Bench Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bench 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
54%
15932
17%
46719
13%
15893
58%
15893
30%
29149

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
48%
896
11%
2506
8%
895
50%
895
24%
1730
January16th to endNight
48%
812
18%
2540
13%
812
55%
812
27%
1616
February1st to 15thNight
45%
697
10%
2288
9%
697
49%
697
21%
1553
February16th to endNight
43%
632
11%
2028
9%
632
47%
632
21%
1384
March1st to 15thNight
51%
720
14%
2004
8%
720
57%
720
33%
1266
March16th to endNight
47%
765
12%
2109
8%
765
50%
765
24%
1463
April1st to 15thNight
46%
539
14%
1798
11%
539
51%
539
23%
1159
April16th to endNight
47%
540
14%
1653
12%
540
49%
540
24%
1107
May1st to 15thNight
52%
539
15%
1512
11%
539
54%
539
31%
958
May16th to endNight
56%
563
12%
1498
9%
561
58%
561
27%
1038
June1st to 15thNight
50%
540
11%
1440
7%
540
52%
540
28%
979
June16th to endNight
64%
384
20%
1284
17%
384
69%
384
44%
613
July1st to 15thNight
75%
442
18%
1292
14%
442
78%
442
51%
647
July16th to endNight
69%
564
26%
1452
21%
531
73%
531
60%
708
August1st to 15thNight
74%
539
23%
1611
19%
536
78%
536
48%
805
August16th to endNight
71%
573
30%
1779
27%
573
74%
573
46%
897
September1st to 15thNight
66%
540
26%
1800
25%
540
72%
540
39%
917
September16th to endNight
66%
659
29%
1896
24%
659
68%
659
42%
1061
October1st to 15thNight
55%
660
21%
1980
16%
660
59%
660
28%
1262
October16th to endNight
59%
704
26%
2112
18%
704
65%
704
35%
1219
November1st to 15thNight
50%
864
21%
2409
15%
864
54%
864
34%
1484
November16th to endNight
50%
900
18%
2520
12%
900
55%
900
30%
1619
December1st to 15thNight
48%
900
9%
2520
6%
900
50%
900
23%
1768
December16th to endNight
45%
960
15%
2688
9%
960
50%
960
23%
1896

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%5%25%5%35%5%7%9%43%
48176
Night
5%6%35%5%345%6%9%44%
46891
Cloud CoverDay
5%5%25%5%35%5%6%8%43%
48176
Night
5%6%35%5%35%5%6%9%43%
46893
TransparencyDay
34%8%17%23%12%4
47852
Night
34%8%18%23%12%3
46719
SeeingDay
8%6%30%34%16%2
6043
Night
7%425%44%16%1
15944

By Month

Percentage Cloud Cover NW (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
36%37%7%48%8%11%13%25%
1433
16th to endDay
210%45%6%46%6%8%8%36%
1695
February1st to 15thDay
37%49%7%47%7%8%11%28%
1575
16th to endDay
38%36%8%47%8%7%10%30%
1445
March1st to 15thDay
34346%45%5%8%11%41%
1956
16th to endDay
7%8%37%7%36%6%7%9%34%
2114
April1st to 15thDay
7%7%46%5%35%5%8%9%37%
2150
16th to endDay
8%8%36%6%35%7%7%8%34%
2306
May1st to 15thDay
8%7%245%35%5%6%7%43%
2439
16th to endDay
14%6%35%5%36%5%7%8%32%
2616
June1st to 15thDay
7%5%246%47%5%7%9%39%
2520
16th to endDay
223335%5%6%9%56%
2670
July1st to 15thDay
21135%35%6%7%9%53%
2586
16th to endDay
22134245%5%7%60%
2614
August1st to 15thDay
4213435%6%8%8%51%
2340
16th to endDay
43134235%47%60%
2423
September1st to 15thDay
7%31332335%6%60%
2160
16th to endDay
6%31342445%7%56%
1872
October1st to 15thDay
13%6%35%4345%48%40%
1650
16th to endDay
6%435%42445%7%50%
1760
November1st to 15thDay
46%25%5%345%7%9%44%
1436
16th to endDay
46%37%5%35%5%7%8%42%
1440
December1st to 15thDay
46%49%9%5%8%7%7%8%27%
1440
16th to endDay
46%5%6%7%5%8%8%8%11%26%
1536
January1st to 15thNight
28%46%6%47%6%8%10%33%
2506
16th to endNight
37%37%5%35%6%6%9%40%
2540
February1st to 15thNight
38%5%7%7%46%7%8%9%31%
2277
16th to endNight
39%5%7%7%35%6%7%9%33%
2028
March1st to 15thNight
26%25%5%35%6%7%10%42%
2004
16th to endNight
7%7%36%6%446%5%10%35%
2106
April1st to 15thNight
8%7%46%5%245%5%7%41%
1798
16th to endNight
8%9%36%7%35%45%6%38%
1653
May1st to 15thNight
10%6%25%433448%46%
1511
16th to endNight
13%6%25%5%245%7%11%35%
1498
June1st to 15thNight
9%5%25%5%347%5%9%40%
1440
16th to endNight
321232445%8%59%
1284
July1st to 15thNight
1123245%8%11%57%
1371
16th to endNight
221231246%7%64%
1551
August1st to 15thNight
32133245%8%10%54%
1620
16th to endNight
431332345%6%62%
1779
September1st to 15thNight
8%313222446%60%
1800
16th to endNight
7%323313346%57%
1896
October1st to 15thNight
10%7%25%33445%7%44%
1980
16th to endNight
5%4145%3346%7%51%
2112
November1st to 15thNight
46%26%5%345%6%7%48%
2409
16th to endNight
37%36%6%346%5%9%43%
2520
December1st to 15thNight
27%38%7%46%7%7%9%33%
2520
16th to endNight
48%48%6%45%6%7%10%32%
2688

