Today is such a rainy, headache-inducing day that I thought it would be interesting to see how many primary source collections document rain in some way. Three collections specifically document historic rain and they are all from soldiers, two serving in the Civil War, one in WWI.
Hodgdon, Max R., Papers, 1912-18, documents plane modifications in WWI to cope with rain and mud.
Rogers, Alonzo R. Papers, 1861, 1888, includes a letter to "Sister" from Alonzo in Camp opposite Chattanooga (Tenn.) dated Nov. 6, 1863 describing rain, homesickness, constant skirmishes, food and leaving for Bridgeport.
Young, Charles B. Correspondence, 1864, 1865. His last letter
to "Dear Family", dated Dec. 22, 1864, notes that his comrades drove
Rebels farther back, captured troops and artillery, were on night
manuevers, the battlefields were covered with dead and wounded soldiers,
rain soaked supplies, that he has been sick.