Looking directly up on a clear night from San Jose we are viewing the heavens through a thick blanket of air. About 10 mi into the sky lies 90% of the atmosphere. The very top of the atmosphere is almost 1000 km high. That is a lot of air.
And that is looking straight up.
As you look at objects lower than the horizon you are looking sideways through the thickest parts of the atmosphere. The amount of atmosphere progressively increases the closer you get to the horizon.
One way to reduce this problem is to get high. The Mauna Kea telescopes are at about 4 km elevation. That leaves almost 40% of the earth atmosphere below them. Even Shingletown (elevation 3880 ft. / 1183 m) is above 20% of the atmosphere.