Watson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Watson Observatory clear sky chart from 2005-11-05 to 2017-09-22.

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
56%
15333
24%
47020
20%
15290
62%
15290
32%
28277

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
36%
720
25%
2327
15%
716
49%
716
26%
1429
January16th to endNight
35%
768
26%
2346
16%
768
46%
768
25%
1515
February1st to 15thNight
40%
704
25%
2112
14%
701
51%
701
29%
1330
February16th to endNight
38%
631
25%
1893
15%
631
46%
631
28%
1188
March1st to 15thNight
39%
720
22%
2130
14%
720
45%
720
29%
1338
March16th to endNight
47%
748
23%
2092
16%
748
53%
748
34%
1280
April1st to 15thNight
50%
539
26%
1797
23%
539
58%
539
34%
1002
April16th to endNight
57%
540
28%
1791
23%
540
65%
540
36%
976
May1st to 15thNight
61%
540
23%
1620
18%
540
66%
540
33%
984
May16th to endNight
73%
561
22%
1682
19%
561
75%
561
37%
976
June1st to 15thNight
76%
540
16%
1620
15%
540
76%
540
34%
965
June16th to endNight
83%
540
19%
1620
17%
540
84%
540
38%
921
July1st to 15thNight
78%
510
14%
1530
15%
510
79%
510
35%
917
July16th to endNight
74%
564
16%
1595
15%
531
75%
531
31%
998
August1st to 15thNight
78%
539
19%
1777
23%
536
78%
536
33%
1027
August16th to endNight
70%
573
20%
1910
24%
573
70%
573
26%
1208
September1st to 15thNight
69%
639
20%
1899
19%
639
69%
639
29%
1200
September16th to endNight
70%
685
28%
1892
24%
685
72%
685
36%
1134
October1st to 15thNight
66%
660
34%
1815
29%
660
70%
660
43%
1037
October16th to endNight
57%
704
33%
2120
29%
704
63%
704
38%
1196
November1st to 15thNight
58%
700
31%
2276
29%
700
64%
700
38%
1216
November16th to endNight
51%
720
27%
2340
24%
720
59%
720
33%
1324
December1st to 15thNight
44%
720
23%
2340
19%
720
52%
720
26%
1448
December16th to endNight
38%
768
21%
2496
19%
768
44%
768
22%
1668

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
12%5%25%5%35%5%5%7%41%
47855
Night
11%42442445%7%46%
47219
TransparencyDay
40%5%11%20%18%4
47556
Night
36%410%21%20%6%
47022
SeeingDay
13%313%22%26%21%
5237
Night
13%318%32%26%5%
15335

By Month

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
17%8%34324345%42%
1611
16th to endDay
17%6%34424345%45%
1875
February1st to 15thDay
16%7%36%423345%42%
1750
16th to endDay
18%6%25%5%2433442%
1580
March1st to 15thDay
16%7%35%424346%40%
1828
16th to endDay
14%6%25%5%245%46%41%
2141
April1st to 15thDay
13%5%25%5%2445%6%44%
2151
16th to endDay
9%4245%35%5%5%6%46%
2169
May1st to 15thDay
10%5%25%4245%5%8%43%
2331
16th to endDay
8%426%6%35%5%6%8%42%
2431
June1st to 15thDay
8%5%26%6%37%7%8%9%32%
2340
16th to endDay
5%2245%37%6%9%12%40%
2340
July1st to 15thDay
8%426%7%47%7%9%10%30%
2340
16th to endDay
6%426%6%49%9%8%11%30%
2465
August1st to 15thDay
7%316%6%47%8%9%13%32%
2169
16th to endDay
9%5%26%7%47%7%7%10%29%
2292
September1st to 15thDay
10%5%26%6%46%6%6%7%36%
2047
16th to endDay
11%5%25%425%5%6%6%44%
1881
October1st to 15thDay
11%42442345%6%50%
1815
16th to endDay
11%41442223358%
1749
November1st to 15thDay
12%6%23323235%54%
1582
16th to endDay
14%5%1332335%5%49%
1620
December1st to 15thDay
18%7%24434435%41%
1620
16th to endDay
21%8%5%6%5%2432436%
1728
January1st to 15thNight
16%6%2433335%5%44%
2327
16th to endNight
17%7%3442445%441%
2346
February1st to 15thNight
15%6%24424435%46%
2103
16th to endNight
16%5%24324335%46%
1893
March1st to 15thNight
15%6%34324347%43%
2130
16th to endNight
11%5%245%23348%47%
2092
April1st to 15thNight
11%42342445%8%49%
1797
16th to endNight
8%42442345%6%53%
1791
May1st to 15thNight
9%325%5%35%449%46%
1619
16th to endNight
5%414425%47%9%48%
1682
June1st to 15thNight
42246%37%6%7%12%41%
1620
16th to endNight
3214436%7%8%11%47%
1620
July1st to 15thNight
5%3245%35%7%8%11%41%
1620
16th to endNight
5%325%6%46%6%7%12%39%
1703
August1st to 15thNight
4315%5%45%5%8%11%43%
1788
16th to endNight
9%5%346%35%5%7%9%38%
1910
September1st to 15thNight
9%4246%35%6%7%7%41%
1899
16th to endNight
8%425%42436%7%49%
1892
October1st to 15thNight
8%424333336%56%
1815
16th to endNight
10%42341333358%
2120
November1st to 15thNight
10%323313336%57%
2276
16th to endNight
12%5%24324336%51%
2340
December1st to 15thNight
15%6%35%5%24335%44%
2340
16th to endNight
21%7%25%5%23345%37%
2496

