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I usually hate the spring season here with a passion as that is when those awful "sunny and dry" westerlies start to rev up ensuring the minimum of rain falls
Happily in recent "springs" this has not been the case
We usually tend to get our rainfall maximum from late summer throughout autumn to early winter but it depends yearly of course. That's one thing I like so much about Sydney - the extreme variability - and I find it much more interesting than Melbourne. For example July 1995 had just 5mm /0.1 inches, August zero / zero inches and yet September measured 226mm / 8.8 in" - more than three times the average.
Over the "recent" decade, the earliest I can remember is May 2007 when it were the driest since 1957 with just 9mm / 0.3 in" falling then followed by the wettest June since 1950 with 510mm / 20.1" falling - the turnabout was quite amazing!
I'm also now wondering from looking at the peaks and dips in the rainfall stats alone for a climate, how much you can easily read into it being real effects of the processes that cause the rain (such as shifts seasonal shifts in storm tracks/fronts, convective precipitation in summer, monsoon etc.)
For example, if you saw Miami's stats with over 200 mm in summer months and around 50 mm in winter months, you can tell, along with knowledge of Miami's geography and latitude, that's the monsoonal influence coming through. Or when June goes from a 40mm average to a 20 mm July average in Seattle, you know that's the winter-type storm track influence diminishing and the subtropical high building up.
However, for something like NYC's climate, if you see a number closer to 90 mm one month and the next month averages 80 mm and then the next, a number more like 100 mm, could you really say it's anything other than it's capturing chance fluctuations based on binning by month, and it's just even year round. If you saw 115 mm in July and 80 mm for February for the climate averaged from 30 years of data, does that difference let you ever infer meaningful like "that's a summer peak in rainfall"?
If you look at Paris, France and see 50 mm in Jan, then 40 mm in Feb, then 40 mm, 50 mm, 65 mm etc. in Mar, Apr., May and so on, it would seem no discernable trend is there based on those numbers, though if you look at number of rainy days 2-3 fewer rainy days per month in summer, you can kind of make out a "Mediterranean" pattern.
In this country, with in the main fairly modest seasonal variabilities, it's not hard to detect the influences that produce them.
I'm also now wondering from looking at the peaks and dips in the rainfall stats alone for a climate, how much you can easily read into it being real effects of the processes that cause the rain (such as shifts seasonal shifts in storm tracks/fronts, convective precipitation in summer, monsoon etc.)
For example, if you saw Miami's stats with over 200 mm in summer months and around 50 mm in winter months, you can tell, along with knowledge of Miami's geography and latitude, that's the monsoonal influence coming through. Or when June goes from a 40mm average to a 20 mm July average in Seattle, you know that's the winter-type storm track influence diminishing and the subtropical high building up.
However, for something like NYC's climate, if you see a number closer to 90 mm one month and the next month averages 80 mm and then the next, a number more like 100 mm, could you really say it's anything other than it's capturing chance fluctuations based on binning by month, and it's just even year round. If you saw 115 mm in July and 80 mm for February for the climate averaged from 30 years of data, does that difference let you ever infer meaningful like "that's a summer peak in rainfall"?
If you look at Paris, France and see 50 mm in Jan, then 40 mm in Feb, then 40 mm, 50 mm, 65 mm etc. in Mar, Apr., May and so on, it would seem no discernable trend is there based on those numbers, though if you look at number of rainy days 2-3 fewer rainy days per month in summer, you can kind of make out a "Mediterranean" pattern.
That’s the tough part – in many climate zones trying to identify a rhythm or pattern can be very difficult. It can be frustrating to figure out cause and effect. In the case of NYC’s climate – the rhythm of seasonal precip has several things that impact it – and they overlap so much, that it can be difficult to decipher where one seasonal climate control ends and another begins.
From what I understand, a true “Mediterranean pattern” is more than just a modest decline in precip at the time of high sun (summer months); In a true Mediterranean climate like Los Angeles for example – precip just doesn’t slacken at the time of high sun (like it does in Paris) it sharply halts: In Mediterranean climates, summer is almost without exception, a period of intense and brilliant sun, cloudless skies, desert like humidity values, and a dearth of atmospheric disturbances. Some Mediterranean stations like Los Angeles or Haifa receive absolute zero precipitation in the summer months – a far cry from Paris where some rain always falls in summer.
