BX Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BX Observatory clear sky chart from 2005-11-05 to 2017-09-23.

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
44%
14854
9%
46658
8%
14827
46%
14827
16%
34315

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
24%
748
4%
2593
4%
745
25%
745
6%
2235
January16th to endNight
29%
768
3%
2688
3%
768
31%
768
7%
2256
February1st to 15thNight
36%
698
6%
2399
6%
698
38%
698
10%
1913
February16th to endNight
44%
632
9%
1963
10%
632
46%
632
15%
1454
March1st to 15thNight
34%
720
5%
2160
4%
720
35%
720
11%
1739
March16th to endNight
37%
768
9%
2005
4%
768
40%
768
16%
1552
April1st to 15thNight
45%
719
8%
1799
4%
719
47%
719
17%
1408
April16th to endNight
42%
492
12%
1572
10%
492
45%
492
16%
1144
May1st to 15thNight
51%
359
11%
1440
17%
359
53%
359
17%
943
May16th to endNight
54%
374
7%
1439
8%
374
54%
374
16%
971
June1st to 15thNight
46%
360
2%
1173
0%
360
46%
360
19%
813
June16th to endNight
53%
360
4%
1080
0%
360
53%
360
21%
741
July1st to 15thNight
68%
340
7%
1142
2%
340
69%
340
38%
598
July16th to endNight
72%
376
12%
1416
18%
354
76%
354
37%
664
August1st to 15thNight
78%
360
21%
1432
31%
358
80%
358
39%
668
August16th to endNight
72%
614
24%
1760
21%
614
73%
614
42%
954
September1st to 15thNight
69%
720
21%
1800
13%
720
70%
720
42%
1091
September16th to endNight
58%
688
20%
1862
14%
688
61%
688
28%
1231
October1st to 15thNight
53%
660
12%
1980
13%
660
54%
660
23%
1336
October16th to endNight
41%
704
9%
2277
7%
704
44%
704
15%
1668
November1st to 15thNight
35%
700
9%
2389
8%
700
37%
700
10%
1907
November16th to endNight
30%
819
5%
2619
2%
819
33%
819
8%
2204
December1st to 15thNight
32%
900
5%
2700
4%
900
33%
900
9%
2267
December16th to endNight
26%
975
3%
2970
2%
975
27%
975
7%
2558

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
10%8%37%6%46%5%6%7%33%
48260
Night
10%10%47%6%45%5%5%7%31%
46815
Cloud CoverDay
12%9%37%6%35%5%6%7%31%
48260
Night
12%11%47%6%35%5%5%6%31%
46818
TransparencyDay
49%10%15%16%6%1
47921
Night
51%9%14%16%6%
46658
SeeingDay
15%13%24%26%15%3
6206
Night
17%8%18%31%20%3
14863

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
15%15%7%8%7%5%5%6%7%6%13%
1351
16th to endDay
10%14%5%9%9%48%7%6%7%17%
1536
February1st to 15thDay
9%13%412%9%5%5%6%5%5%21%
1462
16th to endDay
9%11%48%7%46%7%7%7%24%
1511
March1st to 15thDay
10%11%5%8%9%5%6%6%6%7%21%
1800
16th to endDay
10%11%5%8%7%47%6%7%7%22%
2217
April1st to 15thDay
10%11%6%8%7%5%6%6%6%8%23%
2149
16th to endDay
10%10%48%8%36%5%5%7%27%
2387
May1st to 15thDay
10%8%47%6%45%5%6%7%34%
2511
16th to endDay
14%7%47%6%36%6%5%7%30%
2669
June1st to 15thDay
16%7%47%6%35%6%6%7%28%
2784
16th to endDay
13%5%36%6%36%5%6%7%34%
2880
July1st to 15thDay
5%314435%5%6%8%51%
2745
16th to endDay
42134245%47%57%
2657
August1st to 15thDay
3313435%46%8%54%
2520
16th to endDay
5%314425%5%6%8%50%
2439
September1st to 15thDay
6%415%335%45%8%50%
2160
16th to endDay
10%7%25%5%36%5%6%8%37%
1930
October1st to 15thDay
11%8%26%7%36%5%7%6%33%
1650
16th to endDay
18%9%47%6%25%5%5%7%28%
1595
November1st to 15thDay
16%13%5%8%6%45%6%5%6%21%
1470
16th to endDay
10%13%7%8%8%5%9%5%6%7%15%
1329
December1st to 15thDay
8%14%7%13%11%47%5%6%6%14%
1260
16th to endDay
12%14%5%9%8%46%6%7%7%17%
1248
January1st to 15thNight
16%17%6%10%7%5%5%5%5%6%12%
2593
16th to endNight
10%15%7%12%11%5%6%6%47%13%
2688
February1st to 15thNight
14%15%5%8%7%45%5%5%6%20%
2387
16th to endNight
8%12%5%8%6%345%5%8%30%
1963
March1st to 15thNight
10%12%5%8%7%46%5%5%6%25%
2160
16th to endNight
9%10%5%8%6%46%5%6%7%30%
2001
April1st to 15thNight
9%8%36%7%6%5%6%6%8%31%
1799
16th to endNight
10%9%47%7%46%446%34%
1572
May1st to 15thNight
6%9%27%6%45%5%5%7%38%
1439
16th to endNight
12%7%37%5%36%6%5%8%34%
1439
June1st to 15thNight
13%6%35%6%3446%9%35%
1173
16th to endNight
9%6%25%5%45%6%6%7%38%
1080
July1st to 15thNight
331332445%8%57%
1212
16th to endNight
421232445%7%61%
1512
August1st to 15thNight
411242345%8%60%
1440
16th to endNight
33133235%5%7%58%
1760
September1st to 15thNight
5%31332345%7%57%
1800
16th to endNight
9%5%25%5%245%5%8%46%
1862
October1st to 15thNight
9%6%26%5%46%45%6%42%
1980
16th to endNight
14%7%37%6%37%46%7%32%
2277
November1st to 15thNight
17%12%5%6%6%4445%6%23%
2389
16th to endNight
14%14%7%9%7%47%5%5%5%17%
2619
December1st to 15thNight
10%15%7%11%8%47%6%5%5%16%
2700
16th to endNight
12%17%6%10%8%47%6%46%14%
2970

