How can one summer day be flat calm, and an almost identical day be super windy?
The flat calm days usually have high pressure sitting overhead...with little difference in air pressure from one location to the next. Windy days see a big difference in pressure over a short distance-that's like an inflated balloon at one location. When you stop holding the neck of the balloon...the higher pressure air moves out. Winds can also be generated along the shoreline when it's sunny out; because land heats up much more rapidly, that generates a difference in pressure which then leads to wind.