Borderline Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Borderline Observatory clear sky chart from 2005-11-06 to 2024-01-11.

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
27%
22869
17%
74657
8%
22825
39%
22825
18%
48726

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
14%
760
19%
2649
5%
757
33%
757
14%
1683
January11th to 20thNight
17%
724
13%
2532
4%
724
30%
724
11%
1784
January21st to endNight
16%
792
18%
2767
6%
792
33%
792
13%
1763
February1st to 10thNight
10%
720
13%
2313
1%
720
25%
720
12%
1609
February11th to 20thNight
13%
703
18%
2100
3%
700
30%
700
12%
1419
February21st to endNight
13%
592
20%
1776
5%
592
31%
592
13%
1194
March1st to 10thNight
17%
720
21%
2160
7%
720
36%
720
16%
1417
March11th to 20thNight
19%
720
17%
2111
6%
720
30%
720
14%
1436
March21st to endNight
22%
792
22%
1980
6%
792
34%
792
20%
1369
April1st to 10thNight
18%
720
18%
1800
4%
720
27%
720
20%
1262
April11th to 20thNight
24%
711
16%
1775
3%
711
31%
711
20%
1238
April21st to endNight
21%
594
17%
1674
5%
594
30%
594
17%
1161
May1st to 10thNight
23%
540
14%
1620
6%
540
30%
540
16%
1144
May11th to 20thNight
26%
389
17%
1451
11%
389
38%
389
16%
930
May21st to endNight
38%
389
15%
1551
13%
388
48%
388
18%
957
June1st to 10thNight
34%
360
15%
1440
12%
360
45%
360
17%
905
June11th to 20thNight
39%
360
12%
1440
12%
360
47%
360
18%
896
June21st to endNight
41%
360
13%
1440
13%
360
52%
360
21%
869
July1st to 10thNight
48%
350
13%
1400
14%
350
56%
350
22%
812
July11th to 20thNight
50%
348
15%
1392
18%
348
55%
348
23%
814
July21st to endNight
51%
394
19%
1486
19%
372
59%
372
26%
812
August1st to 10thNight
52%
389
18%
1453
14%
387
60%
387
27%
829
August11th to 20thNight
48%
649
15%
1713
7%
649
51%
649
28%
1140
August21st to endNight
50%
792
21%
1980
10%
792
54%
792
34%
1273
September1st to 10thNight
47%
720
20%
1800
8%
720
52%
720
32%
1171
September11th to 20thNight
47%
720
25%
1918
14%
720
53%
720
34%
1168
September21st to endNight
41%
720
24%
1980
15%
720
50%
720
29%
1279
October1st to 10thNight
34%
720
16%
2020
9%
720
42%
720
20%
1389
October11th to 20thNight
30%
720
20%
2194
10%
720
43%
720
23%
1395
October21st to endNight
22%
792
14%
2574
6%
792
33%
792
15%
1775
November1st to 10thNight
28%
740
22%
2407
11%
740
43%
740
21%
1484
November11th to 20thNight
23%
760
22%
2470
9%
760
38%
760
17%
1578
November21st to endNight
23%
760
19%
2556
7%
760
39%
760
17%
1581
December1st to 10thNight
15%
760
15%
2660
4%
760
29%
760
11%
1821
December11th to 20thNight
17%
760
15%
3239
4%
759
31%
759
11%
2087
December21st to endNight
15%
829
11%
4836
3%
817
30%
817
6%
3282

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
16%7%36%6%45%5%6%7%35%
74765
Night
15%7%35%5%35%5%6%8%38%
74925
TransparencyDay
45%6%13%21%13%2
74470
Night
43%6%14%22%13%2
74754
SeeingDay
24%18%28%19%8%3
13763
Night
23%16%28%24%8%1
22876

