Below is something which I had clipped from a website article about the unique weather pattern Truckee/Donner Lake has:
"These factors contribute to Truckee’s consistently lower temperatures during spring and fall seasons when the area records the bulk of its “coldest in the nation” readings. High pressure tends to dominate West Coast weather during the summer and autumn months. This large semi-stationary air mass forces vigorous cold fronts to track far to the north, providing California with a sunny, dry climate. The air pressing down on the Sierra Nevada during these dry summer months contains very little moisture. Without much humidity in the air, there is nothing to trap and hold the sun’s heat. As soon as the sun sets, temperatures plummet. The same principle applies to deserts, known for scorching heat during the day followed by frigid nights. This high pressure cell also fosters calm, windless evenings in the Sierra, which inhibits atmospheric mixing. Without breezes to stir up the night air, the dense cooler air settles to the ground with warmer air above. This nocturnal inversion, known as such because air aloft is normally colder than at the surface, keeps Truckee’s average minimal summer temperatures hovering in the upper thirties. In addition, Truckee’s typical cloud-free evening sky allow accumulated daytime heat to escape rapidly back into the atmosphere, a dynamic known as radiational cooling. Another component that contributes to extreme temperature fluctuations is distance from any large body of water. Oceans and large lakes, like Tahoe, heat up and cool off slowly, while land surfaces gain and lose heat much more rapidly. Truckee’s afternoon temperatures are usually much higher than those recorded near Lake Tahoe, but at night plunge quickly, and are often 10 to 15 degrees colder by sunrise."