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
36%38%7%5%6%8%10%12%26%
1432
16th to endDay
29%37%7%36%6%7%9%35%
1695
February1st to 15thDay
48%48%7%47%6%8%10%29%
1575
16th to endDay
36%5%6%8%5%8%6%8%7%30%
1445
March1st to 15thDay
2436%6%35%6%7%10%42%
1956
16th to endDay
6%7%36%6%46%6%6%9%33%
2115
April1st to 15thDay
7%7%45%5%26%6%7%9%37%
2150
16th to endDay
7%8%27%6%36%7%6%9%34%
2306
May1st to 15thDay
8%7%26%4345%6%7%43%
2439
16th to endDay
13%7%35%5%36%6%7%8%32%
2616
June1st to 15thDay
7%5%25%5%36%6%6%9%40%
2520
16th to endDay
2212335%5%6%8%57%
2670
July1st to 15thDay
2213435%6%7%9%54%
2586
16th to endDay
321332445%8%60%
2614
August1st to 15thDay
4213425%6%7%8%53%
2340
16th to endDay
421342445%8%60%
2423
September1st to 15thDay
6%313223347%60%
2160
16th to endDay
6%223424446%57%
1872
October1st to 15thDay
11%7%25%5%35%45%6%41%
1650
16th to endDay
6%415%6%245%5%7%49%
1760
November1st to 15thDay
46%25%5%345%6%9%46%
1436
16th to endDay
47%36%6%35%5%5%8%42%
1440
December1st to 15thDay
46%310%9%49%7%7%9%26%
1440
16th to endDay
36%47%9%5%9%8%8%11%24%
1536
January1st to 15thNight
39%46%7%46%6%7%10%33%
2506
16th to endNight
38%36%6%45%47%10%39%
2540
February1st to 15thNight
37%5%9%7%47%5%7%9%31%
2277
16th to endNight
38%6%8%7%35%5%7%9%33%
2028
March1st to 15thNight
26%35%5%36%5%7%11%41%
2004
16th to endNight
6%9%36%5%36%6%5%10%35%
2109
April1st to 15thNight
7%9%36%5%345%5%7%41%
1798
16th to endNight
8%8%36%7%345%6%8%37%
1653
May1st to 15thNight
8%7%245%2445%7%48%
1511
16th to endNight
12%7%344245%7%11%35%
1497
June1st to 15thNight
10%4245%36%6%7%8%40%
1440
16th to endNight
3213325%45%8%59%
1284
July1st to 15thNight
2122346%6%11%59%
1371
16th to endNight
221231336%6%64%
1551
August1st to 15thNight
32133146%7%10%54%
1620
16th to endNight
43133135%46%61%
1779
September1st to 15thNight
8%313213346%61%
1800
16th to endNight
7%42322345%7%57%
1896
October1st to 15thNight
10%7%2443445%7%43%
1980
16th to endNight
45%234245%7%8%51%
2112
November1st to 15thNight
45%35%4345%6%8%48%
2409
16th to endNight
37%35%6%35%6%5%8%43%
2520
December1st to 15thNight
27%48%8%47%7%8%9%32%
2520
16th to endNight
48%48%7%47%6%6%10%31%
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
40%13%23%18%3
1433
16th to endDay
40%9%19%22%6%1
1695
February1st to 15thDay
47%8%18%19%5%
1575
16th to endDay
44%11%19%17%7%1
1445
March1st to 15thDay
31%10%26%25%6%
1956
16th to endDay
42%10%21%20%4
2115
April1st to 15thDay
39%9%21%22%6%
2150
16th to endDay
42%9%18%23%6%
2306
May1st to 15thDay
37%7%17%27%9%
2439
16th to endDay
46%10%18%17%6%
2620
June1st to 15thDay
35%10%19%23%10%
2520
16th to endDay
20%6%15%26%21%9%
2670
July1st to 15thDay
23%7%11%26%22%9%
2436
16th to endDay
20%5%8%18%29%17%
2450
August1st to 15thDay
24%6%12%26%18%11%
2325
16th to endDay
21%410%25%26%12%
2423
September1st to 15thDay
23%5%9%25%27%8%
2160
16th to endDay
25%5%14%30%22%2
1872
October1st to 15thDay
41%6%17%24%9%1
1650
16th to endDay
31%6%20%27%11%1
1760
November1st to 15thDay
32%8%17%27%12%1
1436
16th to endDay
