City-Data Forum Measure of precipitation duration (eg. hours of rain per month) (climate, snow)
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01-08-2011, 11:34 AM
 Location: Toronto 3,338 posts, read 6,560,420 times Reputation: 2408

I was just wondering about statistical measures of raininess (partly influenced by the city climate rankings thread) and was wondering if there is a measurement that takes into account precipitation duration, rather than total amount.

This would take into account the fact that 30 mm falling over half a month in a maritime climate could be "rainier" than a single stray thunderstorm event dropping 30 mm in a dry season's otherwise rainless month in a tropical climate.

I know it'd be obviously impractical for any weather station to note the stop and start time of rainfall "events" and sum them up for the day, in minutes, hours or something, and throughout the months to give say hours per month, as cool as that may be (in an ideal world, you could also measure things like average strength of precipitation in mm per hour).

However, since weather stations usually take hourly measurements, a good stand-in could be if it was raining that hour or not.
Thus, if you checked once an hour and there was rain at least at 11:00, 12:00, 13:00, 14:00 that day, you'd put 4 "rain hours" that day.

Are there summary statistics for this kind of thing online (I haven't seen any) or any similar kind of variable that gives an idea of "duration"?. It seems like it'd be a useful measure.

01-08-2011, 12:14 PM
 Location: Western Massachusetts 46,080 posts, read 48,878,100 times Reputation: 15088
You could compare the number of days of rain (> 0.01 in) to the total rainfall and make a ratio out of that. This gives how much rain the average rainy day has.

California comes out quite high.

01-08-2011, 01:50 PM
 Location: Yorkshire, England 5,599 posts, read 9,669,450 times Reputation: 3100
My local weather station records 'wet hours', but I haven't seen any monthly/annual figures or averages ever quoted. The data doesn't appear to be that accurate though. November 2009 (a very mild, wet month) recorded 149.4mm of rain over 147 wet hours, yet according to this on the 1st there was 12.3mm of rain but no wet hours but on the 30th 0.4mm fell over the space of 20 hours Five hours a day for that month sounds plausible even for London. Where I was living then between the mountains of North Wales and the coast it would probably have been more.

Somewhere like Manchester (or Buxton in particular) would give some really terrible stats for rain duration in some months I can think of.

Edit - I tried to find a possible day with 24 hours of rain in London, but 22/11/2003 comes out worst I could find with 23 hours. That week must have been pretty bad:

01-08-2011, 04:43 PM
 Location: USA East Coast 4,445 posts, read 9,677,664 times Reputation: 2136
Quote:
 Originally Posted by Stumbler. I was just wondering about statistical measures of raininess (partly influenced by the city climate rankings thread) and was wondering if there is a measurement that takes into account precipitation duration, rather than total amount. This would take into account the fact that 30 mm falling over half a month in a maritime climate could be "rainier" than a single stray thunderstorm event dropping 30 mm in a dry season's otherwise rainless month in a tropical climate. I know it'd be obviously impractical for any weather station to note the stop and start time of rainfall "events" and sum them up for the day, in minutes, hours or something, and throughout the months to give say hours per month, as cool as that may be (in an ideal world, you could also measure things like average strength of precipitation in mm per hour). However, since weather stations usually take hourly measurements, a good stand-in could be if it was raining that hour or not. Thus, if you checked once an hour and there was rain at least at 11:00, 12:00, 13:00, 14:00 that day, you'd put 4 "rain hours" that day. Are there summary statistics for this kind of thing online (I haven't seen any) or any similar kind of variable that gives an idea of "duration"?. It seems like it'd be a useful measure.
That’s the problem when anyone compares two climates based on just a few monthly climate variables – a lot of things about the true nature of the climate can be lost (or or worse, hidden).

The only one that I saw recently (not the small durations you really want)... was a map of Average Annual Number of Days with Measurable Precipitation. It looks like they took each day (24 hrs) and if there was ANY measurable precip (rain or snow) it was counted as 1 day. The duration on any measurable precip in 24 hours is likely a fair estimated of what smaller duration time frames might be.

Although the map is of the USA - areas right near the US/Canadian border are likely somewhat close (like Toronto). One look at the map and you can see where the greatest number of days (24 hours) stands out with measurable precip…the windward side of the Appalachian Highlands and the Pacific Northwest. (Page 104 at the top)

Extreme weather: a guide & record book - Google Books

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01-08-2011, 05:09 PM
 Location: Toronto 3,338 posts, read 6,560,420 times Reputation: 2408
Quote:
 Originally Posted by wavehunter007 That’s the problem when anyone compares two climates based on just a few monthly climate variables – a lot of things about the true nature of the climate can be lost (or or worse, hidden). The only one that I saw recently (not the small durations you really want)... was a map of Average Annual Number of Days with Measurable Precipitation. It looks like they took each day (24 hrs) and if there was ANY measurable precip (rain or snow) it was counted as 1 day. The duration on any measurable precip in 24 hours is likely a fair estimated of what smaller duration time frames might be. Although the map is of the USA - areas right near the US/Canadian border are likely somewhat close (like Toronto). One look at the map and you can see where the greatest number of days (24 hours) stands out with measurable precip…the windward side of the Appalachian Highlands and the Pacific Northwest. (Page 104 at the top) Extreme weather: a guide & record book - Google Books .
That seems like an interesting book that I should go check out. Thanks for sharing.

