North Bay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the North Bay clear sky chart from 2005-11-06 to 2024-03-28.

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%
23345
11%
81216
8%
23263
36%
23263
13%
61168

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 10thNight
18%
941
4%
4744
4%
926
20%
926
4%
3989
January11th to 20thNight
23%
859
7%
4565
5%
847
26%
847
5%
3643
January21st to endNight
31%
830
6%
4525
9%
814
35%
814
6%
3780
February1st to 10thNight
23%
760
8%
2470
5%
760
28%
760
10%
1948
February11th to 20thNight
23%
741
10%
2405
5%
740
30%
740
11%
1877
February21st to endNight
24%
628
10%
2041
7%
628
29%
628
11%
1605
March1st to 10thNight
25%
760
15%
2100
7%
760
32%
760
18%
1564
March11th to 20thNight
27%
760
15%
2090
7%
760
34%
760
17%
1577
March21st to endNight
30%
821
16%
2256
8%
821
36%
821
22%
1608
April1st to 10thNight
31%
720
13%
1816
7%
720
34%
720
19%
1404
April11th to 20thNight
32%
605
17%
1669
8%
605
37%
605
21%
1198
April21st to endNight
33%
540
14%
1620
9%
540
38%
540
18%
1190
May1st to 10thNight
40%
460
17%
1540
15%
460
46%
460
21%
1022
May11th to 20thNight
44%
354
18%
1416
19%
354
53%
354
19%
893
May21st to endNight
47%
388
12%
1551
15%
388
54%
388
22%
938
June1st to 10thNight
42%
360
9%
1321
11%
360
46%
360
17%
897
June11th to 20thNight
46%
360
15%
1260
15%
360
52%
360
25%
767
June21st to endNight
48%
360
9%
1260
9%
360
51%
360
20%
824
July1st to 10thNight
56%
350
15%
1225
16%
350
60%
350
30%
700
July11th to 20thNight
54%
348
16%
1245
17%
348
61%
348
26%
748
July21st to endNight
57%
431
13%
1521
17%
407
58%
407
23%
929
August1st to 10thNight
51%
537
13%
1600
12%
534
53%
534
24%
1076
August11th to 20thNight
59%
534
16%
1598
12%
534
62%
534
26%
1036
August21st to endNight
45%
634
15%
1822
8%
634
48%
634
24%
1232
September1st to 10thNight
50%
720
17%
1800
9%
720
52%
720
32%
1187
September11th to 20thNight
47%
720
18%
1885
12%
720
48%
720
29%
1265
September21st to endNight
44%
720
18%
2024
13%
720
48%
720
25%
1366
October1st to 10thNight
36%
720
15%
2160
12%
720
41%
720
18%
1569
October11th to 20thNight
24%
720
11%
2160
8%
720
29%
720
13%
1689
October21st to endNight
20%
792
10%
2398
6%
792
26%
792
10%
1939
November1st to 10thNight
25%
740
11%
2590
9%
740
31%
740
13%
1892
November11th to 20thNight
17%
760
10%
2660
6%
760
23%
760
9%
2113
November21st to endNight
18%
760
5%
2660
4%
760
20%
760
7%
2236
December1st to 10thNight
15%
760
6%
2660
2%
760
19%
760
5%
2286
December11th to 20thNight
18%
816
8%
3370
4%
815
24%
815
7%
2735
December21st to endNight
16%
1036
4%
5189
2%
1026
19%
1026
4%
4446

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%11%48%6%3445%7%30%
78531
Night
18%13%5%8%6%34446%27%
81472
TransparencyDay
54%6%12%19%8%1
78228
Night
59%6%11%16%8%1
81309
SeeingDay
21%9%27%28%12%3
14156
Night
23%8%28%29%11%1
23380

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 10thDay
17%24%11%9%6%4333416%
2208
11th to 20thDay
