When you go higher into the air, air pressure starts to decrease and when you are at sea level, air pressure very high. When the temperature is really high, air molecules start bouncing around...
Air density decreases with temperature so that the force needed to push the air out of the way is decreased, thereby decreasing resistance. Conversely, air viscocity increases with temperature...
The sun affects our air temperature by Heating the outer layers on the earth until it reaches the layer that we breath, that's how the sun affects air tempurature.