Meso- and Micro-scale climate factors
Here in Denver, we have to do a lot of mental re-figuring to convert NOAA's airport-centered weather data into "city" terms. The DIA weather station is pretty far out on the prairie, about 30 km away from Denver's core urban heat island and subject to different wind patterns than UCD's weather station in the inner city.
Meso-scale factors affecting Christchurch include foehn winds and sea breezes.
The foehns off the eastern slope of the Alps were mentioned in the first post. Called the Chinook in our part of the world, and the Nor'wester in New Zealand, they're a product of the temperature and pressure difference between the windward and leeward side of the mountain range. Humid air masses off the ocean cool rapidly as they ascend the western slope, but the cooling rate slows as moisture condenses and releases latent heat. As the air mass descends on the leeward side, having released its moisture as rain or snow, it now warms more quickly at the dry adiabatic lapse rate. The relatively low pressure on the eastern plain sucks this air quickly across the landscape, often warming the region by several degrees in a short time.
Sea Breezes
Another meso-scale factor affecting the Christchurch Aero weather station is the sea breezes. In the spring and summer, as the island receives increasing insolation, inland areas heat up faster than the ocean. The resulting low pressure over land draws cooler air westward, lowering temperatures in Christchurch.
The effect isn't as pronounced for Christchurch Aero as it is for coastal stations, like New Brighton Pier about 16 km east of the airport, so people on the eastern side of the city would want to obtain their forecasts from coastal rather than inland weather stations.
As mentioned in previous posts, the Canterbury Plain is arid relative to the Southern Alps range on its west, which wrings out moisture from the weather systems that the prevailing Westerlies push across the island. Although the Plain is in a humid maritime climate zone, it's also in a "rain shadow":
Temperature Trends Over Time
This weather station has recorded data going back to 1954, but unfortunately Niwa's CliFlo database doesn't provide it in spreadsheet format. (Don't even ask about Weather Underground, because they have some pretty silly ideas about effective data presentation.) Below are the monthly average minimum and maximum temperatures for January and July from 1985 to 2014:
In Christchurch that disparity isn't quite as pronounced. The airport is in the western suburbs about 8 km from the city center, so although its temperatures will be a bit lower than those recorded by a weather station downtown, the difference isn't as large as the difference between DIA and UCD.
Looking at this satellite image, it appears that the station's proximity to large areas of irrigated agriculture might be an anthropogenic micro-scale factor affecting its readings:
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New Zealand Aero (station 4843). Weather Underground pinpoints its location at 43°29'34.80"S and 172°32'13.20"E, about halfway between downtown and the Waimakariri river. Image: screen capture from Google Earth |
Actually, however, pretty much the entire Canterbury plain is irrigated agriculture. (They grow all the produce that sustains the Northern Hemisphere through the winter.) Zooming out reveals that the larger factor by far is the ocean, which provides humidity and moderates seasonal and diurnal temperatures for the whole region.
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Let's get some perspective on the situation. Oh hi ocean... you're probably adding more moisture to the air than the irrigated fields do.Fun Fact: Pegasus Bay is a popular retirement spot for My Little Ponies. Image: screen capture from Google Earth. |
The foehns off the eastern slope of the Alps were mentioned in the first post. Called the Chinook in our part of the world, and the Nor'wester in New Zealand, they're a product of the temperature and pressure difference between the windward and leeward side of the mountain range. Humid air masses off the ocean cool rapidly as they ascend the western slope, but the cooling rate slows as moisture condenses and releases latent heat. As the air mass descends on the leeward side, having released its moisture as rain or snow, it now warms more quickly at the dry adiabatic lapse rate. The relatively low pressure on the eastern plain sucks this air quickly across the landscape, often warming the region by several degrees in a short time.
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I'm including this diagram because it's so bad it's good, and thus so good it's perfect. A high school physical sciences teacher from "my day" (or before) evidently made this on a typewriter, a feat which I can guarantee, having done similar things myself in the days before regular people had computers, takes some excellent carriage-return skills. Nostalgia is a powerful thing. Image from http://www.learninginfo.org/sandbox/foehn-wind.htm (and before that, maybe Mr. Crowell's banker-box of overhead transparencies). |
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| And now for Something Completely Different: "Get up for the downslope [winds]; everybody get up"... OK, OK, no need for the cane; I'll see myself out. |
Another meso-scale factor affecting the Christchurch Aero weather station is the sea breezes. In the spring and summer, as the island receives increasing insolation, inland areas heat up faster than the ocean. The resulting low pressure over land draws cooler air westward, lowering temperatures in Christchurch.