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
17%15%7%10%6%46%5%7%5%12%
1351
16th to endDay
13%14%5%9%7%5%5%6%7%9%16%
1536
February1st to 15thDay
12%14%410%8%46%5%6%421%
1462
16th to endDay
11%13%5%8%6%36%7%6%8%23%
1511
March1st to 15thDay
13%13%6%9%8%35%5%5%6%20%
1800
16th to endDay
13%11%5%8%8%45%6%7%7%21%
2217
April1st to 15thDay
12%13%47%7%46%6%7%7%21%
2149
16th to endDay
12%11%48%7%47%45%7%25%
2387
May1st to 15thDay
12%8%37%6%35%5%5%7%33%
2511
16th to endDay
16%8%37%5%36%5%5%7%28%
2669
June1st to 15thDay
17%8%37%6%46%5%5%7%26%
2784
16th to endDay
14%7%36%6%45%6%6%7%32%
2880
July1st to 15thDay
5%4235%35%5%6%8%49%
2745
16th to endDay
5%21342445%7%56%
2657
August1st to 15thDay
43145%3445%9%53%
2520
16th to endDay
6%414425%5%6%8%48%
2439
September1st to 15thDay
7%425%424448%50%
2160
16th to endDay
14%6%15%5%35%5%5%8%36%
1930
October1st to 15thDay
12%9%37%6%36%46%7%33%
1650
16th to endDay
19%10%46%5%25%5%5%6%27%
1595
November1st to 15thDay
20%13%5%7%6%46%46%5%20%
1470
16th to endDay
14%15%6%8%8%5%6%5%5%8%14%
1329
December1st to 15thDay
11%16%6%12%10%6%6%5%6%5%14%
1260
16th to endDay
16%14%6%9%6%46%6%6%6%16%
1248
January1st to 15thNight
19%18%6%9%7%35%45%5%13%
2592
16th to endNight
13%16%6%10%10%46%5%5%6%14%
2688
February1st to 15thNight
17%15%48%5%46%5%45%21%
2387
16th to endNight
10%14%5%7%6%445%5%8%28%
1963
March1st to 15thNight
14%14%48%7%35%45%7%24%
2160
16th to endNight
11%12%47%6%45%46%7%28%
2005
April1st to 15thNight
11%10%36%7%46%6%6%8%29%
1799
16th to endNight
11%9%47%7%45%45%6%33%
1572
May1st to 15thNight
7%10%38%5%35%5%6%6%37%
1439
16th to endNight
12%8%35%5%45%5%5%8%32%
1439
June1st to 15thNight
15%8%35%6%245%5%8%33%
1173
16th to endNight
11%5%36%426%7%7%6%36%
1080
July1st to 15thNight
432432435%8%56%
1212
16th to endNight
5%21232446%7%60%
1512
August1st to 15thNight
411331445%8%60%
1440
16th to endNight
5%31341345%8%58%
1760
September1st to 15thNight
6%31432445%6%57%
1800
16th to endNight
11%6%15%5%24447%44%
1862
October1st to 15thNight
10%7%26%5%35%5%5%6%40%
1980
16th to endNight
17%8%37%6%445%5%6%31%
2277
November1st to 15thNight
21%13%46%5%3345%6%23%
2389
16th to endNight
16%16%6%7%8%26%6%5%417%
2619
December1st to 15thNight
13%16%6%10%8%45%45%5%18%
2700
16th to endNight
14%19%6%9%8%45%5%45%14%
2970

