chill hours hours below a chilling temperature threshold
chill units chill hours that are weighted by temperature for effectiveness in satisfying species/cultivar requirements

Apples need ~1200 chill hours, blueberries need ~600, while peaches vary (some need as little as ~700 hours). Species with low chill requirements (e.g., peaches) can be susceptible to spring frost because once the chill requirement has been satisfied, a short spell of warm weather induces buds to swell and open.

The AgroClimate Chill Hours Calculator monitors and forecasts chill accumulation (temperature is measured at 6.56 feet above the ground) from October 1–April 30 using the weather station that is the closest to the selected county. During the chill season the current accumulation is shown. During the remaining of the year the prediction for the next season, the historical average, and the past season are shown.

  • Neutral, El Niño (usually cooler winters), La Niña (usually warmer winters), and average conditions can be selected
  • Users can easily view and change the station used for monitoring
  • Users can choose monitoring by 2 types of chill hour

Geographic distribution

Based on the period 1971–2000, rainfall in orange county is not evenly distributed. A semicircular area covering about 40% of the county just west of Durham receives an average of 44–48 inches yearly, while the rest of the county receives about 4 inches less.

annual precipitation NC State Climate Office Normal monthly precipitation totals

Seasonal pattern

seasonal precipitation Northern Piedmont seasonal precipitation totals (10-year moving average)

While yearly precipitation totals are constant, the seasonal distribution of precipitation for our region has changed. The decades of the 1970’s through the 1990’s, which are often used as a reference period, were in fact atypical both for wetness and for evenly distributed seasonal rainfall. The last decade has been very dry, with notable decreases in precipitation in winter and spring. Because many plants have distinct seasonal water needs, the uncertain persistence and /or direction of these trends makes it difficult to predict which plants will thrive here in the future.

  • Summer is usually the wettest season and July is the wettest month. Summer rainfall is also the most variable, occurring mostly in connection with showers and thunderstorms that are highly local.
  • Autumn is historically the driest season, with November historically the driest month. Any significant rain is usually due to hurricanes (the hurricane season is June–November).
  • Precipitation during winter and spring occurs mostly in connection with migratory low pressure storms. Historically, these occur more frequently and are distributed more evenly than in the summer. March is usually our wettest month. Recently, however, our winters have been even drier than our autumns.

Garden-effective rain

The graph below shows the likelihood of different classes of precipitation in Chapel Hill over any 7-day period based on all available historic data (119 years).

KEY  
nominal at least 0.01″
effective at least 0.5″
rainy at least 2″

weekly precipitation

The graph shows that Orange County does not receive reliable, garden-effective rain at any time of year.

  • Reliable rain is defined as having at least an 80% likelihood of occurring.
  • Garden-effective rain is defined as totaling at least 0.5 inches over 7 days.
  • Plant selection and siting should consider ease of irrigation. See Lawns and Water for helpful information.
  • A unique aspect of our Ornamentals page is the integration of our county climate with plant water needs. We have created a list of plants suited to our area, rated by water need. Of course tolerance of our climate is only one criterion and you will find thoughtful lists of plants rated for other qualities as well.
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