Watch Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watch Observatory clear sky chart from 2005-11-05 to 2017-12-20.

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
29%
15020
14%
48046
8%
14991
36%
14991
16%
33907

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
17%
720
9%
2506
5%
716
23%
716
8%
1951
January16th to endNight
19%
768
11%
2688
6%
768
26%
768
10%
1980
February1st to 15thNight
18%
702
9%
2294
5%
701
23%
701
10%
1769
February16th to endNight
22%
631
14%
1901
6%
631
32%
631
13%
1409
March1st to 15thNight
21%
720
13%
2159
8%
720
28%
720
13%
1613
March16th to endNight
29%
768
20%
1992
9%
768
37%
768
21%
1395
April1st to 15thNight
27%
718
17%
1796
6%
718
32%
718
19%
1333
April16th to endNight
26%
638
17%
1718
4%
638
35%
638
19%
1211
May1st to 15thNight
30%
380
17%
1460
13%
380
41%
380
14%
965
May16th to endNight
33%
374
13%
1497
11%
374
45%
374
16%
968
June1st to 15thNight
31%
360
9%
1440
8%
360
40%
360
13%
998
June16th to endNight
38%
360
9%
1440
8%
360
47%
360
16%
897
July1st to 15thNight
49%
340
15%
1360
19%
340
56%
340
23%
771
July16th to endNight
46%
376
13%
1418
14%
354
56%
354
23%
815
August1st to 15thNight
52%
376
15%
1447
15%
374
57%
374
21%
845
August16th to endNight
50%
756
20%
1902
10%
756
55%
756
34%
1217
September1st to 15thNight
48%
720
15%
1800
8%
720
50%
720
28%
1245
September16th to endNight
50%
717
22%
1959
16%
717
54%
717
31%
1232
October1st to 15thNight
34%
720
18%
2136
11%
720
42%
720
21%
1446
October16th to endNight
26%
768
12%
2448
8%
768
32%
768
14%
1782
November1st to 15thNight
26%
760
16%
2472
11%
760
35%
760
15%
1743
November16th to endNight
18%
780
12%
2728
6%
780
26%
780
10%
2019
December1st to 15thNight
16%
780
9%
2730
4%
780
22%
780
8%
2123
December16th to endNight
11%
788
10%
2755
3%
788
19%
788
7%
2180

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
17%7%36%5%3445%6%33%
48806
Night
18%9%36%5%34446%32%
48215
Cloud CoverDay
17%8%36%5%3445%6%33%
48815
Night
18%9%36%5%34446%32%
48221
TransparencyDay
49%412%21%10%1
48493
Night
52%411%20%10%1
48046
SeeingDay
19%13%22%24%14%5%
5843
Night
20%19%22%25%10%
15023

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
22%13%5%8%6%25%435%21%
1432
16th to endDay
13%15%5%7%6%35%447%25%
1536
February1st to 15thDay
14%13%49%6%35%5%5%5%24%
1566
16th to endDay
17%12%49%6%34445%27%
1572
March1st to 15thDay
17%10%5%8%7%35%43428%
1790
16th to endDay
19%6%36%6%24345%38%
2232
April1st to 15thDay
21%8%36%5%24445%32%
2152
16th to endDay
19%6%36%42445%5%36%
2241
May1st to 15thDay
20%6%35%5%3445%6%33%
2502
16th to endDay
19%6%26%43436%7%35%
2618
June1st to 15thDay
22%7%35%42445%6%32%
2520
16th to endDay
17%5%25%446%5%6%8%31%
2520
July1st to 15thDay
12%425%5%25%5%5%8%39%
2520
16th to endDay
12%4246%36%5%7%10%35%
2654
August1st to 15thDay
12%5%36%7%35%6%6%8%35%
2497
16th to endDay
10%3145%346%6%8%45%
2292
September1st to 15thDay
15%5%2442445%7%42%
2160
16th to endDay
13%424424247%48%
2001
October1st to 15thDay
20%7%25%5%34446%35%
1821
16th to endDay
22%10%36%6%345%5%5%27%
1776
November1st to 15thDay
16%8%37%425%5%6%7%31%
1716
16th to endDay
22%8%37%6%44436%27%
1560
December1st to 15thDay
23%13%48%6%345%5%6%18%
1560
16th to endDay
27%13%5%8%6%2434420%
1568
January1st to 15thNight
18%16%5%8%7%34445%20%
2506
16th to endNight
13%15%6%7%7%35%445%26%
2688
February1st to 15thNight
14%13%5%10%8%36%5%45%21%
2292
16th to endNight
16%11%6%8%6%35%445%27%
1901
March1st to 15thNight
20%10%47%6%35%34428%
2146
16th to endNight
17%10%3443324440%
1992
April1st to 15thNight
26%7%25%5%24345%32%
1796
16th to endNight
21%7%35%5%23346%37%
1718
May1st to 15thNight
21%8%36%5%2335%5%35%
1460
16th to endNight
19%5%35%434447%36%
1495
June1st to 15thNight
23%6%25%5%25%5%47%29%
1440
16th to endNight
15%5%25%425%5%6%8%36%
1440
July1st to 15thNight
10%3245%346%6%9%43%
1440
16th to endNight
10%6%345%345%6%8%42%
1514
August1st to 15thNight
11%5%25%5%3447%8%40%
1457
16th to endNight
11%424424448%47%
1902
September1st to 15thNight
15%6%25%5%34446%41%
1800
16th to endNight
16%5%14324347%47%
1959
October1st to 15thNight
17%7%25%434446%37%
2136
16th to endNight
24%7%38%5%34445%29%
2448
November1st to 15thNight
20%7%36%5%34446%32%
2472
16th to endNight
21%11%47%5%34345%27%
2728
December1st to 15thNight
22%15%5%7%5%3434422%
2730
16th to endNight
25%15%5%8%5%2333421%
2755

