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Some cities, like London and Rome (I'm sure there are many others - please mention them), count a rain day when they get over 1mm (0.3in) of precipitation. Though many other cities call it a 'rain day' when their gauge hits the 0.2mm mark (0.01in).
So, London has 110 rainy days (based on 1mm). Sydney has 143 (0.2mm). Shouldn't they stick with one formula, as this could confuse people? For years I thought London got less rainy days than Sydney, when it didn't (Sydney's yearly 1mm rain days are at 100).
BQ: What should be counted as a 'precipitation day'? A day receiving 1mm (0.3in) or 0.2mm (0.01in) of rain?
London gets the same amount of rainy days as Sydney when using 0.2mm though. I think 1mm is better, because using the 0.2 threshold, you can record a precipitation day if morning dew gets onto the sensor.
I prefer precipitation hours based on a 0.1 or 0.2 mm threshold. They give a much better impression of the degree of rain exposure because both, a brief thunderstorm and hours of horrible drizzle, produce just one rain day. Precipitation hours are also better comparable to sunshine hours.
I favour the lower precipitation threshold, particularly living somewhere with a lot of drizzle and light rain. 0.9 mm might not be a lot, but it can easily take us several hours of drizzle to get to that point, and on a day like that in autumn / winter the ground will probably be damp all day. If you'd been outside in that weather you probably wouldn't appreciate somebody telling you it didn't count as a proper rainy day.
With dew getting onto the sensor and counting as trace rainfall, surely there is a way of getting round that somehow? My local site sometimes records trace rainfall when it's foggy, so it's not ideally accurate.
I wish rainfall hours were more widely recorded as well, because it's a useful tool to compare climates and gives extra info when raw rainfall totals don't tell the whole story. The only place I know of which gives figures for it is the Hampstead amateur station. Last month was very wet there with 142 mm of rain, similar to, say, a feasible summer monthly total somewhere on the east coast of the US in a thundery year, but it took 122 hours to get up to that total, and the streets would have been more or less constantly damp - so which place would feel wetter?
I favour the lower precipitation threshold, particularly living somewhere with a lot of drizzle and light rain. 0.9 mm might not be a lot, but it can easily take us several hours of drizzle to get to that point, and on a day like that in autumn / winter the ground will probably be damp all day. If you'd been outside in that weather you probably wouldn't appreciate somebody telling you it didn't count as a proper rainy day.
With dew getting onto the sensor and counting as trace rainfall, surely there is a way of getting round that somehow? My local site sometimes records trace rainfall when it's foggy, so it's not ideally accurate.
I wish rainfall hours were more widely recorded as well, because it's a useful tool to compare climates and gives extra info when raw rainfall totals don't tell the whole story. The only place I know of which gives figures for it is the Hampstead amateur station. Last month was very wet there with 142 mm of rain, similar to, say, a feasible summer monthly total somewhere on the east coast of the US in a thundery year, but it took 122 hours to get up to that total, and the streets would have been more or less constantly damp - so which place would feel wetter?
I was just about to say the opposite, but your point is equally valid. What ever threshold is used, I just wish it was the same every where.
Trace rainfalls (below 1 mm),particularly during the warmer half of the year, can be difficult to detect. It's reached the 0.1C threshold here, about 5 days in the last week (from convective buildup), but talking to people last night (at an event), everyone had thought it only rained on one day. So to say it rained 5 days (at 0.1 mm) , may be technically correct, but could give a false impression.The dews here leave the ground wetter than the average trace rain fall.
Today was Day 60, though that is using a non-standard 1800-1800 recording period that includes six days with trace rainfall and three with 0.2 mm. Today had 20 mm though, which is the joint-wettest of the spell so far.
It's very unusual for weather patterns to get stuck in a rut for this long. The highs this month so far have been ridiculously consistent: 6.2, 6.4, 5.7, 6.4, 6.4, 6.5, 6.0, 6.9, 6.2. Lows have mostly been in the 0-3 C range all winter long, but we've only had one frost since this wet spell started.
I think 1mm is better, because using the 0.2 threshold, you can record a precipitation day if morning dew gets onto the sensor.
I agree, and true.
I think the 0.2mm threshold should only be used for arid places.
Wet cities (or at least non-arid ones) should go by 1mm. I mean, Melbourne (goes by 0.2mm) has brief drizzles in some of its days - looking at a month on its climate table (e.g. '14 rainy days') makes you think, "wow, it rains a lot there" - when it's just a drizzle. Rain over 1mm is a 'true' rain day or event.
I think the 0.2mm threshold should only be used for arid places.
Wet cities (or at least non-arid ones) should go by 1mm. I mean, Melbourne (goes by 0.2mm) has brief drizzles in some of its days - looking at a month on its climate table (e.g. '14 rainy days') makes you think, "wow, it rains a lot there" - when it's just a drizzle. Rain over 1mm is a 'true' rain day or event.
Yes. And for dry places like Alexandra in NZ, dew can cause a precip. event below the 1mm threshold.
In my opinion a "day with rain" is a "day with rain" no matter
how short the time or small the quantity. In my weather
records, days with occurrences (rain,hail,snow) are marked with
a binary flag; - "0" = no, "1" = yes.
So that includes counting any day with measurable rainfall
(0.2mm+) as a "rain day". Going by that number, my town
averages about 240 days per annum with any amount of rain.
If I only considered days with 1.0mm+ of rain, that number
would be considerably lower.
As for the issue of fog drip/dew some have mentioned; I am
generally able to determine if 0.2/0.4mm has collected due to
this as opposed to rain, and a "rain day" will not be
recorded, although the precip quantity value will be added to
the total precipitation.
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