What exactly creates high and low pressure...and is there a tendency for one to be found more frequently during certain times of the year?
High pressure is generated by sinking air, low pressure by rising air. They can be generated on a very small scale by air rising from heated pavement (you see hawks soaring in these thermals) or cold air moving down a mountainside into a valley (why the coldest temperatures are found in such places). Highs and lows can be generated on a much bigger scale by the large scale circulation of the atmosphere. The same basic principles (sinking air/high pressure, rising air/low pressure) apply. In terms of locations, high pressure is most commonly found on a global scale in the tropics and over/near the poles...low pressure over the equator and in what we call the mid-latitudes...essentially spanning the continental U.S. Highs and lows in this zone (the mid-latitudes) tend to shift; for example in the Pacific Northwest, high pressure is most common during the summer and early autumn months, low pressure in the late autumn and winter months.