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
11%9%47%7%5%6%6%7%7%31%
1517
11th to 20thDay
14%8%47%7%47%6%7%7%29%
1440
21st to endDay
10%10%47%6%46%5%7%8%33%
1588
February1st to 10thDay
13%10%48%8%5%5%7%7%8%26%
1646
11th to 20thDay
9%8%49%8%46%5%7%7%33%
1748
21st to endDay
18%11%47%6%35%6%45%31%
1480
March1st to 10thDay
12%9%46%7%35%6%6%9%34%
1797
11th to 20thDay
16%9%46%6%46%5%5%6%34%
1848
21st to endDay
17%8%37%5%34446%38%
2376
April1st to 10thDay
16%9%46%5%35%5%5%6%35%
2160
11th to 20thDay
15%7%36%6%45%6%5%7%35%
2134
21st to endDay
19%8%36%7%45%5%6%7%32%
2286
May1st to 10thDay
23%9%36%5%35%45%6%31%
2340
11th to 20thDay
19%5%35%5%3445%7%38%
2459
21st to endDay
16%5%25%445%46%8%40%
2725
June1st to 10thDay
18%6%35%6%46%6%7%8%31%
2520
11th to 20thDay
18%5%35%6%35%5%5%9%36%
2520
21st to endDay
17%7%36%6%35%6%7%9%31%
2520
July1st to 10thDay
14%5%36%6%45%6%6%9%38%
2520
11th to 20thDay
12%5%35%6%35%5%6%10%38%
2520
21st to endDay
15%5%35%5%45%6%7%8%37%
2744
August1st to 10thDay
11%5%36%6%45%5%9%9%37%
2455
11th to 20thDay
13%5%36%6%46%6%7%10%34%
2183
21st to endDay
13%425%6%36%6%7%9%40%
2376
September1st to 10thDay
13%6%245%46%5%6%7%42%
2160
11th to 20thDay
15%5%25%43445%7%46%
2040
21st to endDay
17%6%245%3445%9%42%
1980
October1st to 10thDay
23%6%35%5%44445%36%
1938
11th to 20thDay
15%7%25%5%5%45%7%6%40%
1764
21st to endDay
21%8%36%5%35%5%6%7%30%
1782
November1st to 10thDay
13%7%35%5%35%447%45%
1671
11th to 20thDay
17%8%35%5%45%5%5%6%39%
1710
21st to endDay
17%7%36%5%35%6%6%6%36%
1620
December1st to 10thDay
16%12%5%8%6%46%45%7%27%
1520
11th to 20thDay
20%9%45%5%34445%36%
1906
21st to endDay
17%12%37%6%36%5%6%8%27%
2772
January1st to 10thNight
12%8%36%6%35%5%6%7%39%
2649
11th to 20thNight
15%10%47%6%45%5%7%7%31%
2532
21st to endNight
9%7%48%6%5%6%5%6%9%35%
2767
February1st to 10thNight
13%9%37%6%47%5%6%8%33%
2313
11th to 20thNight
9%8%46%6%5%6%6%7%9%35%
2098
21st to endNight
15%9%36%6%35%45%8%35%
1776
March1st to 10thNight
12%9%26%6%445%6%9%37%
2150
11th to 20thNight
14%8%346%46%5%7%7%36%
2110
21st to endNight
14%6%36%6%35%46%7%39%
1980
April1st to 10thNight
16%8%35%5%35%45%8%37%
1800
11th to 20thNight
17%6%25%5%35%5%5%8%38%
1775
21st to endNight
21%7%35%425%36%7%36%
1674
May1st to 10thNight
21%9%36%5%35%45%7%32%
1620
11th to 20thNight
17%6%36%6%35%45%7%37%
1450
21st to endNight
16%5%25%5%345%5%9%39%
1550
June1st to 10thNight
18%6%25%5%34449%39%
1440
11th to 20thNight
19%5%245%345%48%41%
1440
21st to endNight
16%5%243345%6%8%43%
1440
July1st to 10thNight
13%5%245%35%5%6%9%44%
1440
11th to 20thNight
14%5%245%35%6%6%10%40%
1440
21st to endNight
12%3245%345%6%9%47%
1574
August1st to 10thNight
11%415%5%35%45%10%48%
1463
11th to 20thNight
13%5%25%5%445%7%8%41%
1713
21st to endNight
16%5%143245%6%7%47%
1980
September1st to 10thNight
13%5%244336%6%8%45%
1800
11th to 20thNight
14%4245%33447%51%
1918
21st to endNight
16%6%25%434446%47%
1980
October1st to 10thNight
22%6%245%24446%40%
2020
11th to 20thNight
14%7%25%5%345%6%7%42%
2194
21st to endNight
21%8%25%5%345%5%7%35%
2574
November1st to 10thNight
14%6%245%35%446%46%
2407
11th to 20thNight
18%5%35%6%35%5%5%8%38%
2470
21st to endNight
13%8%35%4245%6%9%41%
2556
December1st to 10thNight
16%8%46%6%35%5%7%8%32%
2660
11th to 20thNight
18%7%35%5%35%5%7%8%34%
3288
21st to endNight
20%10%48%6%35%5%5%6%28%
4884