That maps seems to show what I was getting at. The "Rainfall intensity" map on the same page is interesting too, showing the greatest intensity in southern Louisiana and Mississipi, as it would also hint that those locations are the ones with the short duration tropical-style rains. "Percent of all rainy days with one inch or more of precipitation" as shown there would actually also be a cool statistic to see city-by-city, if it was collected.

01-08-2011, 05:44 PM
 Location: Western Massachusetts 46,080 posts, read 48,878,100 times Reputation: 15088
Here are the numbers for some cities I found:

Ithaca, NY

35.0" rain/year; 157 days of rain (> 0.01"); 5 days of rain (> 1")

Seattle, WA

38.7" rain/year; 155 days of rain (> 0.01"); 5 days of rain (> 1")

San Rafael, CA

35.7" rain/year; 66 days of rain (> 0.01"); 12 days of rain (> 1")

Honeydew, CA

104.2" rain/year; 101 days of rain (> 0.01"); 33 days of rain (> 1")

Westfield, MA (near where I live)

45.9" rain/year; 109 days of rain (> 0.01"); 13 days of rain (> 1")

Mobile, AL

61.9" rain/year; 105 days of rain (> 0.01"); 20 days of rain (> 1")

01-08-2011, 06:21 PM
 Location: Toronto 3,338 posts, read 6,560,420 times Reputation: 2408
Quote:
 Originally Posted by nei Honeydew, CA 104.2" rain/year; 101 days of rain (> 0.01"); 33 days of rain (> 1") Westfield, MA (near where I live) 45.9" rain/year; 109 days of rain (> 0.01"); 13 days of rain (> 1") Mobile, AL 61.9" rain/year; 105 days of rain (> 0.01"); 20 days of rain (> 1")
Hmm... interesting that a location in the PNW/Northern California has rain that comes in such short, intense bursts.

01-08-2011, 06:34 PM
 Location: Western Massachusetts 46,080 posts, read 48,878,100 times Reputation: 15088
Quote:
 Originally Posted by Stumbler. Hmm... interesting that a location in the PNW/Northern California has rain that comes in such short, intense bursts.
I think most of the rain in California even in the northern parts gets its rain from winter storms. I picked an unusual location (perhaps the close to the wettest in the state? that is on low mountains facing the ocean, increasing the intensity of rain.

01-08-2011, 07:16 PM
 Location: Toronto 3,338 posts, read 6,560,420 times Reputation: 2408
These are stats I got from Environment Canada for a few Canadian cities (1 inch = 0.984 mm):

Prince Rupert, B.C 97.19 inches (2468.5mm) per year; 228.2 days of rain (>= 0.2 mm); 23.5 days of rain (>= 25 mm)

Vancouver 45.46 inches (1154.7 mm) per year; 161.3 days of rain (>= 0.2 mm); 6.4 days of rain (>= 25 mm)

Calgary 12.62 inches (320.6 mm) per year; 67.5 days of rain (>= 0.2 mm); 1.4 days of rain (>= 25 mm)

Medicine Hat, Alberta 9.82 inches (249.6 mm) per year; 60.7 days of rain (>= 0.2 mm); 1 day of rain (>= 25 mm)

Toronto 26.95 inches (684.6mm) per year; 111.8 days of rain (>= 0.2 mm); 4.3 days of rain (>= 25 mm)

Halifax 48.78 inches (1238.9 mm) per year; 131.9 days of rain (>= 0.2 mm); 13.3 days of rain (>= 25 mm)

01-08-2011, 07:45 PM
 Location: Western Massachusetts 46,080 posts, read 48,878,100 times Reputation: 15088
Your precipitation numbers look off. I think you quoted rainfall instead of precipitation. Maybe that was your intent? Perhaps that was what you wanted.

I put precipitation so i'd have to redo them if you want a direct comparison.

Prince Rupert looks awful. Apparently someone likes this climate. From wikipedia:

In 1999, John Viviers and his family moved to Prince Rupert from South Africa specifically because of the weather. Three of the four Viviers children have a rare genetic skin disorder called Porphyria, which is basically an allergy to the Sun. After a 10 year immigration battle with the Canadian Government, the Viviers were granted permanent resident status in April 2008
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