In terms of " rain days", and the way rain days modestly fall in the summer months in Paris - I would think you are not really seeing a “Mediterranean" pattern”. The drop in rain days more likely is just the fact that storms/fronts/jets are retreating northward and high pressure is more common. Most temperate climates in the lower middle latitituds probably show some slight or even modest drop in rain days in mid summer or even early fall when high pressure is at its most intense.
If you look at Paris, France and see 50 mm in Jan, then 40 mm in Feb, then 40 mm, 50 mm, 65 mm etc. in Mar, Apr., May and so on, it would seem no discernable trend is there based on those numbers, though if you look at number of rainy days 2-3 fewer rainy days per month in summer, you can kind of make out a "Mediterranean" pattern.
Most of NW Europe has this pattern; London has a spring precip min and an autumn max, due to the fact that high pressure is more dominant between Feb-May.
The winter/spring maximum of precipitation across much of the interior of the USA can be explained by the prevailing winter storm tracks: Low pressure enters the mainland on the NW coast, moves over the Mts, and then reorganizes somewhere east of the Rockies. Other precipitation (both rain and snow) events occur in the quasi-stationary polar front jet that moves in waves across the central USA in winter. This winter rainfall pattern extends into spring, as low pressure still tracks through the middle of the USA in a west to east fashion. Even in late spring (May or June), storm tracks across the middle of the USA resemble winter in their paths and effects. So many stations will show a winter/spring max in precip.
Come true summer (late June – September) and extending into early fall (October)…there is a marked northward shift in storm tracks well into Canada. By late June (on average) storm tracks/fronts/jets shift northward and the weather is more sun controlled. In addition, areas from Texas eastward come under the influence of the more stable western limb of the subtropical anticyclone. So often when you look at rainfall profiles in the middle of the USA, there is a “rainfall dent” in July and August at some stations You see this in link you posted for San Antonio, TX – May and June are get twice as much rainfall as July and Aug. This is less a bridge in west -east precip patterns and more a change in seasonal storm tracks.
I have Trewatha's The Earth's Problem Climates on my shelf (took it out from the library) and I looked through it to see if any information on the topic. Wavehunter007 said most of Trewatha's explanation but I don't he think he made it clear why the rainfall max is in June rather than sometime in spring or another time of year..
The largest moisture source for the Great Plains is the Gulf of Mexico. Moisture influx is greatest when the plains is at its highest temperatures in the mid-summer, and increasing temperature should lead to more intense precipitation. But, as wavehunter said, a northward storm track movement into Canada causes a rainfall dent in midsummer instead of a rainfall max. At 55°N in Canada, the rainfall max is in midsummer. So even though June is a "summer" month in the Plains, it still has some of the storm track patterns of spring, but is hotter than earlier in the spring with more incoming Gulf of Mexico moisture and therefore more precipitation is possible. It's the hottest part of the year in the Great Plains with a persistent storm track.
However, for something like NYC's climate, if you see a number closer to 90 mm one month and the next month averages 80 mm and then the next, a number more like 100 mm, could you really say it's anything other than it's capturing chance fluctuations based on binning by month, and it's just even year round. If you saw 115 mm in July and 80 mm for February for the climate averaged from 30 years of data, does that difference let you ever infer meaningful like "that's a summer peak in rainfall"?
The best check would be to look at all stations within 50-100 miles away (or at least coastal ones). If they have the same small rainfall fluctuations as NYC, perhaps they're real.
I've noticed in the coastal northeast, at least close to NYC, two small rainfall minimums, one around February and another in early autumn. The February one might because of a cold ocean; the land - ocean contrast is the smallest (= less coastal storms) while frontal storms from the continent tend to be a bit weaker.
The early autumn one might have something to do with storm track patterns? The difference for all these is small, though, but I think the February dryness is real.
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