36%8%24%22%8%
1440
December1st to 15thDay
48%9%22%15%2
1440
16th to endDay
45%11%20%17%5%1
1536
January1st to 15thNight
41%10%23%17%5%
2506
16th to endNight
37%8%21%23%7%1
2540
February1st to 15thNight
43%9%19%19%6%
2288
16th to endNight
43%9%19%17%8%1
2028
March1st to 15thNight
31%10%25%22%9%
2004
16th to endNight
41%10%18%21%7%
2109
April1st to 15thNight
40%7%18%24%8%
1798
16th to endNight
43%7%18%23%7%
1653
May1st to 15thNight
33%9%17%26%10%1
1512
16th to endNight
39%10%21%21%6%
1498
June1st to 15thNight
37%10%18%21%10%1
1440
16th to endNight
20%6%14%31%20%7%
1284
July1st to 15thNight
16%7%15%30%20%9%
1292
16th to endNight
15%49%23%28%19%
1452
August1st to 15thNight
19%6%15%25%21%11%
1611
16th to endNight
21%5%8%23%24%15%
1779
September1st to 15thNight
24%5%10%21%26%10%
1800
16th to endNight
26%5%10%26%27%3
1896
October1st to 15thNight
38%7%15%26%10%2
1980
16th to endNight
29%7%16%30%13%2
2112
November1st to 15thNight
31%7%18%26%12%2
2409
16th to endNight
36%8%22%22%9%
2520
December1st to 15thNight
42%10%23%19%2
2520
16th to endNight
43%10%17%22%5%1
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
421%35%34%4
90
16th to endDay
11%120%50%12%4
96
February1st to 15thDay
26%41%43%6%
90
16th to endDay
46%44%37%7%
81
March1st to 15thDay
6%6%40%35%9%1
246
16th to endDay
12%12%41%27%4
288
April1st to 15thDay
9%11%48%24%5%
269
16th to endDay
14%15%37%24%7%
270
May1st to 15thDay
13%8%29%29%15%3
396
16th to endDay
15%8%23%29%18%4
517
June1st to 15thDay
7%9%26%33%20%2
495
16th to endDay
36%28%38%19%2
495
July1st to 15thDay
4423%36%27%2
475
16th to endDay
2327%40%23%2
473
August1st to 15thDay
7%331%36%18%2
315
16th to endDay
35%25%36%21%7%
332
September1st to 15thDay
7%6%31%33%15%6%
315
16th to endDay
11%7%23%37%16%4
248
October1st to 15thDay
18%426%36%11%2
90
16th to endDay
6%427%40%18%3
96
November1st to 15thDay
3326%37%24%4
90
16th to endDay
14%6%34%42%2
90
December1st to 15thDay
11%41%34%13%
90
16th to endDay
9%125%52%11%1
96
January1st to 15thNight
6%5%28%47%11%1
900
16th to endNight
6%232%47%9%1
812
February1st to 15thNight
5%431%47%10%1
704
16th to endNight
7%330%46%9%1
632
March1st to 15thNight
5%431%43%14%
720
16th to endNight
11%5%28%41%12%
765
April1st to 15thNight
10%7%25%44%10%
539
16th to endNight
12%7%25%41%12%1
540
May1st to 15thNight
12%5%24%36%19%1
540
16th to endNight
14%418%38%24%
563
June1st to 15thNight
11%5%27%35%18%
540
16th to endNight
4320%48%21%
384
July1st to 15thNight
2116%45%34%
442
16th to endNight
3120%47%26%
564
August1st to 15thNight
4113%49%28%1
539
16th to endNight
5%214%44%31%1
573
September1st to 15thNight
9%315%42%24%4
540
16th to endNight
8%318%40%25%3
659
October1st to 15thNight
14%220%43%16%2
660
16th to endNight
6%325%45%16%2
704
November1st to 15thNight
6%332%45%10%1
864
16th to endNight
7%6%29%49%7%
900
December1st to 15thNight
5%430%47%9%1
900
16th to endNight
7%329%47%10%1
960
Page generated at 2017-09-23 04:43:46. Version ac04

Copyright 2017 A. Danko


Bench Observatory Clear Sky Chart | Charts Home