Percentage Cloud Cover SE (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
21%14%5%8%6%34445%21%
1432
16th to endDay
13%15%5%8%6%35%447%25%
1536
February1st to 15thDay
14%13%5%8%7%45%5%5%5%23%
1566
16th to endDay
16%12%5%9%5%34445%27%
1572
March1st to 15thDay
17%10%5%9%6%3444428%
1800
16th to endDay
18%6%35%5%34345%38%
2231
April1st to 15thDay
21%8%45%5%35%445%32%
2152
16th to endDay
18%7%35%5%3445%5%37%
2241
May1st to 15thDay
19%6%35%5%3445%6%33%
2502
16th to endDay
18%6%25%5%3445%7%35%
2618
June1st to 15thDay
21%7%35%5%2445%6%32%
2520
16th to endDay
17%6%25%5%36%5%6%8%31%
2520
July1st to 15thDay
11%5%246%35%5%6%8%40%
2520
16th to endDay
11%4245%45%6%8%10%36%
2654
August1st to 15thDay
11%6%25%7%35%5%6%9%35%
2497
16th to endDay
9%3144246%6%9%46%
2292
September1st to 15thDay
15%5%2443445%7%42%
2160
16th to endDay
13%424423347%48%
2001
October1st to 15thDay
20%6%36%5%335%45%35%
1821
16th to endDay
22%10%36%5%3446%5%27%
1776
November1st to 15thDay
16%8%46%5%3446%7%31%
1716
16th to endDay
21%9%37%6%35%446%27%
1560
December1st to 15thDay
24%13%48%5%35%45%6%18%
1560
16th to endDay
26%13%6%7%5%33345%19%
1568
January1st to 15thNight
17%16%5%9%7%34445%20%
2506
16th to endNight
14%15%5%8%7%34445%26%
2688
February1st to 15thNight
13%13%5%11%8%36%5%46%21%
2285
16th to endNight
15%11%5%9%6%35%445%27%
1901
March1st to 15thNight
20%11%47%6%35%345%27%
2159
16th to endNight
17%10%35%423335%39%
1992
April1st to 15thNight
26%7%25%434345%32%
1796
16th to endNight
21%7%44423337%36%
1718
May1st to 15thNight
21%7%35%5%32445%35%
1460
16th to endNight
19%5%36%435%447%35%
1495
June1st to 15thNight
23%6%25%435%45%7%29%
1440
16th to endNight
15%6%26%4245%7%8%36%
1440
July1st to 15thNight
10%315%5%35%5%6%8%43%
1440
16th to endNight
10%5%24425%47%8%42%
1514
August1st to 15thNight
11%5%25%5%25%47%8%40%
1457
16th to endNight
12%424334448%47%
1902
September1st to 15thNight
15%6%25%5%33446%41%
1800
16th to endNight
16%5%23324346%47%
1959
October1st to 15thNight
17%7%35%4345%46%37%
2136
16th to endNight
23%7%38%6%34445%29%
2448
November1st to 15thNight
20%7%36%5%34445%33%
2472
16th to endNight
21%11%48%5%34445%26%
2728
December1st to 15thNight
22%15%5%7%5%25%345%21%
2730
16th to endNight
24%16%5%7%5%3333421%
2755