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
44%27%16%24%4
1612
16th to endDay
42%25%21%21%6%
1875
February1st to 15thDay
45%26%23%19%3
1752
16th to endDay
45%2416%24%5%
1580
March1st to 15thDay
45%39%16%19%5%
1828
16th to endDay
41%311%20%18%3
2142
April1st to 15thDay
39%39%20%19%7%
2151
16th to endDay
35%39%23%23%3
2169
May1st to 15thDay
37%5%12%20%19%4
2331
16th to endDay
37%414%25%14%3
2435
June1st to 15thDay
41%7%17%20%12%1
2340
16th to endDay
31%8%16%24%15%3
2340
July1st to 15thDay
40%9%19%19%9%1
2201
16th to endDay
40%9%19%21%9%
2310
August1st to 15thDay
37%7%20%21%10%2
2156
16th to endDay
44%8%14%19%10%2
2292
September1st to 15thDay
43%6%13%18%12%5%
2047
16th to endDay
38%5%11%20%19%4
1881
October1st to 15thDay
33%37%21%24%9%
1815
16th to endDay
31%2415%34%10%
1749
November1st to 15thDay
34%2417%31%9%
1582
16th to endDay
35%26%19%27%8%
1620
December1st to 15thDay
45%38%20%16%5%
1620
16th to endDay
54%26%18%12%4
1728
January1st to 15thNight
41%26%19%24%5%
2327
16th to endNight
44%2419%20%7%
2346
February1st to 15thNight
41%25%21%20%8%
2112
16th to endNight
41%2419%25%6%
1893
March1st to 15thNight
40%29%20%19%7%
2130
16th to endNight
36%39%24%22%4
2092
April1st to 15thNight
32%310%23%19%9%
1797
16th to endNight
30%49%25%22%8%
1791
May1st to 15thNight
34%5%11%18%25%3
1620
16th to endNight
29%7%15%27%15%5%
1684
June1st to 15thNight
31%7%19%26%14%1
1620
16th to endNight
26%8%20%23%18%3
1620
July1st to 15thNight
29%8%25%23%9%3
1530
16th to endNight
33%9%23%20%12%1
1595
August1st to 15thNight
31%7%20%24%13%3
1777
16th to endNight
39%7%17%20%13%2
1910
September1st to 15thNight
37%7%16%18%15%4
1899
16th to endNight
31%5%9%23%21%7%
1892
October1st to 15thNight
29%27%22%24%12%
1815
16th to endNight
31%2313%33%15%
2120
November1st to 15thNight
29%25%19%27%15%
2276
16th to endNight
35%17%18%25%11%
2340
December1st to 15thNight
41%39%18%21%5%
2340
16th to endNight
48%26%19%16%5%
2496

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
23%8%28%24%11%3
90
16th to endDay
17%11%48%13%6%2
96
February1st to 15thDay
14%16%36%28%3
90
16th to endDay
19%9%30%35%3
81
March1st to 15thDay
19%7%36%26%7%2
118
16th to endDay
17%326%32%19%1
288
April1st to 15thDay
16%8%21%25%23%4
270
16th to endDay
12%121%32%25%6%
270
May1st to 15thDay
11%5%17%27%21%16%
288
16th to endDay
10%210%29%24%22%
331
June1st to 15thDay
12%414%30%37%
315
16th to endDay
8%5%16%25%43%
315
July1st to 15thDay
8%113%28%47%
305
16th to endDay
8%118%30%50%
326
August1st to 15thDay
9%1118%34%35%
313
16th to endDay
13%17%37%29%
332
September1st to 15thDay
11%26%14%38%27%
315
16th to endDay
11%27%23%38%16%
278
October1st to 15thDay
14%10%29%36%8%
255
16th to endDay
16%212%29%30%8%
195
November1st to 15thDay
18%5%20%24%23%7%
90
16th to endDay
11%6%28%31%16%5%
90
December1st to 15thDay
26%432%24%11%1
90
16th to endDay
23%5%33%26%9%2
96
January1st to 15thNight
21%6%31%33%6%
720
16th to endNight
19%7%33%34%5%
768
February1st to 15thNight
18%5%29%40%6%
704
16th to endNight
18%7%32%36%4
631
March1st to 15thNight
19%6%30%34%8%
720
16th to endNight
14%6%25%35%17%1
748
April1st to 15thNight
12%328%34%19%1
539
16th to endNight
10%321%42%20%1
540
May1st to 15thNight
9%317%35%29%4
540
16th to endNight
9%7%37%39%6%
563
June1st to 15thNight
5%2330%47%11%
540
16th to endNight
21223%52%17%
540
July1st to 15thNight
5%1120%52%17%
510
16th to endNight
6%1317%50%20%
564
August1st to 15thNight
6%1118%54%17%
539
16th to endNight
13%1220%50%12%
573
September1st to 15thNight
10%15%23%47%11%
639
16th to endNight
10%25%32%43%5%
685
October1st to 15thNight
10%114%38%30%4
660
16th to endNight
13%219%34%25%4
704
November1st to 15thNight
12%220%35%25%3
700
16th to endNight
15%424%38%16%1
720
December1st to 15thNight
18%6%23%38%12%1
720
16th to endNight
24%425%35%8%
768
Page generated at 2017-09-23 05:43:44. Version ac04

Copyright 2017 A. Danko


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