The weather during the Battle of Gettysburg

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RebelYank87

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I have often seen in Civil War literature, both fiction and non-fiction, this idea that the weather at the Battle of Gettysburg was unusually hot and humid. The scientific data, however, doesn't bear this out. In fact...the data tells us the weather during parts of the battle, as well as the days leading up to it, was unseasonably cold. The Rev. Dr. Jacobs of Pennsylvania College took measurements and made observations of the temperature/weather conditions throughout the battle and this is what we have in his report:

July 1, 1863: High - 76, Low - 72 (Cloudy skies)
July 2, 1863: High - 81, Low - 74 (Mostly cloudy skies)
July 3, 1863: High - 87, Low - 73 (Partly cloudy skies)

And then the weather leading up to the battle was even cooler:

June 25, 1863: High - 63, Low - 51
June 26, 1863: High - 63, Low - 60
June 27, 1863: High - 67, Low - 61
June 28, 1863: High - 68, Low - 63
June 29, 1863: High - 72, Low - 66
June 30, 1863: High - 79, Low - 68

All of this information can be found at the link below:

Weather at Gettysburg

So my question is, how did we go from unseasonably cool weather to the history books telling us that the weather was "unusually hot"?
 

rjustice21

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Did people have a way to measure the dew points back in 1863? I am curious as to what the dew points were during the days of the Battle of Gettysburg.

Pennsylvania can get really humid in the summer, even when the temperature isn't very hot.
This is a good point. It gets very humid. 87 on the third day could have had a real feel of 90-95. Even 81 could have been pretty bad.
 
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rjustice21

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Does anyone have background how they are even able to study this? I don't have any background in meteorology, so I can't even fathom how they are able to provide this data.

Ah... just noticed that the recordings were taking during the battle. Never mind.
 
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jackt62

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Assuming the statistics taken by that one individual are accurate, they seem to present a temperature pattern that is probably normal for that part of Pennsylvania in the early summer. It is not uncommon for people to perceive the same temperature as being either too cold or too hot, and future recollections by veterans of the battle may also be exaggerated.
 
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RebelYank87

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On top of the oppressive heat, the air was filled with dust. That many men & horses moving around, camp fires & powder smoke hanging in a humidity soaked atmosphere must have been brutal to breath. That would have been an advantage to the defenders. I had not focused on that before. Thanks for the thought provoking question.
The dust might not have been as much of a factor as it had been raining off and on throughout the latter part of June. But the smoke from the gunpowder no doubt was oppressive because there was very little wind recorded during those three days in July. With the air being almost stagnant, it must have been very difficult to see for the combatants.
 

Wallyfish

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Gettysburg is quite a fascinating place for many reasons. There were two colleges in town in 1863 which for a small town is unique by itself. If you open the link in @RebelYank87 original post, you will see the name Rev Dr Jacobs. He was a math profession at Pennsylvania College, now named Gettysburg College.

Dr Rev Michael Jacobs also had an interest in meteorology and he kept daily weather records as found in the original post. I don't recall ever seeing relative humidity data published during the battle. But the temperatures recorded are typical for southern Pennsylvania. Much has been written on the July 4th rainfall that fell and its impact on the confederate retreat and union movements in pursuit. 1.39" of rain fell on July 4th. That is a lot of rain.

There are several sites that discuss Jacobs weather data. A brief overview can be found below.

Jacobs source
 

Tom Elmore

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Don't forget, temperatures are taken in the shade. In direct sun it can be 10-15 degrees hotter, and most of Pickett's Virginians were lying in the direct sun for a few hours on July 3. I suppose hundreds of cannon discharges in close proximity might add something as well. The hottest temperature of the entire battle likely coincided with the hour (around 3 p.m.) of the Pickett-Pettigrew-Trimble assault.

As for humidity, extant accounts infer that it was high. On June 30, light rain encompassed a wide area between Harrisburg, Chambersburg and Winchester. Light rain was even experienced by the Union Eleventh Corps at 5 a.m. on July 1 near Emmitsburg, and also by the First Corps near Marsh Creek starting at 6 a.m.; over the next couple of hours it began clearing up. Upwards of 30 men in the 150th Pennsylvania alone could not keep up with the brisk pace set by their regiment on the way to Gettysburg that morning.

As noted, it was cooler for several days in the latter part of June and nights were chilly, but sources in both armies uniformly concur that the period June 15 -18 was intensely hot, with daytime highs estimated to reach perhaps 100 degrees in the shade.
 
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cake1979

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My wife and I were at Gettysburg for the 150th back in 2013. The weather was similar to the conditions in 1863, with temps between 82 on the 1st and 88 on July 4th. Probably a little more overcast than back then, but the temperature was similar. When the sun was out, it was pretty warm and it was humid the whole time. We drank a lot of water as we walked the field, in hiking clothing and sun hats.

In wool uniforms, marching and fighting (or just lying in the sun) would have been miserable. Couple that with the dust mentioned above and the stories of unbearable heat seem pretty plausible.
 

infomanpa

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Did people have a way to measure the dew points back in 1863? I am curious as to what the dew points were during the days of the Battle of Gettysburg.

Pennsylvania can get really humid in the summer, even when the temperature isn't very hot.
Yes...the first instrument to measure dew point was invented a hundred years earlier.
 
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