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April 19, 2014



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Gettysburg Campaign - Meteorology

The Gettysburg Campaign took place during the last weeks of spring and the first weeks of summer, traditionally the ideal campaigning season, with generally fine weather and long days.

Weather

For the early part of the campaign the weather was generally seasonable. It was rather hot until 25 June, when there was a cooling trend. For the critical days of the campaign we have fairly detailed information as a result of the efforts of the Rev. Dr. M. Jacobs of Pennsylvania --Gettysburg-- College, and the more casual observations of Lt. Frank Haskell.

Climatic Conditions, 25 June - 4 July 1865
Temperature (F)Precipitation
Time ->Day070014002100
25 June Thu 59 51 63Intermittent rains
26 June Fri 60 63 62from 0800 25 June
27 June Sat 61 63 67to 0700 27 June.
28 June Sun 63 67 68
29 June Mon 66 72 69
30 June Tue 68 79 71
1 July Wed 72 76 74Very cloudy.
2 July Thu 74 81 76Foggy dawn; humid; PM, "Mizzling rain."
3 July Fri 73 87 76Very humid; thunderstorms nearby.
4 July Sat 69 72 70Very humid; intermittent showers, 0600-1200; heavier rains, 1300-1400; very heavy rains from 2100.

With the rains on 4 July the temperature fell sharply. Rains on the morning of 5 July helped cover Lee's retreat from the field. The next day was clear, but the rains resumed on the 7th, and grew quite heavy on the 8th, by which time Lee had reached the security of the Antietam area.

Note that during the battle the temperature was at its highest at 1400 on 3 July, during the Confederate bombardment proceeding "Pickett's Charge."

Sunrise/Sunset.

Local sunrise and sunset were the principal determinants of when fighting could take place, since engagements at night were risky at best. Gettyburg lies at 39* 50' North and 77* 16' West, which yields the following times for sunrise and sunset during the period of the battle:

30 June1 July 2 July 3 July 4 July
Civil Twilight Begins 0403 0403 0404 0404 0405
Sunrise 0435 0435 0436 0436 0437
Sunset 1933 1933 1933 1932 1932
Civil Twilight Ends 2006 2006 2006 2005 2005

These are local solar times. The time at Richmond, which is considerably south of Gettysburg, is a few seconds later, while that at Washington is about one minute faster. To convert to Eastern Standard Time add nine minutes.

Under moonless conditions it is theoretically possible to engage in military operations from about the beginning of morning nautical twilight. This occurs some 30 minutes earlier than the beginning of civil twilight, and is essentially when visibility is about 400 yards under ideal conditions. Operations can continue until the end of evening nautical twilight, about 30 minutes after the end of civil twilight. In practice, of course, conditions are rarely sufficiently ideal for operations.

The Moon

By coincidence, the Gettysburg Campaign lasted almost exactly two lunar months.

Phases of the Moon, June-July 1863
Phase June July
Full Moon 1
Last Quarter 7 7
New Moon 15 15
First Quarter 23 23
Full Moon 30 31

The Army of Northern Virginia began to march north in early June by the light of the full moon. By the time Ewell's II Corps reached Winchester there was virtually no moon, which may have helped his initial night attack on the place. As the Confederate army began to spread out and plunder southeastern Pennsylvania the moon began to wax, reaching full just before the armies clashed at Gettysburg. The cloudy conditions and fog on the night of 1-2 July largely obscured the moon, but it appears to have been fairly bright on the night of 2-3 July, with sufficient light to cause Ewell concern about continuing the advance of Johnson's division on Culp's Hill; there was certianly enough moonlight to permit Alpheus Williams to site the batteries for his planned morning attack against Johnson's positions. There was little moonlight during the retreat.

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