A Fresh Look at the Weather of the Battle of Gettysburg

More than sixty years ago, noted weather historian David Ludlum published The Weather at Gettysburg in Weatherwise Magazine. Primarily using observations taken by Professor Michael Jacobs at Pennsylvania College (now Gettysburg University), Ludlum reconstructed some of the weather conditions during this historic battle in which one of every 200 Americans alive at that time fought.

Professor Jacobs measured temperature, pressure, winds, and precipitation, but other than his observations of clouds, he did not provide any humidity-related information. Nonetheless, Ludlum writes that on the third [of July, 1863] “the air became increasingly humid but not excessively so,” and also “with tropical air now present and humidities running high, the discomfort index stood high.” Given typical summer weather in south-central Pennsylvania, these are reasonable descriptions, but no actual observations are provided in support.

We fill this gap in knowledge about the weather of the Battle of Gettysburg using moisture-related observations from nearby observers in Harrisburg, PA, as well as other nearby observers, to estimate the dew point and heat index during the battle. In addition, results from a newly developed reanalysis product from NOAA/CIRES/DOE that reconstructs weather conditions in the 19^th century will be compared with the available temperature and humidity observations.