Irkutsk originally had a borderline subarctic climate. Since 2000, the temperatures have resembled a humid continental climate. Snow cover disappeared earlier, from late April in the 1930s to late March in the 1980s. Discontinuous permafrost depth had decreased from 200 m to 100 m during the same period.
Irkutsk is characterized by an extreme variation of temperatures between seasons. It can be very warm in the summer, and very cold in the winter. However, Lake Baikal has a tempering effect thanks to which temperatures in Irkutsk are not as extreme as elsewhere in Siberia. The warmest month of the year is July, when the average temperature is +18 °C (64 °F), the highest temperature recorded being +37.2 °C (99.0 °F). The coldest month of the year is January, when the average temperature is −18 °C (0 °F), and record low of −49.7 °C (−57.5 °F). Precipitation also varies widely throughout the year, with July also being the wettest month, when precipitation averages 113 millimeters (4.4 in). The driest month is February, when precipitation averages only 7.6 millimeters (0.30 in). Almost all precipitation during the Siberian winter falls as flurry, dry snow.