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 2018-10-08.

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%
17365
28%
51036
15%
17322
59%
17322
40%
26906

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
28%
974
29%
2716
10%
970
45%
970
39%
1387
January16th to endNight
28%
874
34%
2746
13%
874
51%
874
36%
1347
February1st to 15thNight
21%
758
25%
2457
7%
757
43%
757
29%
1300
February16th to endNight
25%
684
30%
2050
7%
684
49%
684
33%
1107
March1st to 15thNight
36%
780
30%
2278
11%
780
53%
780
44%
1157
March16th to endNight
38%
744
27%
2200
11%
744
53%
744
38%
1203
April1st to 15thNight
37%
584
24%
1948
12%
584
54%
584
35%
1039
April16th to endNight
40%
579
23%
1929
14%
579
55%
579
31%
1085
May1st to 15thNight
45%
585
20%
1716
13%
585
53%
585
29%
1078
May16th to endNight
61%
611
18%
1626
12%
609
66%
609
41%
950
June1st to 15thNight
68%
585
24%
1560
17%
585
72%
585
52%
817
June16th to endNight
78%
585
28%
1560
22%
585
81%
585
59%
779
July1st to 15thNight
78%
555
20%
1480
16%
555
80%
555
43%
840
July16th to endNight
81%
612
26%
1574
24%
579
81%
579
48%
846
August1st to 15thNight
81%
575
22%
1760
23%
572
82%
572
48%
864
August16th to endNight
78%
621
28%
2070
32%
621
79%
621
43%
1022
September1st to 15thNight
70%
585
30%
1950
29%
585
76%
585
42%
976
September16th to endNight
70%
587
35%
2088
33%
587
78%
587
48%
926
October1st to 15thNight
49%
752
28%
2256
15%
752
59%
752
41%
1228
October16th to endNight
43%
795
34%
2331
15%
795
57%
795
42%
1215
November1st to 15thNight
41%
950
32%
2470
13%
950
53%
950
48%
1310
November16th to endNight
36%
975
35%
2629
12%
975
53%
975
50%
1330
December1st to 15thNight
28%
975
25%
2730
7%
975
44%
975
36%
1450
December16th to endNight
29%
1040
27%
2912
8%
1040
45%
1040
33%
1650

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%
52113
Night
441332335%7%59%
51211
TransparencyDay
27%5%14%26%19%6%
51790
Night
23%413%28%22%7%
51036
SeeingDay
9%8%32%29%15%4
7283
Night
8%9%30%32%16%3
17372

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
5%3432335%8%59%
1552
16th to endDay
43342335%7%60%
1840
February1st to 15thDay
5%25%424448%57%
1734
16th to endDay
5%2442335%7%59%
1710
March1st to 15thDay
242443345%8%55%
2008
16th to endDay
5%6%2432445%10%48%
2376
April1st to 15thDay
6%5%3232446%10%51%
2330
16th to endDay
10%9%24435%5%6%7%40%
2306
May1st to 15thDay
11%7%335%36%6%6%8%37%
2562
16th to endDay
10%5%245%35%6%7%10%39%
2840
June1st to 15thDay
4213326%6%8%10%49%
2730
16th to endDay
33134346%6%8%53%
2730
July1st to 15thDay
6%4135%36%7%7%9%44%
2730
16th to endDay
42145%37%7%8%10%44%
2844
August1st to 15thDay
5%4245%37%6%8%10%41%
2444
16th to endDay
5%314425%5%7%9%48%
2484
September1st to 15thDay
7%31332347%8%54%
2340
16th to endDay
5%323322346%61%
2198
October1st to 15thDay
6%41332345%7%56%
1873
16th to endDay
35%12312346%64%
1879
November1st to 15thDay
131332336%7%61%
1718
16th to endDay
132321336%9%60%
1661
December1st to 15thDay
15%25%22446%8%56%
1560
16th to endDay
25%14314448%58%
1664
January1st to 15thNight
324313348%63%
2716
16th to endNight
1423323346%63%
2746
February1st to 15thNight
6%2442345%7%58%
2447
16th to endNight
7%25%32345%7%58%
2050
March1st to 15thNight
1423223348%62%
2278
16th to endNight
46%23323347%58%
2200
April1st to 15thNight
46%2332436%6%56%
1948
16th to endNight
12%6%1332235%7%51%
1929
May1st to 15thNight
14%6%2332445%7%44%
1715
16th to endNight
8%6%1331445%7%53%
1625
June1st to 15thNight
421222345%8%61%
1560
16th to endNight
31121336%9%66%
1560
July1st to 15thNight
42123245%7%11%52%
1560
16th to endNight
321333446%9%57%
1672
August1st to 15thNight
321232446%10%57%
1769
16th to endNight
5%21342445%7%58%
2070
September1st to 15thNight
7%31342345%6%58%
1950
16th to endNight
5%21312346%66%
2088
October1st to 15thNight
7%412222346%59%
2256
16th to endNight
45%13323246%61%
2331
November1st to 15thNight
2314312348%64%
2470
16th to endNight
131321335%7%65%
2629
December1st to 15thNight
2424323248%60%
2730
16th to endNight
15%25%32345%9%55%
2912

