The Rock Bottom Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Rock Bottom Observatory clear sky chart from 2005-11-05 to 2017-09-17.

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
46%
15957
27%
46871
15%
15914
59%
15914
40%
24735

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
29%
899
29%
2506
11%
895
46%
895
38%
1275
January16th to endNight
27%
807
34%
2535
12%
807
51%
807
34%
1250
February1st to 15thNight
22%
698
25%
2264
7%
697
43%
697
29%
1202
February16th to endNight
25%
632
29%
1894
6%
632
48%
632
33%
1022
March1st to 15thNight
35%
720
28%
2103
10%
720
52%
720
42%
1092
March16th to endNight
38%
687
27%
2031
11%
687
53%
687
38%
1110
April1st to 15thNight
40%
539
23%
1798
13%
539
55%
539
35%
944
April16th to endNight
39%
540
23%
1800
15%
540
54%
540
30%
1010
May1st to 15thNight
45%
540
20%
1584
13%
540
54%
540
29%
991
May16th to endNight
60%
563
18%
1498
12%
561
65%
561
42%
873
June1st to 15thNight
67%
540
22%
1440
16%
540
71%
540
50%
763
June16th to endNight
79%
540
29%
1440
23%
540
82%
540
60%
710
July1st to 15thNight
77%
510
18%
1360
14%
510
79%
510
41%
785
July16th to endNight
81%
564
27%
1444
24%
531
81%
531
48%
777
August1st to 15thNight
80%
539
22%
1650
22%
536
81%
536
47%
816
August16th to endNight
78%
573
29%
1910
33%
573
80%
573
43%
939
September1st to 15thNight
69%
540
28%
1800
27%
540
75%
540
41%
903
September16th to endNight
73%
502
35%
1787
33%
502
81%
502
50%
773
October1st to 15thNight
49%
660
29%
1980
16%
660
59%
660
41%
1075
October16th to endNight
43%
729
33%
2137
14%
729
57%
729
41%
1116
November1st to 15thNight
41%
875
33%
2275
13%
875
54%
875
48%
1203
November16th to endNight
36%
900
35%
2427
13%
900
53%
900
50%
1227
December1st to 15thNight
27%
900
25%
2520
6%
900
43%
900
35%
1350
December16th to endNight
28%
960
26%
2688
7%
960
45%
960
32%
1529

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
5%4244245%6%8%51%
47921
Night
441332335%8%59%
47046
TransparencyDay
27%5%14%26%19%6%
47598
Night
23%413%28%21%7%
46871
SeeingDay
9%8%30%29%16%5%
5872
Night
8%9%30%31%16%3
15964

By Month

Percentage Cloud Cover SW (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
43432335%9%60%
1432
16th to endDay
43342335%7%61%
1698
February1st to 15thDay
5%25%424447%56%
1597
16th to endDay
6%2441335%7%58%
1580
March1st to 15thDay
2425%5%335%5%8%54%
1854
16th to endDay
5%6%243245%6%10%48%
2193
April1st to 15thDay
6%5%2232446%10%51%
2150
16th to endDay
10%8%34435%5%6%7%40%
2160
May1st to 15thDay
10%8%335%36%6%5%8%38%
2364
16th to endDay
10%5%345%35%6%7%10%39%
2616
June1st to 15thDay
5%313326%6%8%11%48%
2520
16th to endDay
2213335%6%7%8%55%
2520
July1st to 15thDay
6%4145%36%7%7%9%43%
2520
16th to endDay
42145%37%7%8%10%44%
2623
August1st to 15thDay
5%4245%37%6%8%10%41%
2301
16th to endDay
5%314425%5%7%9%48%
2292
September1st to 15thDay
7%31332346%8%54%
2160
16th to endDay
4323312447%62%
1876
October1st to 15thDay
6%4133235%5%7%57%
1650
16th to endDay
45%12312346%63%
1723
November1st to 15thDay
241332337%7%61%
1583
16th to endDay
232321336%8%60%
1533
December1st to 15thDay
15%25%23446%8%54%
1440
16th to endDay
26%14414448%56%
1536
January1st to 15thNight
323313349%63%
2506
16th to endNight
1423423346%63%
2535
February1st to 15thNight
6%2442345%6%58%
2254
16th to endNight
7%2432345%7%58%
1894
March1st to 15thNight
142322435%9%59%
2103
16th to endNight
46%23323347%57%
2031
April1st to 15thNight
45%2332436%6%56%
1798
16th to endNight
12%6%1332235%7%51%
1800
May1st to 15thNight
13%6%2332445%7%45%
1583
16th to endNight
8%5%1322445%8%53%
1497
June1st to 15thNight
421222345%9%60%
1440
16th to endNight
11122336%9%67%
1440
July1st to 15thNight
5%2123245%8%12%50%
1440
16th to endNight
321333446%8%56%
1542
August1st to 15thNight
32133245%6%10%56%
1659
16th to endNight
5%21342445%7%58%
1910
September1st to 15thNight
8%31242345%6%58%
1800
16th to endNight
41131235%6%67%
1787
October1st to 15thNight
7%423223346%59%
1980
16th to endNight
5%5%13313246%60%
2137
November1st to 15thNight
2314313448%63%
2275
16th to endNight
1312223347%66%
2427
December1st to 15thNight
15%2432435%8%59%
2520
16th to endNight
15%25%32445%8%55%
2688