11%15%8%15%11%44447%17%
2370
21st to endDay
17%21%8%10%7%44335%17%
3124
February1st to 10thDay
11%18%8%12%8%45%5%5%6%19%
1710
11th to 20thDay
13%16%7%10%8%4445%6%24%
1663
21st to endDay
16%15%6%10%7%46%44423%
1413
March1st to 10thDay
15%13%5%10%6%34446%29%
2075
11th to 20thDay
14%11%5%10%10%35%5%46%28%
2090
21st to endDay
17%9%48%5%3445%5%37%
2255
April1st to 10thDay
20%11%5%8%5%4446%6%27%
2143
11th to 20thDay
20%10%36%5%3435%6%35%
2255
21st to endDay
22%11%36%5%24445%35%
2340
May1st to 10thDay
20%8%45%6%345%47%34%
2418
11th to 20thDay
18%9%36%6%45%45%7%34%
2478
21st to endDay
15%6%35%5%35%46%8%40%
2725
June1st to 10thDay
18%8%37%5%45%46%7%34%
2636
11th to 20thDay
15%6%35%6%35%5%6%9%38%
2700
21st to endDay
17%7%37%6%345%6%9%32%
2700
July1st to 10thDay
13%425%5%45%5%5%9%43%
2700
11th to 20thDay
14%5%36%5%35%5%6%9%38%
2669
21st to endDay
13%5%36%5%35%5%7%9%38%
2704
August1st to 10thDay
14%6%25%6%345%7%9%38%
2327
11th to 20thDay
14%5%25%5%445%7%8%39%
2303
21st to endDay
15%6%36%5%445%6%9%36%
2516
September1st to 10thDay
17%7%35%5%3445%8%39%
2160
11th to 20thDay
17%6%26%6%3445%8%40%
2070
21st to endDay
23%7%36%6%3445%6%33%
1936
October1st to 10thDay
19%9%26%6%45%5%5%5%34%
1800
11th to 20thDay
27%13%38%6%34446%22%
1800
21st to endDay
29%14%5%8%6%3345%5%20%
1954
November1st to 10thDay
22%12%47%5%345%5%7%26%
1487
11th to 20thDay
21%15%6%9%8%345%46%19%
1520
21st to endDay
27%21%6%10%6%33335%14%
1520
December1st to 10thDay
19%22%7%11%7%345%35%13%
1520
11th to 20thDay
19%18%6%10%6%34345%22%
1811
21st to endDay
19%22%8%14%7%3333414%
2431
January1st to 10thNight
16%20%8%13%8%4444414%
4748
11th to 20thNight
13%16%6%14%10%45%5%5%5%17%
4589
21st to endNight
14%24%9%11%7%4443415%
4532
February1st to 10thNight
12%15%7%13%9%45%5%5%5%21%
2470
11th to 20thNight
13%17%8%10%8%45%44424%
2403
21st to endNight
15%18%6%10%9%44435%22%
2041
March1st to 10thNight
14%13%6%9%7%44345%31%
2090
11th to 20thNight
16%14%5%6%7%345%5%5%30%
2089
21st to endNight
16%12%47%5%34436%36%
2256
April1st to 10thNight
21%13%6%6%6%24445%30%
1816
11th to 20thNight
20%10%46%5%35%345%35%
1669
21st to endNight
22%10%36%6%35%446%31%
1620
May1st to 10thNight
17%9%37%5%33446%38%
1540
11th to 20thNight
17%6%25%5%5%5%46%8%37%
1415
21st to endNight
15%6%36%5%34449%41%
1550
June1st to 10thNight
13%8%46%7%46%46%6%35%
1321
11th to 20thNight
16%6%25%5%2345%9%43%
1260
21st to endNight
16%6%27%6%345%6%8%36%
1260
July1st to 10thNight
11%4244346%7%10%45%
1260
11th to 20thNight
13%5%26%5%345%5%8%44%
1287
21st to endNight
11%6%35%6%3447%8%44%
1611
August1st to 10thNight
14%7%346%35%5%6%6%41%
1611
11th to 20thNight
12%5%25%5%45%6%6%6%44%
1598
21st to endNight
17%7%35%6%3445%8%39%
1822
September1st to 10thNight
16%5%25%5%33447%47%
1800
11th to 20thNight
16%6%25%43445%7%43%
1885
21st to endNight
19%7%35%5%2345%8%39%
2024
October1st to 10thNight
21%8%37%5%445%5%6%33%
2160
11th to 20thNight
28%11%36%6%34345%26%
2160
21st to endNight
28%13%47%6%3435%5%22%
2398
