08-26-2008, 08:07 AM
Hi,
Yes the leeward side is always drier, the question is simply how much drier.
The reason cutting effects rainfall patters is pretty simple really. The island makes its own rain. The average rainfall only a few miles offshore is only about 20 to 30 inches a year. The reason it rains on the island so much is a factor of topography--the rapid climb in altitude and vertical relief in general sweeps relatively moisture laden marine air up, which cools, and the water load drops as rain. This is why you have a band around the islands at a more or less equal altitude with more or less equal rainfall. Below, the air doesn't drop moisture because it hasn't cooled enough(due to climb in altitude and the resultant drop in pressure) and above the rain band, the climate is arid as most of the water has already been wrung out. As well, the heavy vegetation locally affects weather, both by simply absorbing rainfall, which evaporates to some degree, and re-rains--and by moderating the daily rise in temperature, as forested areas warm slower and in general maintain more constant temperatures than do croplands, structures, roads, and the like. This is important, because thermals are generated above hot surfaces--this punctures holes in the TWI(trade wind inversion) that generates most of the rain that sneaks over to the Kona side, and potential rainfall is simply swept up into the upper atmosphere and blown away from the island. You can see the effect if you look. You will see clouds spawn in certain areas, others rain, and above clearings and the like you will see in low cloud structure the obvious turbulence caused by rising air. It will look just like I describe, like you were blowing air through smoke or a fog. Every time that happens, that percentage of precipitation is lost from the island.
It's a matter of both the rain we get from the trades, and the rain we keep on the island to make up our total rainfall.
As atmospheric temperatures increase, these small effect play much more significant roles.
Yes the leeward side is always drier, the question is simply how much drier.
The reason cutting effects rainfall patters is pretty simple really. The island makes its own rain. The average rainfall only a few miles offshore is only about 20 to 30 inches a year. The reason it rains on the island so much is a factor of topography--the rapid climb in altitude and vertical relief in general sweeps relatively moisture laden marine air up, which cools, and the water load drops as rain. This is why you have a band around the islands at a more or less equal altitude with more or less equal rainfall. Below, the air doesn't drop moisture because it hasn't cooled enough(due to climb in altitude and the resultant drop in pressure) and above the rain band, the climate is arid as most of the water has already been wrung out. As well, the heavy vegetation locally affects weather, both by simply absorbing rainfall, which evaporates to some degree, and re-rains--and by moderating the daily rise in temperature, as forested areas warm slower and in general maintain more constant temperatures than do croplands, structures, roads, and the like. This is important, because thermals are generated above hot surfaces--this punctures holes in the TWI(trade wind inversion) that generates most of the rain that sneaks over to the Kona side, and potential rainfall is simply swept up into the upper atmosphere and blown away from the island. You can see the effect if you look. You will see clouds spawn in certain areas, others rain, and above clearings and the like you will see in low cloud structure the obvious turbulence caused by rising air. It will look just like I describe, like you were blowing air through smoke or a fog. Every time that happens, that percentage of precipitation is lost from the island.
It's a matter of both the rain we get from the trades, and the rain we keep on the island to make up our total rainfall.
As atmospheric temperatures increase, these small effect play much more significant roles.