Like all aspects of the weather, there are so many variables that nailing down any particular item, or group of items as to causes of gustiness is near impossible.
I'm guessing that, over open water
, a lot of it has to do primarily with the source of the wind, in combination with localized environmental conditions, such as water
temperature, time of year, time of day, cloud cover, etc.
For instance, while the wind is always to some extent gusty, around here (30N) when the wind is from the south, i.e. a warm wind, the gusts seem to be shorter, with a larger range in velocity, whilst a wind from the north, i.e. a cold wind, seems to have longer gusts that have a comparatively smaller range in velocity. I assume the situation is reversed in the Southern Hemisphere.
Speculating even further, while the primary 'cause' of the wind is pressure differences, there other contributing factors. Running with the warm/cold scenario above, with the warm wind in the south, the air is lighter, tending to rise as the sun warms it, there are generally scattered clouds causing uneven heating
, which results in different areas rising at different rates, with the air replacing it doing so at different rates (velocities).
Contrast this with a north, cool, dense wind. Typically areas of high pressure are much larger than lows. The coolness and large volume tend to add stability to the air mass; the generally clear weather cuts down on the uneven heating
induced by variable cloudiness. This stability seems to (usually) moderate the intensity of the change over the 'base' wind, and generally lengthen the duration, at least in my experience in my part of the world.
Proximity to land also has huge effects on gustiness, the differential heating of land and water and the relative smoothness of water as compared to land are probably primary drivers. We used to sail on the south shore of Lake Pontchartrain, in what were the gustiest conditions I've ever seen, before or since. It only happens in the spring, and has never been adequately explained to me, but while the lake water is still cool, apparently the heating of the land generates enough of a thermal for the (highly variable) wind to stay at the surface for 2-3 miles into the lake. I'm talking about near 0 to 30-35 knots, for gusts lasting for 15 seconds if you're unlucky, to a minute or a minute and a half if you were lucky (we were on windsurfers). After that, 2 or 3 miles into the lake, the wind would die off to a more steady but less than shortboardable 10 knots or so, and if you got out that far, you had to swim back, get rescued, or drift 24 miles across the lake (the wind was from the south, directly offshore).
Obviously these are my subjective opinions. I hope someone with some hard scientific information will chime in to shed some 'official' light on the subject...