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
23%319%33%16%4
1433
16th to endDay
22%311%37%19%5%
1698
February1st to 15thDay
26%421%33%10%3
1598
16th to endDay
24%213%33%22%3
1580
March1st to 15thDay
26%314%30%20%3
1854
16th to endDay
29%5%14%29%16%4
2193
April1st to 15thDay
28%415%28%16%6%
2150
16th to endDay
39%414%25%14%2
2160
May1st to 15thDay
40%6%15%23%12%2
2364
16th to endDay
36%7%19%20%11%4
2620
June1st to 15thDay
26%8%17%19%19%8%
2520
16th to endDay
21%6%11%23%26%10%
2520
July1st to 15thDay
31%6%13%19%15%11%
2371
16th to endDay
29%6%16%26%13%7%
2458
August1st to 15thDay
32%7%15%19%15%8%
2286
16th to endDay
26%6%14%24%21%6%
2292
September1st to 15thDay
26%411%23%22%11%
2160
16th to endDay
21%28%28%33%5%
1876
October1st to 15thDay
24%5%13%23%26%5%
1650
16th to endDay
20%212%29%27%6%
1723
November1st to 15thDay
20%315%27%23%9%
1583
16th to endDay
19%310%32%25%7%
1533
December1st to 15thDay
25%419%32%16%1
1440
16th to endDay
26%417%28%19%4
1536
January1st to 15thNight
19%317%32%22%4
2506
16th to endNight
22%310%33%24%6%
2535
February1st to 15thNight
26%415%34%15%3
2264
16th to endNight
25%410%29%23%6%
1894
March1st to 15thNight
22%411%31%25%4
2103
16th to endNight
25%412%31%21%3
2031
April1st to 15thNight
26%413%28%20%6%
1798
16th to endNight
33%411%28%18%3
1800
May1st to 15thNight
36%6%14%23%16%2
1584
16th to endNight
27%5%22%25%13%5%
1498
June1st to 15thNight
20%417%27%16%12%
1440
16th to endNight
13%5%10%24%31%15%
1440
July1st to 15thNight
25%7%16%22%17%9%
1360
16th to endNight
22%5%15%30%16%9%
1444
August1st to 15thNight
20%420%25%19%9%
1650
16th to endNight
23%412%25%25%8%
1910
September1st to 15thNight
25%414%22%19%15%
1800
16th to endNight
17%310%28%32%7%
1787
October1st to 15thNight
25%5%11%21%27%8%
1980
16th to endNight
24%28%28%27%7%
2137
November1st to 15thNight
19%411%28%25%11%
2275
16th to endNight
17%310%31%28%8%
2427
December1st to 15thNight
23%417%32%17%4
2520
16th to endNight
27%5%16%30%17%2
2688

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
6%7%28%32%15%8%
90
16th to endDay
311%36%37%11%
96
February1st to 15thDay
8%24%44%21%1
90
16th to endDay
9%17%35%29%6%1
81
March1st to 15thDay
8%18%42%26%31
144
16th to endDay
13%20%44%18%2
288
April1st to 15thDay
14%21%39%17%5%1
269
16th to endDay
20%19%33%20%41
270
May1st to 15thDay
15%9%34%27%10%2
420
16th to endDay
18%7%26%25%17%5%
517
June1st to 15thDay
5%329%30%23%7%
495
16th to endDay
3331%31%22%7%
495
July1st to 15thDay
10%318%33%23%10%
475
16th to endDay
6%315%32%34%8%
482
August1st to 15thDay
6%5%33%36%13%4
315
16th to endDay
7%323%40%21%3
332
September1st to 15thDay
10%5%33%30%15%5%
315
16th to endDay
4329%34%21%7%
146
October1st to 15thDay
6%5%33%22%22%10%
90
16th to endDay
9%9%28%37%10%5%
96
November1st to 15thDay
5%324%33%23%10%
90
16th to endDay
15%8%46%28%
90
December1st to 15thDay
8%21%47%21%1
90
16th to endDay
19%50%34%41
96
January1st to 15thNight
9%19%36%22%9%2
900
16th to endNight
8%18%40%23%7%1
807
February1st to 15thNight
7%18%45%22%6%
704
16th to endNight
8%15%46%24%5%
632
March1st to 15thNight
5%11%42%28%10%1
720
16th to endNight
9%10%37%34%6%1
687
April1st to 15thNight
10%7%37%35%7%
539
16th to endNight
15%7%32%30%12%1
540
May1st to 15thNight
17%325%35%15%2
540
16th to endNight
11%317%39%23%3
563
June1st to 15thNight
6%218%46%22%3
540
16th to endNight
2113%45%34%2
540
July1st to 15thNight
6%17%33%40%9%
510
16th to endNight
41432%48%8%
564
August1st to 15thNight
5%5%39%43%5%
539
16th to endNight
6%17%36%37%10%
573
September1st to 15thNight
9%112%37%30%8%
540
16th to endNight
5%314%43%23%9%
502
October1st to 15thNight
10%6%30%32%16%4
660
16th to endNight
8%10%32%30%13%4
729
November1st to 15thNight
5%11%37%31%10%2
875
16th to endNight
6%11%43%30%8%
900
December1st to 15thNight
10%16%40%25%6%
900
16th to endNight
8%16%41%24%8%1
960
Page generated at 2017-09-18 06:38:27. Version ac04

Copyright 2017 A. Danko


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