Observatoire Iris
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatoire Iris clear sky chart from 2005-11-05 to 2017-08-12.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
32%
14469
13%
46463
8%
14440
40%
14440
17%
32667

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 15thNight
21%
726
13%
2541
6%
722
31%
722
13%
1844
January16th to endNight
31%
768
14%
2688
8%
768
41%
768
14%
1859
February1st to 15thNight
28%
701
14%
2377
7%
701
38%
701
17%
1635
February16th to endNight
25%
631
15%
1893
7%
631
35%
631
15%
1394
March1st to 15thNight
25%
720
15%
2142
9%
720
33%
720
17%
1522
March16th to endNight
30%
768
18%
2046
7%
768
38%
768
20%
1455
April1st to 15thNight
27%
691
17%
1769
5%
691
34%
691
18%
1312
April16th to endNight
31%
540
16%
1620
9%
540
38%
540
18%
1133
May1st to 15thNight
32%
468
12%
1548
9%
468
38%
468
16%
1091
May16th to endNight
32%
375
10%
1395
7%
374
41%
374
15%
986
June1st to 15thNight
40%
360
7%
1260
5%
360
44%
360
16%
887
June16th to endNight
41%
360
6%
1260
5%
360
43%
360
16%
898
July1st to 15thNight
46%
340
7%
1248
9%
340
49%
340
19%
827
July16th to endNight
51%
376
9%
1418
11%
354
55%
354
17%
919
August1st to 15thNight
52%
414
11%
1463
12%
412
55%
412
18%
989
August16th to endNight
46%
553
15%
1603
9%
553
49%
553
26%
1048
September1st to 15thNight
45%
660
11%
1708
5%
660
48%
660
25%
1182
September16th to endNight
43%
657
18%
1815
12%
657
48%
657
28%
1197
October1st to 15thNight
34%
660
18%
1911
9%
660
44%
660
22%
1303
October16th to endNight
29%
704
11%
2288
5%
704
37%
704
16%
1620
November1st to 15thNight
30%
700
17%
2277
10%
700
39%
700
17%
1560
November16th to endNight
30%
747
14%
2613
9%
747
39%
747
14%
1848
December1st to 15thNight
23%
750
10%
2700
5%
750
31%
750
11%
2033
December16th to endNight
23%
800
13%
2880
5%
800
33%
800
10%
2125

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
15%8%36%5%35%5%5%7%33%
47518
Night
15%8%36%5%34446%36%
46628
TransparencyDay
49%9%13%18%8%1
47201
Night
48%8%12%20%9%1
46463
SeeingDay
21%12%28%23%10%2
6090
Night
20%10%28%28%10%
14472

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
11%12%47%5%24445%35%
1394
16th to endDay
7%8%49%6%48%5%5%7%33%
1536
February1st to 15thDay
5%10%5%8%9%48%5%5%6%30%
1483
16th to endDay
11%12%49%6%35%447%30%
1580
March1st to 15thDay
12%11%48%6%35%45%5%31%
1807
16th to endDay
9%11%5%6%7%36%5%45%35%
2175
April1st to 15thDay
17%11%45%5%24445%33%
2179
16th to endDay
19%9%25%5%24345%36%
2340
May1st to 15thDay
22%6%25%5%35%446%34%
2414
16th to endDay
19%6%36%5%25%7%5%7%30%
2717
June1st to 15thDay
20%6%245%46%5%6%7%30%
2700
16th to endDay
20%5%35%6%46%6%5%7%26%
2700
July1st to 15thDay
14%435%6%46%6%7%9%32%
2635
16th to endDay
14%425%5%46%6%7%9%32%
2654
August1st to 15thDay
14%5%25%6%46%6%7%9%29%
2389
16th to endDay
11%4245%36%5%7%9%39%
2245
September1st to 15thDay
16%425%5%34447%40%
1922
16th to endDay
15%31442446%7%44%
1815
October1st to 15thDay
20%5%25%5%35%346%37%
1717
16th to endDay
20%9%36%5%3446%5%30%
1584
November1st to 15thDay
15%10%37%5%45%45%5%32%
1581
16th to endDay
17%10%46%5%25%45%6%31%
1347
December1st to 15thDay
16%14%5%7%435%33331%
1260
16th to endDay
17%15%37%7%4433429%
1344
January1st to 15thNight
11%11%5%8%6%36%44432%
2541
16th to endNight
8%9%48%6%45%5%5%7%32%
2688
February1st to 15thNight
5%9%48%8%36%45%7%36%
2376
16th to endNight
13%12%5%8%6%33446%31%
1893
March1st to 15thNight
13%11%45%424446%35%
2129
16th to endNight
10%11%46%5%34445%40%
2046
April1st to 15thNight
18%11%36%5%1345%6%33%
1769
16th to endNight
21%8%24424435%38%
1620
May1st to 15thNight
23%8%25%423445%35%
1548
16th to endNight
22%7%345%25%445%33%
1393
June1st to 15thNight
18%7%35%5%25%45%6%33%
1260
16th to endNight
18%6%35%5%35%5%5%7%34%
1260
July1st to 15thNight
17%424425%5%6%7%38%
1323
16th to endNight
13%5%24436%5%5%8%38%
1514
August1st to 15thNight
14%4245%35%5%6%8%38%
1473
16th to endNight
13%414434448%45%
1603
September1st to 15thNight
17%5%2342445%6%42%
1708
16th to endNight
18%5%13323436%48%
1815
October1st to 15thNight
17%6%24435%346%41%
1911
16th to endNight
22%7%35%324346%35%
2288
November1st to 15thNight
17%8%25%4245%5%7%37%
2277
16th to endNight
16%9%45%5%33447%34%
2613
December1st to 15thNight
15%12%5%7%6%3434331%
2700
16th to endNight
16%13%47%5%35%35%430%
2880

