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I was just thinking about how if you live in a climate with lack of seasonality in precipitation you take it for granted as the default and see Mediterranean or monsoonal climates as more exotic.
But thinking about it another way, it is interesting how many climates seem to not have a strong difference in average precipitation per month over the year, despite the sources of the precipitation differing (frontal vs. convective, for instance, as well as other influences like hurricanes) and the other climate factors like temperature over the year that result in how much potential precipitation can come out. For instance, the East coast of the United States is very even in rainfall. Many places have just 80-100 mm in every month. Likewise, even in western Europe, you have climates with 40-50 mm most months.
It's interesting for instance, that if you have a climate, perhaps in the Mid-Atlantic with say 80-90 mm in one really average winter month, it can also have 80-90 mm in an average summer month, even though most of the precipitation can take place in totally different ways (eg. Nor'easters and winter storms, frontal rain, hurricanes and their remnants, summer convection), it all manages to even out on average. It's not like summer is that much wetter in many Eastern US climates due to more convection compared to drizzly fronts and snow in winter, or vice versa, once everything is averaged. That in itself, come to think about it, that they lack seasonality, is just as interesting or perhaps more so as the fact that Mediterranean or monsoon climates do have seasonality. After all, Mediterranean and monsoonal seasonality seems more easily explained in differences between seasonal rains as one or two things or forces drive the seasonality.
What factors cause evenness in precipitation over the year?
I think in most cases, several factors work together to create precipitation seasonality/a lack of precipitation seasonality.
In Western Europe's case, I'd say its relatively flat topography and western continental position are key factors. If there was a high (4500 m/15k feet) mountain range running north-south from the UK through France down to Africa, the areas west of the mountains would have considerably wetter winters than summers for obvious reasons and areas just to the east would have wetter summers than winters due to the rain shadow in winter and a higher percentage of rain coming from convective activity in the warmer months. Then as you travel farther east the precipitation seasonality would decrease.
For more oceanic locations such as Midway Island, Micronesia, Hawaii, Kerguelen, Campbell Island, Norfolk Island, Easter Island, etc...wind direction and ocean currents are most likely the dominant factors.
Hawaii is an odd case for islands though since it has places with dry summers/wet winters, dry(er) winters/wet(ter) summers (very rare there), and everything in between, including little to no seasonality. Most areas do receive most rainfall in winter though. According to WRCC only the Kona coast has wetter summers than winters although the contrast isn't extremely high.
An interesting question, and one that is difficult to answer. The majority of locations in NZ see summer rainfall dropping to about half of the winter /spring total, but there are a few locations with even precipitation, or even summer rainfall maximums. I think that points to a change in dominant wind direction, or where high pressure sits in relation to the country.
What factors cause evenness in precipitation over the year?
In my area the evenness is by virtue of being a transition area between the left coast (west of the Coast/Cascade mountains) where 80% of the precipitation comes in the winter thanks to large wet frontal storms and continental climate (east of the Rockies) where 80% of their precipitation comes in the summer thanks to convective activity. The transition area between these two mountain ranges is about 400 km wide, and contains several mountain smaller mountain ranges, deep valleys, and plateaus.
An interesting question, and one that is difficult to answer. The majority of locations in NZ see summer rainfall dropping to about half of the winter /spring total, but there are a few locations with even precipitation, or even summer rainfall maximums. I think that points to a change in dominant wind direction, or where high pressure sits in relation to the country.
There is really only quite small part of the country with a 50% or lower ratio, even given all 12 choices of "seasons". And a large portion of the South Island is on the upside of 75%, ranging up to 90% or so in a few patches.
Here are some numbers from LTAs (all "seasons" considered):
I live in Columbia, MD between DC and Baltimore - that very Mid-Atlantic climate the OP spoke of with very even annual precip. Actually a SLIGHT peak in the May/June timeframe, I think, but it's no more than an inch/25mm variation from the driest to wettest month.
However, I do wonder if we'd end up with higher summer precip if not for the Appalachians. Most of our fall through spring precip comes from frontal systems, enhanced by tropical systems (and their remnants) and the famed "Noreaster" type storms (whether rainy, snowy, or both) barrelling up the coast, which dump large amounts at once and contribute to a big chunk of our precip.
However, I think during more convective and "straight" frontal precip events, there is a slight rain shadow caused by the appalachians, sucking out some of the moisture as systems move from west to east. Evidence of this is that places to our west, TN, KY, OH especially, tend to have slightly higher annual precip (10-15% more) despite being further inland. I know the Shenandoah Valley has a more marked rain shadow since it has mountains on both sides, but I think that, due to the general west-to-east flow of weather systems, there's still a bit of a rainshadow here east of the mountains, too.
I live in Columbia, MD between DC and Baltimore - that very Mid-Atlantic climate the OP spoke of with very even annual precip. Actually a SLIGHT peak in the May/June timeframe, I think, but it's no more than an inch/25mm variation from the driest to wettest month.
However, I do wonder if we'd end up with higher summer precip if not for the Appalachians. Most of our fall through spring precip comes from frontal systems, enhanced by tropical systems (and their remnants) and the famed "Noreaster" type storms (whether rainy, snowy, or both) barrelling up the coast, which dump large amounts at once and contribute to a big chunk of our precip.
However, I think during more convective and "straight" frontal precip events, there is a slight rain shadow caused by the appalachians, sucking out some of the moisture as systems move from west to east. Evidence of this is that places to our west, TN, KY, OH especially, tend to have slightly higher annual precip (10-15% more) despite being further inland. I know the Shenandoah Valley has a more marked rain shadow since it has mountains on both sides, but I think that, due to the general west-to-east flow of weather systems, there's still a bit of a rainshadow here east of the mountains, too.
Doesn't the wind flow from the South/East during the summer? If that was true the Appalachians won't effect precipitation.
There is really only quite small part of the country with a 50% or lower ratio, even given all 12 choices of "seasons". And a large portion of the South Island is on the upside of 75%, ranging up to 90% or so in a few patches.
Here are some numbers from LTAs (all "seasons" considered):
My figures for Riwaka show about 50% - 79 mm/164 mm.
Even with the 62% figure, that's quite a difference between there and Nelson (79%), considering the two towns are in sight of each other.
You may be using 30-year means; the above are from LTAs (about 60 years for Riwaka from memory). I prefer LTAs for this sort of thing because even 30-year span can be "atypical". Wellington had a 30-year period (1906-1935) that was about 150mm/year drier than the average of the rest of its record. It is true that some of the numbers above would be higher if the 30-year means are used; but also a few would be lower.
PS edit: I'm talking 3-month seasons - your numbers are for months!!!
The most extreme values I have in a collection of LTAs for months are 41% at Ophir and 42% at Alexandra. Ath the other end, for Riverside in North Canterbury the number is 84%.
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