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
68%14%10%5%1
1351
16th to endDay
60%13%16%6%11
1536
February1st to 15thDay
61%12%14%8%2
1462
16th to endDay
55%12%15%12%3
1511
March1st to 15thDay
63%10%14%10%1
1800
16th to endDay
60%9%15%10%3
2217
April1st to 15thDay
60%10%13%13%1
2149
16th to endDay
57%8%15%14%3
2387
May1st to 15thDay
49%7%14%18%8%
2511
16th to endDay
54%8%17%16%3
2673
June1st to 15thDay
56%9%12%14%5%1
2784
16th to endDay
49%9%12%18%9%
2880
July1st to 15thDay
32%8%14%23%17%4
2586
16th to endDay
25%7%17%26%20%2
2490
August1st to 15thDay
27%8%18%28%13%2
2503
16th to endDay
30%8%17%28%13%1
2439
September1st to 15thDay
30%7%14%30%15%1
2160
16th to endDay
41%10%18%20%6%1
1930
October1st to 15thDay
46%11%19%18%4
1650
16th to endDay
55%10%18%13%1
1595
November1st to 15thDay
63%11%12%9%21
1470
16th to endDay
66%12%11%7%1
1329
December1st to 15thDay
66%15%8%8%
1260
16th to endDay
64%13%14%7%
1248
January1st to 15thNight
70%9%11%6%2
2593
16th to endNight
66%10%13%6%1
2688
February1st to 15thNight
61%9%14%11%3
2399
16th to endNight
53%9%17%14%5%
1963
March1st to 15thNight
60%10%15%11%2
2160
16th to endNight
53%9%16%15%4
2005
April1st to 15thNight
50%13%16%14%5%
1799
16th to endNight
50%9%14%20%5%
1572
May1st to 15thNight
45%8%13%19%12%
1440
16th to endNight
47%8%16%20%6%
1439
June1st to 15thNight
46%8%15%18%9%1
1173
16th to endNight
42%11%15%20%9%
1080
July1st to 15thNight
26%5%17%25%19%6%
1142
16th to endNight
20%8%14%31%22%2
1416
August1st to 15thNight
21%6%15%31%19%5%
1432
16th to endNight
23%6%17%31%18%1
1760
September1st to 15thNight
26%7%14%27%20%3
1800
16th to endNight
38%7%13%26%9%3
1862
October1st to 15thNight
41%9%18%21%9%
1980
16th to endNight
52%8%17%18%2
2277
November1st to 15thNight
60%10%11%11%4
2389
16th to endNight
65%11%12%8%2
2619
December1st to 15thNight
66%9%13%9%1
2700
16th to endNight
68%11%12%7%
2970

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
10%8%13%25%26%16%
60
16th to endDay
23%7%18%29%15%4
64
February1st to 15thDay
18%22%16%29%9%3
122
16th to endDay
13%13%27%31%11%2
227
March1st to 15thDay
27%24%27%16%3
270
16th to endDay
22%22%28%20%6%
288
April1st to 15thDay
27%16%27%16%9%1
269
16th to endDay
18%21%28%19%9%2
270
May1st to 15thDay
15%17%30%26%6%2
288
16th to endDay
20%10%18%22%20%7%
384
June1st to 15thDay
18%12%20%27%18%3
579
16th to endDay
14%10%25%28%17%3
675
July1st to 15thDay
7%10%20%35%24%2
520
16th to endDay
412%29%33%14%4
327
August1st to 15thDay
7%8%21%32%20%9%
315
16th to endDay
8%10%23%37%13%6%
332
September1st to 15thDay
11%12%26%27%18%4
315
16th to endDay
18%10%30%20%16%2
284
October1st to 15thDay
18%18%26%21%13%1
255
16th to endDay
19%421%29%19%4
107
November1st to 15thDay
28%15%23%18%14%
71
16th to endDay
13%16%25%20%11%13%
60
December1st to 15thDay
23%15%13%20%20%8%
60
16th to endDay
10%7%26%35%12%6%
64
January1st to 15thNight
29%11%19%23%13%2
749
16th to endNight
19%8%22%28%16%3
768
February1st to 15thNight
24%10%14%30%16%2
704
16th to endNight
16%7%20%29%20%3
632
March1st to 15thNight
21%14%21%29%12%1
720
16th to endNight
16%12%25%30%13%2
768
April1st to 15thNight
15%11%18%31%20%2
719
16th to endNight
15%8%24%34%16%
492
May1st to 15thNight
13%7%16%38%22%1
360
16th to endNight
16%5%14%32%26%4
375
June1st to 15thNight
16%8%20%30%21%2
360
16th to endNight
12%7%18%39%19%3
360
July1st to 15thNight
5%315%32%38%4
340
16th to endNight
6%214%35%38%2
376
August1st to 15thNight
5%28%27%45%9%
360
16th to endNight
7%310%38%35%5%
614
September1st to 15thNight
7%313%29%34%10%
720
16th to endNight
15%6%13%30%28%5%
688
October1st to 15thNight
13%5%18%34%23%4
660
16th to endNight
20%9%19%29%17%3
704
November1st to 15thNight
25%12%19%31%9%1
700
16th to endNight
23%11%20%26%14%4
819
December1st to 15thNight
21%10%20%32%13%2
900
16th to endNight
22%9%26%29%10%1
975
Page generated at 2017-09-24 06:15:49. Version ac04

Copyright 2017 A. Danko


BX Observatory Clear Sky Chart | Charts Home