Space Grant Consortium - University of Wisconsin - Green Bay

numerical weather prediction (NWP) models and high-resolution satellite observation, along
with a greater understanding of the dynamical and physical processes that influence maritime
climates. Lake breezes in the vicinity of Lake Michigan and Lake Superior are particularly
worthy of study due to the sink of cold air they access during the spring and early summer.
Pneumonia Fronts
There is a unique manifestation of the lake breeze from Lake Superior that occasionally impacts
the western shore of Lake Michigan in the presence of a typically weak synoptic cold front. This
feature, known as a pneumonia front, has been loosely and variably defined by National Weather
Service forecasters, herein referred to as the working definition, to be a ―rapid wind [shift] to the
northeast‖, an increase in surface wind speed, and a ―rapid [decrease] in surface temperature‖
(Behnke 2005). For the purpose of his study, Behnke offered a less subjective definition of a
pneumonia front, which has since been adopted by the National Weather Service. The quantified
Behnke definition of a pneumonia front has two parts in order to narrow his research. First,
hourly temperature falls at least 16 degrees Fahrenheit are required at a lakeshore observing
station, and no more than 10 degrees Fahrenheit at a station well inland. Second, this
temperature fall must be during the months of May through August.
There are some stipulations that both the working definition and Behnke definition make which
can be problematic in application. First, there are three types of boundaries which can produce a
fairly large temperature drop over a short period of time other than a lake breeze: an outflow
boundary separating rain-cooled air from a warm, moist, conditionally-unstable environment; a
strong, sharp cold front, particularly during the spring and fall months; an early spring lake
breeze where lake-modified air is substantially colder than the air over land heated diurnally.
Second, Behnke quantifies a complex dynamical and physical process via a point change in
temperature at two locations. This approach can lead to difficulties in producing climatology, as
Behnke partially relents, because high-resolution temperature data at one-minute intervals is
generally not available. Therefore, a pneumonia front crossing the observing station at reporting
time may fail to meet the criteria if the adjacent reports are one hour prior and later. In addition,
it is possible that a temperature decrease of less than the quantified amount could be experienced
with a replica pneumonia front under a different synoptic situation or after sunset as the ground
cools, even discounting the lack of high-resolution data. While Behnke’s attempt to reconcile an
overbroad working definition of the pneumonia front is a good forward step to respect the
responsible coordinate dynamical and physical processes, more investigation is needed. In
addition, his attempts to objectively narrow his set of potential case studies proved to be
somewhat fruitless, as he eventually examined 94 potential days surface sea-level pressure and
wind speed from the National Centers’ (NCEP/NCAR) reanalysis data subjectively.
Behnke’s method in finding potential pneumonia front cases focuses solely on the temperature
drops at General Mitchell International Airport in Milwaukee, Wisconsin, (KMKE) without
similar declines at Dane County Regional Airport outside Madison, Wisconsin, (KMSN). In his
study, he features one case of a particularly viable pneumonia front on 17 July 2003. On this
day, the synoptic cold front was synoptically quite weak though discernable. Earlier, surface
observations suggested there was little in the form of a cross-frontal temperature gradient.
However, later in the day, there was a strong temperature drop and increase in wind at KMKE
during the evening hours, when a one-hour temperature decrease of 85 degrees Fahrenheit to 65
degrees was realized associated with the pneumonia front enhanced by Lake Michigan.
12