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
46%6%11%24%12%1
1518
11th to 20thDay
50%8%13%16%9%3
1440
21st to endDay
45%6%14%22%12%3
1588
February1st to 10thDay
53%6%15%16%10%1
1646
11th to 20thDay
47%7%11%20%13%2
1757
21st to endDay
54%5%10%17%13%2
1480
March1st to 10thDay
46%5%15%18%13%3
1800
11th to 20thDay
49%5%12%15%15%3
1848
21st to endDay
47%5%9%17%17%5%
2376
April1st to 10thDay
49%5%10%21%13%3
2160
11th to 20thDay
44%7%13%19%15%2
2134
21st to endDay
50%6%10%18%13%3
2286
May1st to 10thDay
53%5%9%19%13%2
2340
11th to 20thDay
45%6%10%21%16%3
2463
21st to endDay
41%7%16%23%11%2
2725
June1st to 10thDay
48%8%13%20%11%1
2520
11th to 20thDay
44%7%13%20%14%1
2520
21st to endDay
48%8%13%20%11%
2520
July1st to 10thDay
41%8%12%22%15%1
2450
11th to 20thDay
39%7%14%28%10%
2445
21st to endDay
41%7%14%24%13%1
2591
August1st to 10thDay
40%10%15%23%11%1
2441
11th to 20thDay
41%8%16%23%10%1
2183
21st to endDay
38%7%15%25%14%2
2376
September1st to 10thDay
38%5%14%27%15%1
2160
11th to 20thDay
36%6%10%25%21%2
2040
21st to endDay
39%6%12%24%17%3
1980
October1st to 10thDay
50%410%20%13%3
1938
11th to 20thDay
40%7%14%27%10%2
1764
21st to endDay
51%5%12%18%10%3
1782
November1st to 10thDay
38%5%15%23%15%2
1671
11th to 20thDay
44%5%12%20%16%2
1710
21st to endDay
45%5%17%21%9%2
1620
December1st to 10thDay
54%6%15%14%10%1
1520
11th to 20thDay
50%413%19%13%1
1906
21st to endDay
53%7%14%16%10%
2772
January1st to 10thNight
42%6%12%22%14%2
2649
11th to 20thNight
51%8%14%17%8%2
2532
21st to endNight
43%7%13%19%16%2
2767
February1st to 10thNight
48%7%15%18%10%2
2313
11th to 20thNight
45%7%12%21%13%2
2100
21st to endNight
47%413%22%11%3
1776
March1st to 10thNight
43%6%12%25%13%2
2160
11th to 20thNight
44%8%11%21%13%3
2111
21st to endNight
43%5%12%17%18%5%
1980
April1st to 10thNight
44%6%12%22%14%1
1800
11th to 20thNight
43%7%11%23%15%1
1775
21st to endNight
47%5%11%19%14%3
1674
May1st to 10thNight
52%411%19%13%2
1620
11th to 20thNight
45%5%14%18%15%3
1451
21st to endNight
41%8%13%24%12%2
1551
June1st to 10thNight
45%6%14%25%11%1
1440
11th to 20thNight
41%8%17%21%12%1
1440
21st to endNight
37%10%15%24%13%1
1440
July1st to 10thNight
35%9%16%25%14%1
1400
11th to 20thNight
36%9%15%29%11%
1392
21st to endNight
32%8%17%28%14%2
1486
August1st to 10thNight
32%9%17%29%13%1
1453
11th to 20thNight
39%8%18%25%10%
1713
21st to endNight
34%6%14%29%16%1
1980
September1st to 10thNight
35%8%14%26%17%1
1800
11th to 20thNight
34%412%26%22%2
1918
21st to endNight
38%5%10%27%17%3
1980
October1st to 10thNight
46%6%12%22%12%2
2020
11th to 20thNight
40%6%16%26%11%1
2194
21st to endNight
49%6%14%20%10%1
2574
November1st to 10thNight
39%6%12%23%17%3
2407
11th to 20thNight
43%5%13%22%15%3
2470
21st to endNight
40%6%19%24%11%1
2556
December1st to 10thNight
48%7%15%20%9%1
2660
11th to 20thNight
47%6%17%16%13%2
3288
21st to endNight
55%5%12%20%8%1
4884