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
24%318%32%16%4
1553
16th to endDay
22%312%36%20%5%
1840
February1st to 15thDay
25%422%32%10%3
1735
16th to endDay
24%212%33%23%3
1710
March1st to 15thDay
25%313%30%22%4
2008
16th to endDay
29%5%15%30%15%4
2376
April1st to 15thDay
27%5%14%29%16%7%
2330
16th to endDay
39%414%25%14%2
2306
May1st to 15thDay
40%6%15%22%12%2
2562
16th to endDay
36%7%19%20%12%4
2844
June1st to 15thDay
25%8%16%18%19%10%
2730
16th to endDay
23%6%10%23%25%10%
2730
July1st to 15thDay
30%6%13%19%17%11%
2581
16th to endDay
29%6%15%27%13%6%
2679
August1st to 15thDay
31%7%16%19%15%8%
2429
16th to endDay
26%6%14%24%21%6%
2484
September1st to 15thDay
25%411%23%21%12%
2340
16th to endDay
23%28%25%32%6%
2198
October1st to 15thDay
25%5%12%22%26%6%
1873
16th to endDay
20%212%29%27%7%
1879
November1st to 15thDay
20%414%28%24%8%
1718
16th to endDay
19%310%32%26%7%
1661
December1st to 15thDay
24%417%32%17%2
1560
16th to endDay
24%416%29%20%3
1664
January1st to 15thNight
19%417%32%21%4
2716
16th to endNight
21%310%33%24%6%
2746
February1st to 15thNight
26%5%16%33%14%3
2457
16th to endNight
25%49%28%24%6%
2050
March1st to 15thNight
20%411%31%27%4
2278
16th to endNight
25%412%31%21%3
2200
April1st to 15thNight
27%5%12%28%19%6%
1948
16th to endNight
33%411%28%19%3
1929
May1st to 15thNight
37%6%14%23%15%2
1716
16th to endNight
27%5%21%25%14%5%
1626
June1st to 15thNight
20%416%27%17%14%
1560
16th to endNight
14%5%10%24%30%14%
1560
July1st to 15thNight
24%7%16%23%18%9%
1480
16th to endNight
21%5%16%29%17%8%
1574
August1st to 15thNight
20%420%26%18%9%
1760
16th to endNight
23%413%26%24%8%
2070
September1st to 15thNight
24%414%22%18%16%
1950
16th to endNight
18%39%27%30%9%
2088
October1st to 15thNight
25%5%11%21%26%9%
2256
16th to endNight
24%38%28%27%8%
2331
November1st to 15thNight
18%410%29%25%10%
2470
16th to endNight
17%310%31%28%7%
2629
December1st to 15thNight
23%316%31%19%5%
2730
16th to endNight
26%5%16%30%18%2
2912

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%6%37%33%10%5%
135
16th to endDay
210%42%33%11%
144
February1st to 15thDay
9%22%43%22%1
135
16th to endDay
6%13%46%28%4
120
March1st to 15thDay
7%16%44%27%31
193
16th to endDay
13%19%44%19%2
352
April1st to 15thDay
13%20%42%17%41
329
16th to endDay
20%17%32%20%6%1
318
May1st to 15thDay
16%7%33%27%12%2
491
16th to endDay
17%6%26%26%17%4
597
June1st to 15thDay
5%330%31%22%7%
570
16th to endDay
5%331%31%20%7%
570
July1st to 15thDay
9%217%32%25%11%
550
16th to endDay
6%316%32%33%7%
559
August1st to 15thDay
6%432%36%14%4
359
16th to endDay
7%327%38%19%3
396
September1st to 15thDay
9%428%34%17%5%
375
16th to endDay
8%332%35%15%5%
240
October1st to 15thDay
10%8%31%28%14%7%
157
16th to endDay
7%10%35%32%9%4
144
November1st to 15thDay
45%31%34%17%6%
135
16th to endDay
11%9%47%31%
135
December1st to 15thDay
6%18%45%25%2
135
16th to endDay
29%53%30%3
144
January1st to 15thNight
8%18%39%22%8%2
975
16th to endNight
8%17%41%23%7%1
874
February1st to 15thNight
7%17%46%22%6%
764
16th to endNight
7%15%46%25%4
684
March1st to 15thNight
5%11%43%29%9%1
780
16th to endNight
9%10%38%34%5%1
744
April1st to 15thNight
10%7%40%34%6%
584
16th to endNight
16%7%31%30%12%2
579
May1st to 15thNight
19%323%35%15%2
585
16th to endNight
11%317%38%25%3
611
June1st to 15thNight
6%218%45%22%4
585
16th to endNight
3112%46%32%2
585
July1st to 15thNight
6%17%33%42%9%
555
16th to endNight
41433%46%8%
612
August1st to 15thNight
5%5%39%44%5%
575
16th to endNight
6%17%37%37%10%
621
September1st to 15thNight
8%111%36%32%8%
585
16th to endNight
6%316%44%21%8%
587
October1st to 15thNight
10%5%29%34%15%4
752
16th to endNight
8%9%33%30%12%4
795
November1st to 15thNight
5%11%38%32%10%2
950
16th to endNight
5%11%44%30%7%
975
December1st to 15thNight
10%15%41%26%6%
975
16th to endNight
7%15%41%25%7%1
1040
Page generated at 2018-10-09 08:39:17. Version ac04

Copyright 2018 A. Danko


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