Comparison of this Event with a Similar Event with Lower Heat Indices
Earlier in July 2010, a 3-day period of
excessive heat occurred from 6-8 July which generally resulted in higher maximum
temperatures across central North Carolina then the 23-25 July heat wave. It is
interesting to note that the heat index values across central North Carolina for that event were considerably lower and
that in general; conditions did not reach heat advisory criteria despite highs in excess of 100 degrees.
Precipitable water values generally ranged between 1.75 and 2.25 inches at KGSO from 23-26
July (1.87" on 23/00 UTC, 2.29" on 23/12 UTC,
2.00" on 24/00 UTC, 1.88" on 24/12 UTC, 1.57" on 25/00 UTC, 2.03" on 25/12 UTC,
and 2.21" on 26/00 UTC) which all exceeded the 75th percentile
for KGSO. In fact, a precipitable water value of around 2.00
inches for mid to late July is around two standard deviations from normal. On 7 July, the
precipitable water values were less than 0.75 inches across much of central NC.
When comparing the surface and upper air charts from the two different time periods (6-8 July and 23-25 July), a few hypotheses
can be drawn as to why the 6-8 July event was hot and relatively dry while the 23-25 July event was hot and very humid with
relatively higher heat indices. The main difference between the two cases is the location and orientation of the high pressure ridge
across the eastern U.S. and the associated flow around it. The hot and dry case had the mid and upper level
ridge axis centered over the central Appalachians with the ridge axis extending south to north and the upper level jet
across the Northern Plains. In contrast, the hot and humid case had a broad, flat, high pressure ridge
that extended from west to east across the southeastern U.S. with the upper level jet located over the Great Lakes and New England.
Similar patterns are present in the lower troposphere with the ridge at 700 hPa and 850 hPa located over the
southern Appalachians for the dry case and an west to east ridge axis located over the southeast Atlantic coast
for the humid case. The flow around this low level ridge off the Southeast coast allowed warm moist air from the Gulf of Mexico
to be transported across the lower Mississippi Valley and into the Tennessee Valley and the Carolinas.
The surface high was centered well off the Southeast U.S. coast, in a "Bermuda High" type pattern with a ridge
extending west into eastern Florida. The surface pattern for the less humid case featured a surface high over the
southern Appalachians that extend along a north-south axis with a light, mainly northerly flow in the Carolinas.
The image above compares the locations of record high temperatures on 7 July and 25 July 2010. The image is from the
National Climatic Data Center which has a nice records look up website (http://www.ncdc.noaa.gov/extremes/records/)
that allows users to search for various temperature, precipitation, and snow record. Note that the data may be preliminary
and has the potential to include some errors but it is a helpful web site.
The low level thicknesses in the 1000 hPa - 850 hPa layer in the 23-25 July humid case (shown in yellow) were much greater
than those for the 6-8 July dry case (shown in red) in the chart to the right. This is interesting since
the maximum temperatures during the dry case (shown in red squares) were several degrees warmer than the maximum
temperatures during the humid event (shown in yellow diamonds), even with thickness values 10-30m lower than the humid case.
Keep in mind that on a humid day, the low level thickness values should be greater due to the presence of
additional water vapor and the impact on the virtual temperature. On a less humid or drier day, the reduced amount of water vapor would lead to lower
thicknesses. For example, on a hot and humid day, the difference between the low level dry bulb temperature (T)
and the virtual temperature (Tv) can be up to 3 degrees C (nearly 6 degrees F). For an extreme case, comparing
an extremely dry air mass with a very humid environment, a 5-degree C difference between T and Tv would give
about 23 meters of difference in the 1000-850 hPa thickness (Lackmann, 2010). This would likely be
the upper limit of that effect given the comparison between the two extremes. More typical values would
likely range on the order of 5 to 10 meters.
While the amount of water vapor in the lower troposphere would alter the low level thicknesses
directly through changes in the virtual temperature, the changes in moisture would also impact surface
temperatures by altering the amount of cloudiness. The additional moisture would likely lead to enhanced cloudiness
and reducing the amount of insolation, this would work to cool surface temperatures. While on a dry day, the
reduced moisture should lead to less cloudiness and more insolation.
There are several other factors that impact maximum temperatures in addition to the low level thickness values.
These include the amount of cloudiness and insolation, the amount of moisture in
the soil which influences how much short wave radiation is converted to sensible heating,
horizontal thermal advection, the amount of wind and turbulence, and more. So when comparing heat waves or
maximum temperatures, the amount of moisture in the low levels that alters the low level
thicknesses is one of many sources of uncertainty.