Fig 1.
The southeastern United States and the apparent-temperature regions within it.
The 63 weather stations and 63 ozone monitors for which data were obtained are shown in addition to the urbanization of 1-km2 cells based on percent impervious surface. Impervious-surface data, originally at 30-m resolution, were obtained from the Multi-Resolution Land Characteristics consortium.
Fig 2.
Flowchart for calculating apparent temperature (in °F).
Temperature (T) is in degrees Fahrenheit and relative humidity (RH) is in percent. The flowchart is based on previous representations of the equations used to calculate the heat index [33,35].
Table 1.
Apparent temperature (AT) characteristics for the eight regions.
Fig 3.
Mean apparent temperatures for extreme heat days.
Values are °C and the isotherms are deviations (°C) from the mean seasonal apparent temperature.
Fig 4.
Circulation characteristics of extreme heat days.
(a) Mean 500-hPa geopotential heights (black lines) on July-August extreme heat days (EHDs) for the eight regions are shown as solid black lines. (b) Mean 850-hPa geopotential heights (black lines) on July-August EHDs for the eight regions are shown as solid black lines. Positive and negative deviations from mean July-August heights are shown as solid red lines and dashed blue lines, respectively. The red numbers are the maximum positive deviations. (c) Three-day air-mass back trajectories for the eight regions. The red lines are trajectories for 30 random extreme heat days. The blue lines are trajectories for 30 random days that are not extreme heat days.
Table 2.
Air-mass characteristics at representative stations in the eight regions.
Fig 5.
Mean daily maximum 8-hr average ozone concentrations on extreme heat days in July-August from 2000–2015.
The black lines are mean daily maximum 8-hr average ozone concentrations on all days in July-August.
Fig 6.
Correlation between June-August apparent temperature and 500-hPa heights from 1979–2015.
The grey circles are the centroids of the eight regions.
Fig 7.
Time series and trends for apparent temperatures for the eight regions.
(a) Interannual variability in June (blue), July (orange), August (green), and June-August (red) apparent temperatures and frequency of extreme heat days. (b) Trends (°C decade-1) in apparent temperature for June (blue), July (orange), August (green), and June-August (red). Stars denote significant (α = 0.01; one-tailed) trends.
Fig 8.
Projections of summer apparent temperatures in 2050.
Projections under the RCP2.6, RCP4.5, and RCP8.5 scenarios are derived from temperature and relative humidity projections from model output using those scenarios. The solid black (dashed black) line is the projected (present-day) 31°C isotherm.
Fig 9.
Trend in 500-hPa geopotential heights from 1979–2015.
The values are Kendall-Tau correlation coefficients. Significant (α = 0.01; one-tailed) trends are denoted by stippling.