November1st to 10thNight
22%12%46%6%34445%30%
2590
11th to 20thNight
19%15%5%9%7%45%5%45%22%
2660
21st to endNight
26%18%5%7%6%45%445%17%
2660
December1st to 10thNight
21%21%8%11%6%3444414%
2660
11th to 20thNight
21%19%7%9%7%34445%19%
3381
21st to endNight
21%21%7%14%8%3333414%
5236

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 10thDay
72%5%10%10%2
2209
11th to 20thDay
68%412%10%5%1
2370
21st to endDay
71%49%10%4
3124
February1st to 10thDay
65%6%10%13%5%1
1710
11th to 20thDay
62%5%11%14%8%1
1672
21st to endDay
64%48%15%8%1
1413
March1st to 10thDay
55%414%16%9%1
2078
11th to 20thDay
56%5%10%19%9%1
2090
21st to endDay
49%5%10%18%14%3
2255
April1st to 10thDay
56%6%11%17%9%1
2143
11th to 20thDay
51%49%23%12%1
2255
21st to endDay
51%5%9%18%13%3
2340
May1st to 10thDay
50%5%9%19%14%3
2418
11th to 20thDay
49%6%9%20%13%2
2482
21st to endDay
42%7%15%21%13%3
2725
June1st to 10thDay
49%6%10%23%11%1
2636
11th to 20thDay
41%9%12%24%13%1
2700
21st to endDay
46%9%13%20%10%1
2700
July1st to 10thDay
35%8%16%29%12%1
2626
11th to 20thDay
40%6%15%29%8%1
2588
21st to endDay
41%8%15%27%9%
2553
August1st to 10thDay
40%9%16%25%9%
2314
11th to 20thDay
40%9%18%24%7%1
2303
21st to endDay
42%8%17%25%8%
2516
September1st to 10thDay
43%8%14%24%11%
2160
11th to 20thDay
43%6%15%28%8%
2070
21st to endDay
52%6%13%24%4
1936
October1st to 10thDay
51%8%12%19%9%1
1800
11th to 20thDay
63%7%14%12%5%
1800
21st to endDay
68%6%10%10%5%
1954
November1st to 10thDay
58%7%14%17%5%
1487
11th to 20thDay
65%8%11%10%5%1
1520
21st to endDay
76%5%9%7%3
1520
December1st to 10thDay
74%6%8%10%2
1520
11th to 20thDay
67%7%10%13%3
1810
21st to endDay
76%47%12%2
2431
January1st to 10thNight
74%6%11%7%3
4748
11th to 20thNight
68%7%11%11%3
4589
21st to endNight
73%5%7%9%5%1
4532
February1st to 10thNight
64%6%10%14%5%1
2470
11th to 20thNight
64%5%10%15%5%1
2405
21st to endNight
65%49%14%7%1
2041
March1st to 10thNight
56%412%14%12%2
2100
11th to 20thNight
56%6%11%16%10%2
2090
21st to endNight
50%5%10%18%14%3
2256
April1st to 10thNight
58%48%17%11%2
1816
11th to 20thNight
53%37%21%14%2
1669
21st to endNight
55%5%9%14%14%3
1620
May1st to 10thNight
48%49%22%14%2
1540
11th to 20thNight
44%7%10%21%15%3
1416
21st to endNight
40%7%14%19%17%3
1551
June1st to 10thNight
48%8%10%21%10%3
1321
11th to 20thNight
38%6%12%26%17%1
1260
21st to endNight
43%8%15%20%11%1
1260
July1st to 10thNight
31%9%16%27%16%2
1225
11th to 20thNight
38%6%14%28%13%1
1245
21st to endNight
37%10%16%28%10%
1521
August1st to 10thNight
41%9%16%23%10%
1600
11th to 20thNight
37%10%16%25%11%1
1598
21st to endNight
44%7%15%23%10%1
1822
September1st to 10thNight
37%7%15%28%12%2
1800
11th to 20thNight
41%6%16%28%9%
1885
21st to endNight
44%5%16%28%7%
2024
October1st to 10thNight
52%6%11%20%10%
2160
11th to 20thNight
62%5%11%17%5%
2160
21st to endNight
66%5%10%11%7%
2398
November1st to 10thNight
56%6%14%18%6%
2590
11th to 20thNight
65%6%12%11%5%1
2660
21st to endNight
71%7%11%9%3
2660