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
64%39%14%6%
1433
16th to endDay
58%313%16%8%
1536
February1st to 15thDay
61%311%19%4
1568
16th to endDay
58%311%16%9%1
1572
March1st to 15thDay
58%212%16%8%1
1800
16th to endDay
47%310%18%15%4
2232
April1st to 15thDay
53%36%21%12%2
2152
16th to endDay
47%29%20%15%4
2241
May1st to 15thDay
50%49%20%13%2
2502
16th to endDay
47%411%21%12%1
2622
June1st to 15thDay
51%5%10%20%11%
2520
16th to endDay
47%5%13%21%10%1
2520
July1st to 15thDay
37%6%15%28%12%
2371
16th to endDay
39%6%18%25%9%
2487
August1st to 15thDay
42%5%18%24%8%
2483
16th to endDay
31%418%31%12%
2292
September1st to 15thDay
40%413%28%12%
2160
16th to endDay
35%312%34%12%
2001
October1st to 15thDay
49%410%24%9%
1821
16th to endDay
56%315%17%7%
1776
November1st to 15thDay
49%415%20%8%
1716
16th to endDay
57%5%10%19%6%
1560
December1st to 15thDay
65%5%11%13%3
1560
16th to endDay
67%49%12%5%
1568
January1st to 15thNight
64%49%16%4
2506
16th to endNight
59%310%17%7%
2688
February1st to 15thNight
62%411%15%4
2294
16th to endNight
58%39%17%10%
1901
March1st to 15thNight
58%39%18%8%1
2159
16th to endNight
47%38%19%16%4
1992
April1st to 15thNight
54%36%19%14%1
1796
16th to endNight
49%37%18%16%4
1718
May1st to 15thNight
50%37%18%16%4
1460
16th to endNight
48%49%21%14%1
1497
June1st to 15thNight
52%5%10%20%10%
1440
16th to endNight
43%5%14%20%14%
1440
July1st to 15thNight
34%6%12%29%16%
1360
16th to endNight
36%6%17%26%13%
1418
August1st to 15thNight
37%6%20%23%11%
1447
16th to endNight
35%415%30%13%
1902
September1st to 15thNight
42%5%10%26%14%
1800
16th to endNight
37%410%31%14%
1959
October1st to 15thNight
46%49%26%11%
2136
16th to endNight
56%413%19%6%
2448
November1st to 15thNight
53%312%19%9%1
2472
16th to endNight
59%411%17%5%
2728
December1st to 15thNight
64%411%16%3
2730
16th to endNight
66%49%13%5%
2755

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
25%18%20%20%15%1
60
16th to endDay
35%7%28%18%6%3
64
February1st to 15thDay
32%10%39%16%1
89
16th to endDay
26%14%26%26%5%
231
March1st to 15thDay
20%19%30%22%7%
270
16th to endDay
23%17%22%24%10%2
288
April1st to 15thDay
26%20%24%19%8%1
270
16th to endDay
17%20%24%22%12%3
270
May1st to 15thDay
23%18%20%20%12%4
288
16th to endDay
19%16%27%18%13%5%
331
June1st to 15thDay
23%14%20%28%10%2
315
16th to endDay
17%13%19%33%15%2
315
July1st to 15thDay
13%9%22%29%18%7%
305
16th to endDay
13%10%15%32%21%6%
326
August1st to 15thDay
13%8%15%31%21%8%
313
16th to endDay
10%7%19%27%22%13%
332
September1st to 15thDay
15%11%16%22%18%15%
315
16th to endDay
13%11%18%20%24%11%
314
October1st to 15thDay
18%12%21%23%19%4
315
16th to endDay
23%15%27%23%10%
300
November1st to 15thDay
21%18%21%23%13%1
280
16th to endDay
28%8%30%22%10%
90
December1st to 15thDay
25%5%33%26%7%1
90
16th to endDay
41%6%34%13%2
72
January1st to 15thNight
19%25%24%24%4
720
16th to endNight
18%23%28%21%7%
768
February1st to 15thNight
18%23%28%24%5%
704
16th to endNight
19%22%26%24%6%
631
March1st to 15thNight
24%23%23%24%5%
720
16th to endNight
20%20%21%27%9%1
768
April1st to 15thNight
22%22%22%26%5%
718
16th to endNight
20%23%24%21%9%
638
May1st to 15thNight
21%20%20%25%11%1
380
16th to endNight
14%19%26%29%9%
375
June1st to 15thNight
22%20%19%30%8%
360
16th to endNight
14%20%19%29%13%1
360
July1st to 15thNight
10%14%19%33%20%1
340
16th to endNight
11%17%17%35%15%1
376
August1st to 15thNight
13%10%12%38%23%1
376
16th to endNight
12%10%19%31%25%1
756
September1st to 15thNight
15%10%17%35%18%1
720
16th to endNight
17%12%15%27%24%2
717
October1st to 15thNight
18%14%24%27%13%
720
16th to endNight
24%14%27%26%6%
768
November1st to 15thNight
21%20%24%22%10%
760
16th to endNight
22%24%26%16%10%
780
December1st to 15thNight
29%23%22%20%4
780
16th to endNight
29%23%24%16%41
788
Page generated at 2017-12-21 07:04:53. Version ac04

Copyright 2017 A. Danko


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