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
34%19%29%14%3
180
11th to 20thDay
29%24%31%9%5%1
160
21st to endDay
22%30%32%11%41
178
February1st to 10thDay
36%31%25%8%1
266
11th to 20thDay
32%26%31%10%2
416
21st to endDay
32%26%28%13%2
346
March1st to 10thDay
29%23%32%15%2
420
11th to 20thDay
32%25%28%11%31
420
21st to endDay
29%24%28%10%6%2
462
April1st to 10thDay
32%22%27%10%6%2
420
11th to 20thDay
33%20%27%10%8%1
412
21st to endDay
31%20%28%15%42
420
May1st to 10thDay
33%20%27%12%6%2
423
11th to 20thDay
24%18%33%18%5%2
447
21st to endDay
19%15%32%21%10%3
485
June1st to 10thDay
20%16%32%21%9%2
450
11th to 20thDay
19%14%33%22%10%1
450
21st to endDay
20%17%32%18%10%3
450
July1st to 10thDay
16%10%29%27%15%2
445
11th to 20thDay
16%7%32%30%13%2
444
21st to endDay
13%11%38%25%10%3
488
August1st to 10thDay
11%9%28%29%17%5%
446
11th to 20thDay
15%9%28%28%12%8%
439
21st to endDay
15%11%26%28%13%7%
495
September1st to 10thDay
20%11%21%24%17%7%
450
11th to 20thDay
18%12%24%22%15%9%
450
21st to endDay
21%13%28%22%12%6%
450
October1st to 10thDay
29%14%23%18%12%4
450
11th to 20thDay
20%20%27%22%9%2
432
21st to endDay
33%15%25%20%6%1
396
November1st to 10thDay
24%21%24%22%9%1
365
11th to 20thDay
29%25%24%16%5%1
370
21st to endDay
32%18%27%19%41
280
December1st to 10thDay
29%26%25%15%41
180
11th to 20thDay
41%18%32%7%2
180
21st to endDay
34%22%27%14%41
198
January1st to 10thNight
25%22%33%18%3
760
11th to 20thNight
26%19%32%19%21
724
21st to endNight
20%24%33%20%3
792
February1st to 10thNight
25%21%36%15%3
720
11th to 20thNight
24%25%34%17%1
703
21st to endNight
29%22%31%17%1
592
March1st to 10thNight
24%24%32%18%2
720
11th to 20thNight
26%22%28%20%4
720
21st to endNight
24%18%30%21%6%
792
April1st to 10thNight
27%20%31%18%4
720
11th to 20thNight
23%21%28%21%7%
711
21st to endNight
29%18%27%21%5%
594
May1st to 10thNight
29%13%28%22%8%1
540
11th to 20thNight
21%15%30%29%5%
389
21st to endNight
22%7%27%30%13%2
389
June1st to 10thNight
21%11%28%29%12%
360
11th to 20thNight
19%9%29%31%12%
360
21st to endNight
16%9%27%34%13%1
360
July1st to 10thNight
13%8%25%39%15%
350
11th to 20thNight
15%424%41%15%2
348
21st to endNight
11%6%26%39%17%1
394
August1st to 10thNight
11%425%38%21%2
389
11th to 20thNight
15%5%24%32%20%3
649
21st to endNight
17%6%20%34%20%2
792
September1st to 10thNight
15%7%25%33%19%2
720
11th to 20thNight
16%8%23%29%18%5%
720
21st to endNight
19%9%23%29%16%2
720
October1st to 10thNight
28%12%20%25%13%2
720
11th to 20thNight
21%12%30%25%11%1
720
21st to endNight
29%18%27%22%5%
792
November1st to 10thNight
21%19%28%24%8%1
740
11th to 20thNight
24%18%29%20%7%1
760
21st to endNight
23%20%28%23%6%1
760
December1st to 10thNight
25%23%32%15%5%
760
11th to 20thNight
29%19%29%19%4
760
21st to endNight
29%24%28%17%2
836
Page generated at 2024-01-08 11:14:18. Version ac07

Copyright 2024 Attilla Danko


Borderline Observatory Clear Sky Chart |Charts Home