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
51%6%11%18%11%
1395
16th to endDay
48%7%12%19%11%
1536
February1st to 15thDay
52%8%11%15%10%1
1486
16th to endDay
54%7%8%16%10%1
1580
March1st to 15thDay
52%7%10%11%14%2
1817
16th to endDay
49%7%9%18%11%3
2175
April1st to 15thDay
51%7%9%17%10%4
2179
16th to endDay
50%6%10%17%13%2
2340
May1st to 15thDay
50%7%11%18%10%1
2414
16th to endDay
52%10%12%17%7%
2721
June1st to 15thDay
49%11%12%16%7%1
2700
16th to endDay
52%12%13%14%7%
2700
July1st to 15thDay
43%13%17%19%6%
2481
16th to endDay
45%14%15%17%7%
2487
August1st to 15thDay
45%16%18%15%4
2375
16th to endDay
35%13%21%23%5%
2245
September1st to 15thDay
42%9%17%19%10%
1922
16th to endDay
38%8%16%25%11%
1815
October1st to 15thDay
47%8%11%22%9%
1717
16th to endDay
54%9%14%15%6%
1584
November1st to 15thDay
52%8%10%20%7%
1581
16th to endDay
53%8%12%18%6%
1347
December1st to 15thDay
59%6%8%17%7%
1260
16th to endDay
59%6%8%15%9%
1344
January1st to 15thNight
54%7%10%16%9%
2541
16th to endNight
50%7%11%20%9%
2688
February1st to 15thNight
47%7%12%20%10%1
2377
16th to endNight
54%6%9%16%11%1
1893
March1st to 15thNight
49%7%10%18%12%2
2142
16th to endNight
45%7%10%19%12%3
2046
April1st to 15thNight
52%5%7%18%13%2
1769
16th to endNight
49%5%9%19%13%3
1620
May1st to 15thNight
50%6%11%17%11%1
1548
16th to endNight
52%6%13%17%8%
1395
June1st to 15thNight
50%7%13%19%7%1
1260
16th to endNight
49%9%12%18%8%
1260
July1st to 15thNight
42%10%16%24%6%
1248
16th to endNight
42%10%16%21%9%
1418
August1st to 15thNight
42%12%18%22%4
1463
16th to endNight
36%10%20%26%5%
1603
September1st to 15thNight
40%9%16%21%10%
1708
16th to endNight
38%7%14%28%9%
1815
October1st to 15thNight
43%7%13%24%10%
1911
16th to endNight
48%9%14%20%7%
2288
November1st to 15thNight
45%9%14%19%9%
2277
16th to endNight
49%9%12%20%8%
2613
December1st to 15thNight
57%7%11%16%6%
2700
16th to endNight
56%7%10%15%8%1
2880

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 15thDay
20%13%36%26%3
60
16th to endDay
17%20%32%17%10%1
64
February1st to 15thDay
13%13%32%26%12%
131
16th to endDay
23%13%29%22%8%1
239
March1st to 15thDay
21%18%30%20%10%
270
16th to endDay
18%17%25%23%14%2
288
April1st to 15thDay
25%17%27%19%7%1
270
16th to endDay
25%13%35%17%43
270
May1st to 15thDay
29%10%29%17%10%2
288
16th to endDay
22%15%29%23%8%1
430
June1st to 15thDay
25%12%25%24%11%1
495
16th to endDay
22%9%29%25%12%1
495
July1st to 15thDay
18%9%30%25%13%2
415
16th to endDay
15%8%31%32%10%1
326
August1st to 15thDay
13%6%30%28%16%3
297
16th to endDay
14%13%27%22%18%2
268
September1st to 15thDay
20%8%29%23%12%6%
255
16th to endDay
16%12%27%25%11%6%
254
October1st to 15thDay
21%15%24%28%8%2
244
16th to endDay
27%12%26%22%8%2
240
November1st to 15thDay
25%13%25%22%11%1
235
16th to endDay
27%17%20%22%10%1
132
December1st to 15thDay
31%10%36%15%5%1
60
16th to endDay
31%29%25%12%1
64
January1st to 15thNight
19%9%37%27%5%
726
16th to endNight
14%12%30%33%8%
768
February1st to 15thNight
10%10%37%30%10%
704
16th to endNight
22%11%31%27%6%
631
March1st to 15thNight
19%14%35%23%7%
720
16th to endNight
17%16%29%26%9%1
768
April1st to 15thNight
25%11%27%25%9%
691
16th to endNight
26%12%25%25%9%
540
May1st to 15thNight
25%9%25%28%10%
468
16th to endNight
26%8%26%28%9%
375
June1st to 15thNight
20%6%23%35%12%1
360
16th to endNight
20%5%25%32%15%
360
July1st to 15thNight
19%5%23%33%17%
340
16th to endNight
15%224%41%14%
376
August1st to 15thNight
16%418%35%22%2
414
16th to endNight
15%428%36%15%
553
September1st to 15thNight
19%6%24%33%14%2
660
16th to endNight
20%6%27%32%12%1
657
October1st to 15thNight
20%10%28%27%11%1
660
16th to endNight
27%10%27%25%9%
704
November1st to 15thNight
22%14%26%25%9%1
700
16th to endNight
22%15%23%25%10%1
747
December1st to 15thNight
22%14%30%26%5%
750
16th to endNight
24%13%27%24%8%1
800
Page generated at 2017-08-13 14:16:23. Version ac04

Copyright 2017 A. Danko


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