5–6 minute read
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)
45c (Carolina Slate Belt)
NC climate division
Average first frost
Average last frost
It is best to wait until 2 weeks after Apr 20 to plant tender transplants.
190 ± 14 days (reference period 1950–1980)
Avg. sunny days per year
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.
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.
- Climate Division Data is more flexible with respect to time period and offers multiple output formats.
- GHCN (Global Historical Climatology Network) lets you easily visualize trends and makes it easy to see departures from the trend, in addition to plotting raw data.
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.
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.
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.
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.
NC CES Jet streams
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.
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.
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.
modified from NOAA Multivariate ENSO Index
The influence of the oscillaton is greatest in winter.
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