U.S. CitiesCity-Data Forum Index
Go Back   City-Data Forum > General Forums > Weather
 [Register]
Please register to participate in our discussions with 2 million other members - it's free and quick! Some forums can only be seen by registered members. After you create your account, you'll be able to customize options and access all our 15,000 new posts/day with fewer ads.
View detailed profile (Advanced) or search
site with Google Custom Search

Search Forums  (Advanced)
Reply Start New Thread
 
Old 08-24-2012, 09:06 PM
 
Location: Melbourne AUS
1,156 posts, read 1,730,532 times
Reputation: 836

Advertisements

So often we see things like "Number of cloudy days", "Clear days" and all that in climate datasets, but what is the actual definition of all this? What in your opinion constitutes an overcast, clear, partly cloudy, mostly cloudy day, in terms of cloudiness, sunshine/cloud cover duration and amount of cloud cover? Also what benchmarks do weather authorities use?

Personally my definitions are as follows:

Overcast.... 1 hour or less sunshine recorded for the day

Cloudy.... 3 hours or less sunshine

Mostly Cloudy.... 3-5 hours of sunshine

Partly Cloudy..... 5-7 hours of sunshine

Mostly Sunny..... over 7 hours of sunshine

Clear....... no more than 1/8th of the sky obscured at any time during the day.
Rate this post positively Reply With Quote Quick reply to this message

 
Old 08-24-2012, 10:09 PM
 
Location: Leeds, UK
22,256 posts, read 26,571,240 times
Reputation: 8760
If I'm reporting on weather forums and say it is 'clear' or 'sunny' then the sky will usually be clear with a few clouds in the distance.. mostly sunny when there's cumulus in the sky in great numbers but does not obscure the sun very often.. partly cloud is when the sun is obscured for long periods of time, the sky could be covered by up to 50% of cloud, as long as the sun is shining more than it isn't, mostly cloudy is when the sun comes out every now and again throughout the day (aka sunny spells), cloudy is when, well, the sky is more or less completely obscured by cloud, maybe some blue patches, and days like this usually end up with an hour or so of sunshine before the sun sets.. and overcast is when the sky is obscured by a usually featureless grey blanket with no sun all day (v. common in winter)
Rate this post positively Reply With Quote Quick reply to this message
 
Old 08-25-2012, 12:35 AM
 
25,058 posts, read 25,520,490 times
Reputation: 11727
I don't go by sunshine hours, I go by cloud content in the sky:

Overcast: Flat sheet of light gray obscuring the whole sky

Cloudy: Any of the cumulus type clouds obscuring the whole sky (aka uneven cloud surfaces)

Mostly Cloudy: 1/4 of the sky has blue patches

Partly Cloudy: About half the sky is unobscured

Mostly Sunny: 3/4-99% of the sky is unobscured and sky mostly has cirrus type clouds.

Clear: 100% of sky is cloudless
Rate this post positively Reply With Quote Quick reply to this message
 
Old 08-25-2012, 01:25 AM
 
Location: Wellington and North of South
5,127 posts, read 7,583,583 times
Reputation: 2627
Quote:
Originally Posted by Flight Simmer View Post
So often we see things like "Number of cloudy days", "Clear days" and all that in climate datasets, but what is the actual definition of all this? What in your opinion constitutes an overcast, clear, partly cloudy, mostly cloudy day, in terms of cloudiness, sunshine/cloud cover duration and amount of cloud cover? Also what benchmarks do weather authorities use?

Personally my definitions are as follows:

Overcast.... 1 hour or less sunshine recorded for the day

Cloudy.... 3 hours or less sunshine

Mostly Cloudy.... 3-5 hours of sunshine

Partly Cloudy..... 5-7 hours of sunshine

Mostly Sunny..... over 7 hours of sunshine

Clear....... no more than 1/8th of the sky obscured at any time during the day.
I think you need to refine it by allowing for day length (so use % values say) - I think the above is OK for a 12-hour day.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 08-25-2012, 02:02 AM
 
Location: Melbourne AUS
1,156 posts, read 1,730,532 times
Reputation: 836
Quote:
Originally Posted by RWood View Post
I think you need to refine it by allowing for day length (so use % values say) - I think the above is OK for a 12-hour day.
Yeah it gets tricky using hours as day length varies across the year. A day with 7 hours of sunshine in winter where the max daylight length is perhaps 9 hours would "feel" somewhat sunnier than 7 hours in summer with 15 hours of daylight.

