Based on archival materials and scientific literature, the article studies weather conditions in the territory of the Chuvash Republic during the construction of defensive lines during the Great Patriotic War. The relevance of the chosen topic is due to insufficient exploration degree of this aspect in the scientific literature. The novelty of the research consists in attracting new sources: tables of meteorological and agro-meteorological observations TM-1 and BSH-1 made by meteorological stations of Alatyr, Kanash, Poretskoye, Cheboksary in late 1941 – early 1942. The purpose of the work is to study the air temperature values, snow cover height and soil freezing depth in Chuvashia in the winter of 1941–1942 based on the analysis of different groups of historical sources: 1) documents of the authorities controlling the progress of construction; 2) memories of the builders of defensive lines and their relatives; 3) observation data of air temperature, precipitation, snow cover height, depth of soil freezing compiled by weather stations in the territory of the republic and in neighboring cities. Comparison of information obtained from different sources helped to conclude about their reliability and determine in what weather conditions the construction of defensive structures in the territory of the republic took place during the Great Patriotic War. Comparison of historical sources of different groups does not confirm the data on the air temperature below –44°C during the construction period in the territory of Chuvashia. It should be taken into consideration that the minimum temperature indicators refer to the night time when the work was not carried out. The construction lasted a long time, and if we keep in mind the period from October 1941 to February 1942, then the weather was different within these time limits and it is impossible to accurately define only one value of air temperature. It can be unequivocally stated that winter of 1941–1942 was one of the coldest in the XX century.