December1st to 10thNight
75%6%8%9%2
2660
11th to 20thNight
68%6%8%13%5%
3381
21st to endNight
77%46%10%3
5236

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 10thDay
27%16%27%20%9%1
180
11th to 20thDay
18%13%32%26%9%2
189
21st to endDay
26%9%27%26%9%2
297
February1st to 10thDay
26%14%38%16%5%1
270
11th to 20thDay
23%13%35%26%3
270
21st to endDay
32%15%33%21%
225
March1st to 10thDay
21%12%35%27%5%
449
11th to 20thDay
17%12%35%28%7%1
460
21st to endDay
22%11%29%29%6%2
490
April1st to 10thDay
22%9%34%24%9%2
420
11th to 20thDay
27%12%24%22%11%5%
412
21st to endDay
27%10%27%27%8%2
420
May1st to 10thDay
26%5%25%29%13%1
423
11th to 20thDay
21%8%25%26%17%3
447
21st to endDay
17%7%28%28%17%3
485
June1st to 10thDay
22%5%28%33%11%2
566
11th to 20thDay
15%7%27%32%14%5%
630
21st to endDay
17%8%23%35%13%4
630
July1st to 10thDay
11%421%41%19%3
621
11th to 20thDay
16%422%31%22%4
587
21st to endDay
12%5%21%37%21%5%
488
August1st to 10thDay
17%419%34%22%5%
446
11th to 20thDay
17%413%32%26%8%
439
21st to endDay
17%8%21%30%15%8%
495
September1st to 10thDay
18%7%24%28%18%6%
450
11th to 20thDay
17%5%23%31%19%5%
450
21st to endDay
25%8%21%25%16%4
450
October1st to 10thDay
23%10%26%25%14%2
450
11th to 20thDay
27%15%31%21%5%1
450
21st to endDay
33%13%29%22%31
469
November1st to 10thDay
28%18%32%18%41
180
11th to 20thDay
23%21%35%16%5%1
180
21st to endDay
36%15%31%16%21
180
December1st to 10thDay
26%16%30%26%2
180
11th to 20thDay
28%12%38%18%31
180
21st to endDay
29%15%33%17%5%1
198
January1st to 10thNight
28%7%29%28%7%
950
11th to 20thNight
29%8%26%30%8%
867
21st to endNight
19%6%28%33%11%2
836
February1st to 10thNight
17%10%36%30%6%
760
11th to 20thNight
21%10%32%29%7%1
743
21st to endNight
23%11%29%33%4
628
March1st to 10thNight
21%8%35%30%6%
760
11th to 20thNight
21%10%34%27%7%1
760
21st to endNight
21%7%33%32%7%1
821
April1st to 10thNight
28%8%25%33%6%
720
11th to 20thNight
24%9%29%27%10%1
605
21st to endNight
24%7%26%31%10%1
540
May1st to 10thNight
22%427%33%12%2
460
11th to 20thNight
19%429%35%12%1
354
21st to endNight
18%327%38%13%1
389
June1st to 10thNight
15%430%36%15%
360
11th to 20thNight
18%423%37%16%1
360
21st to endNight
17%5%20%35%22%1
360
July1st to 10thNight
11%323%47%15%1
350
11th to 20thNight
12%322%39%23%1
348
21st to endNight
11%420%43%21%1
431
August1st to 10thNight
17%319%36%23%2
537
11th to 20thNight
14%217%38%26%3
534
21st to endNight
21%6%21%31%18%4
634
September1st to 10thNight
18%5%21%32%22%2
720
11th to 20thNight
18%6%23%29%20%3
720
21st to endNight
22%6%23%29%18%3
720
October1st to 10thNight
24%7%26%30%11%1
720
11th to 20thNight
33%12%25%23%6%1
720
21st to endNight
34%12%28%22%4
792
November1st to 10thNight
27%10%31%24%7%1
740
11th to 20thNight
27%14%33%19%7%
760
21st to endNight
35%10%28%22%5%
760
December1st to 10thNight
31%13%31%19%6%
760
11th to 20thNight
28%9%34%22%5%2
816
21st to endNight
31%9%29%25%6%1
1045
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