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Friendship is Magic in Pegasus Bay! And sea breezes are delightful! Twilight Sparkle seems concerned by Pinkie Pie's trajectory, perhaps because she lacks wings. But no ponies were harmed in the making of this graphic. Diagram from https://www.blendspace.com/lessons/SNmjP2UrHdIDzQ/global-and-local-wind; MLPs added in Photoshop from miscellaneous fandom blogs. |
As mentioned in previous posts, the Canterbury Plain is arid relative to the Southern Alps range on its west, which wrings out moisture from the weather systems that the prevailing Westerlies push across the island. Although the Plain is in a humid maritime climate zone, it's also in a "rain shadow":
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The South Island's outline overlaid in Photoshop on a May 6, 2015 Weather Underground map. Precipitation totals from weather stations around the island indicate the rain shadow. To the west, the mountain stations are showing moisture accumulation, while the eastern plains, including the airport station (highlighted) are dry. |
Climographs reveal the magnitude of the difference in yearly rainfall on the mountains versus the plains:
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Arrows point to May precipitation levels for Milford Sound and Christchurch Aero. Milford Sound averages 650 mm while Christchurch averages 60.Climographs from https://www.niwa.co.nz; image from Google Earth |
Temperature Trends Over Time
This weather station has recorded data going back to 1954, but unfortunately Niwa's CliFlo database doesn't provide it in spreadsheet format. (Don't even ask about Weather Underground, because they have some pretty silly ideas about effective data presentation.) Below are the monthly average minimum and maximum temperatures for January and July from 1985 to 2014:
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Seasonal minimum and maximum average temperatures at Christchurch Aero.
Data from https://www.niwa.co.nz plotted in Excel
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The temperatures have declined slightly over the past thirty years, on average. Is global warming a myth? No, it's just hard to prove with only thirty years of data from one weather station in an isolated geographic area. One thing to notice is the pronounced dip and spike in the summer (January) maximum around 1997-1999. It turns out there was a strong El Nino-La Nina event during that period:
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| ENSO events plotted using NOAA's OSI (Oceanic Nino Index), which is based on 3-month overlapping mean sea surface temperature anomalies. 1997-1998 was a "strong" El Nino year. Graph from http://ggweather.com/enso/oni.htm |
Global warming is definitely affecting New Zealand, however. According to the New Zealand government, average temperatures have risen 0.9C over the past century - 0.2 degrees more than the worldwide average increase. According to the WWF (that's the Panda WWF, not the "Gorilla Monsoon" WWF), average sea levels in the islands' four major ports have risen 16 cm in the past 100 years, and glaciers in the Southern Alps have lost 11% of their ice in just the past thirty years.
Comparison to A Location From a Classmate's Blog
Comparison to A Location From a Classmate's Blog
Some people in this class should clearly be meteorologists. (I'm not one of them.) Some other people should probably not blog at all. (I'm probably one of those.) A few others used Tumblr for this assignment, but I'm too old to get a Tumblr account even with a course grade at stake. The latter two criteria eliminate about 75% of the class blogs, including some locations I'd hoped to compare with mine.
Therefore, I chose Bangkok, Thailand, because Chris's blog is really well done, and because while Bangkok's terrestrial geography is similar to New Zealand's, its climate is very very different.
Similarities between Bangkok and Christchurch:
- Both are at a low elevation
- Both have mountains to the west of the city
- Both are next to a large ocean area that moderates temperatures ranges
- Both are dominated by maritime air masses
- Both are next to a large ocean area that moderates temperatures ranges
- Both are dominated by maritime air masses
Differences between Bangkok and Christchurch:
Everything else!
- Bangkok is between the Tropic of Cancer and the Equator, so it's much hotter than Christchurch, with highs between 95 and 100F, and lows between 70 and 80F
- Due to Bangkok's tropical location, those temperatures prevail throughout the year, whereas Christchurch, in the midlatitudes, has cold winters
- The ITCZ is Bangkok's primary macro-scale climate control; the location of the Antarctic polar front is Christchurch's
- The migration of the ITCZ brings a monsoon season to Bangkok from May through October; Christchurch gets most of its moisture in the winter, but it sure as heck isn't anything close to monsoonal
- Bangkok's prevailing winds are the Southeast Trades, while New Zealand, much farther from the equator, gets the Westerlies
- Bangkok receives tropical cyclones (typhoons) off the warm equatorial ocean, rather than the midlatitude cold-occluded cyclones that affect Christchurch
- Bangkok's mountains disrupt cold air masses moving south from the Asian continent, so it doesn't tend to get cold fronts
- Continental polar air masses do migrate south over Thailand in the winter when the ITCZ is to the south, bringing high pressure and drier air; Christchurch has no continents nearby that would perform this function
Everything else!
- Bangkok is between the Tropic of Cancer and the Equator, so it's much hotter than Christchurch, with highs between 95 and 100F, and lows between 70 and 80F
- Due to Bangkok's tropical location, those temperatures prevail throughout the year, whereas Christchurch, in the midlatitudes, has cold winters
- The ITCZ is Bangkok's primary macro-scale climate control; the location of the Antarctic polar front is Christchurch's
- The migration of the ITCZ brings a monsoon season to Bangkok from May through October; Christchurch gets most of its moisture in the winter, but it sure as heck isn't anything close to monsoonal
- Bangkok's prevailing winds are the Southeast Trades, while New Zealand, much farther from the equator, gets the Westerlies
- Bangkok receives tropical cyclones (typhoons) off the warm equatorial ocean, rather than the midlatitude cold-occluded cyclones that affect Christchurch
- Bangkok's mountains disrupt cold air masses moving south from the Asian continent, so it doesn't tend to get cold fronts
- Continental polar air masses do migrate south over Thailand in the winter when the ITCZ is to the south, bringing high pressure and drier air; Christchurch has no continents nearby that would perform this function
What, is the blog done so soon? Just like that?
Yes, it's time to leave New Zealand to its weather,
and move on to other projects.
and move on to other projects.