Converted to percent I would consider the following.

Overcast.... less than 10% of possible sunshine during the course of a day

Cloudy.... < 30% sunshine

Mostly Cloudy.... 30-50% sunshine

Partly Cloudy..... 50-70% sunshine

Mostly Sunny..... > 70% sunshine

Clear....... no more than 1/8th of the sky obscured at any time during the day.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 08-25-2012, 04:24 AM
 
Location: North West Northern Ireland.
20,694 posts, read 21,671,248 times
Reputation: 3107
Overcast- when there are no breaks of the cloud.

Partly cloudy- when there are quite a few breaks of cloud with even some sunshine.

Mostly cloudy- almost overcast with a few holes in the cloud.

Clear- no clouds whatsoever.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 08-25-2012, 05:37 AM
 
Location: Laurentia
5,593 posts, read 7,168,237 times
Reputation: 2425
Sunny - No clouds in the sky or a few small ones

Mostly Sunny - Some small clouds in the sky (maybe 10% of the sky covered?)

Partly Cloudy - Sun still shining, but with roughly 25% to 40% of the sky covered; some significant clouds present

Scattered Clouds - Sun and blue sky evenly matched, roughly 40% to 60% cloud cover

Mostly Cloudy - Dominant clouds, blue patches cover over 10% of the sky

Cloudy - Sky completely covered by clouds or a few small blue patches

I think some of the sunshine requirements as put forth by Flight Simmer are too high. Days when the skies are mostly cloudy all day shouldn't have much sunshine. EDIT: I see that it's his descriptions are off, since he has mostly cloudy, cloudy, and then overcast. It seems to just be a case of differing labels. Still, 30-50% seems too high to be "mostly cloudy". Conversely, partly cloudy or mostly sunny days should have sunshine the vast majority of the time.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 08-25-2012, 06:16 AM
 
Location: Buxton, England
7,023 posts, read 10,074,984 times
Reputation: 3628
I just go by sunshine hours alone but by the first mentioned definitions here in Buxton most days would be overcast or mostly cloudy even in summer. We average 3 hours sun a day so far this August and almost all of that has been between 5am-9am when I'm not up yet anyway it is basically cloudy here all day after around 10-11AM, every day apart from 2-3 days maybe.
Rate this post positively Reply With Quote Quick reply to this message
 
Old 08-25-2012, 07:09 AM
 
Location: Buxton, England
7,023 posts, read 10,074,984 times
Reputation: 3628
I looked up the definition of "overcast" in our dictionary and was not surprised, it said "Buxton, Derbyshire a small town in North Western England".
Rate this post positively Reply With Quote Quick reply to this message
 
Old 09-29-2012, 04:43 PM
 
7,237 posts, read 11,816,023 times
Reputation: 5606
The official NWS measure for sky cover is (measured in percentages)...

0-3 = Clear
4-7 = Partly Cloudy
8-10 = Cloudy

Now, how exactly they apply that scale, I have no idea (I assume it's arbitrary based on who's manning the observation deck).
Rate this post positively Reply With Quote Quick reply to this message
Please register to post and access all features of our very popular forum. It is free and quick. Over $68,000 in prizes has already been given out to active posters on our forum. Additional giveaways are planned.

Detailed information about all U.S. cities, counties, and zip codes on our site: City-data.com.


Reply
Please update this thread with any new information or opinions. This open thread is still read by thousands of people, so we encourage all additional points of view.

Quick Reply
Message:


Over $104,000 in prizes was already given out to active posters on our forum and additional giveaways are planned!

Go Back   City-Data Forum > General Forums > Weather

All times are GMT -6.

© 2005-2021, Advameg, Inc. · Please obey Forum Rules · Terms of Use and Privacy Policy · Bug Bounty

City-Data.com - Contact Us - Archive 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37 - Top