Summers are dry with occasional thunderstorms in August. Winters are wet. The mountains receive substantially more. An average year will have 275 to 300 days of sunshine a year. Although the shallow beach areas can warm up to 60 degrees in August, most of the lake stays 40 - 50 degrees year round.
Swimmers can easily suffer from hypothermia so wetsuits are advised.
The only outlet is the Truckee River in Tahoe City. The Truckee flows North to Truckee, California, then East to Reno, Nevada and then Northeast to Pyramid Lake. Pyramid Lake has no outlet, its only water loss being evaporation. The map below shows the areas immediately adjacent to the lake. Just off the map are Reno to the upper right, Carson City directly to the right, the town of Truckee to the upper left and the ski resorts of Alpine Meadows and Squaw Valley to the left.