Williston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Williston clear sky chart from 2005-11-06 to 2024-01-13.

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
41%
25035
17%
74685
10%
24993
47%
24993
27%
47383

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
32%
946
15%
2647
7%
946
39%
946
22%
1843
January11th to 20thNight
34%
911
13%
2549
8%
911
39%
911
21%
1806
January21st to endNight
28%
990
13%
2772
7%
990
33%
990
16%
2081
February1st to 10thNight
31%
855
12%
2475
5%
855
38%
855
18%
1795
February11th to 20thNight
29%
700
14%
2223
8%
700
36%
700
20%
1550
February21st to endNight
45%
592
20%
1776
12%
592
52%
592
25%
1165
March1st to 10thNight
31%
720
13%
2160
8%
720
36%
720
21%
1511
March11th to 20thNight
38%
716
19%
2116
11%
716
45%
716
27%
1383
March21st to endNight
38%
594
15%
1980
13%
594
43%
594
21%
1317
April1st to 10thNight
35%
540
18%
1800
13%
540
44%
540
24%
1124
April11th to 20thNight
30%
533
16%
1756
10%
533
38%
533
19%
1173
April21st to endNight
41%
540
18%
1590
13%
540
47%
540
27%
1031
May1st to 10thNight
47%
540
17%
1440
10%
540
51%
540
33%
925
May11th to 20thNight
48%
531
15%
1418
8%
531
50%
531
31%
937
May21st to endNight
46%
584
13%
1552
6%
582
49%
582
29%
1037
June1st to 10thNight
46%
540
12%
1364
7%
540
49%
540
37%
851
June11th to 20thNight
42%
540
9%
1260
4%
540
45%
540
36%
838
June21st to endNight
59%
540
11%
1349
8%
540
62%
540
44%
791
July1st to 10thNight
60%
525
13%
1400
8%
525
63%
525
53%
739
July11th to 20thNight
54%
522
16%
1392
8%
522
58%
522
46%
776
July21st to endNight
63%
591
24%
1488
14%
558
66%
558
59%
746
August1st to 10thNight
59%
537
21%
1424
12%
534
64%
534
46%
794
August11th to 20thNight
61%
534
28%
1468
20%
534
67%
534
49%
793
August21st to endNight
59%
594
28%
1980
25%
594
69%
594
47%
940
September1st to 10thNight
55%
540
25%
1800
23%
540
64%
540
41%
904
September11th to 20thNight
54%
540
27%
1854
26%
540
62%
540
37%
959
September21st to endNight
47%
540
26%
1980
22%
540
56%
540
29%
1086
October1st to 10thNight
43%
675
22%
2115
14%
675
51%
675
29%
1270
October11th to 20thNight
43%
782
22%
2222
12%
782
49%
782
29%
1391
October21st to endNight
37%
990
20%
2574
8%
990
45%
990
30%
1695
November1st to 10thNight
36%
925
21%
2405
9%
925
42%
925
26%
1655
November11th to 20thNight
33%
950
19%
2641
10%
950
40%
950
26%
1769
November21st to endNight
35%
950
17%
2691
7%
950
43%
950
26%
1781
December1st to 10thNight
34%
950
12%
2850
6%
950
40%
950
21%
1996
December11th to 20thNight
32%
949
15%
3435
7%
948
41%
948
17%
2288
December21st to endNight
30%
1029
12%
4739
7%
1026
38%
1026
14%
2643

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
9%7%36%6%35%5%6%8%42%
74576
Night
8%7%37%6%35%5%6%7%43%
75162
TransparencyDay
41%415%26%13%1
74278
Night
41%415%26%13%2
74982
SeeingDay
12%8%31%30%16%3
14941
Night
11%7%32%36%12%1
25060

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 10thDay
5%9%6%7%6%7%7%7%6%9%30%
1518
11th to 20thDay
6%10%5%11%8%48%7%6%8%28%
1456
21st to endDay
5%10%6%11%9%6%8%7%6%9%24%
1584
February1st to 10thDay
5%10%7%12%10%5%7%5%6%6%28%
1502
11th to 20thDay
8%10%7%11%8%5%6%5%6%8%27%
1624
21st to endDay
6%10%5%10%8%36%6%6%8%31%
1480
March1st to 10thDay
8%13%6%12%9%45%5%5%7%27%
1800
11th to 20thDay
8%9%37%7%45%46%8%39%
1842
21st to endDay
10%10%47%7%45%5%6%7%35%
2376
April1st to 10thDay
7%8%47%6%46%5%6%9%39%
2160
11th to 20thDay
14%13%47%7%45%45%7%33%
2151
21st to endDay
14%9%36%6%36%5%5%7%36%
2366
May1st to 10thDay
14%7%36%5%35%447%43%
2520
11th to 20thDay
13%6%25%6%3446%8%42%
2478
21st to endDay
19%6%36%6%35%5%6%8%33%
2724
June1st to 10thDay
13%5%244346%6%9%44%
2592
11th to 20thDay
10%5%26%5%35%5%7%9%42%
2700
21st to endDay
7%325%5%35%6%6%8%50%
2604
July1st to 10thDay
6%31443447%8%56%
2520
11th to 20thDay
6%31342446%8%58%
2520
21st to endDay
5%2134345%6%9%58%
2742
August1st to 10thDay
8%42443346%8%56%
2507
11th to 20thDay
7%4135%245%6%7%57%
2432
21st to endDay
9%3133235%5%7%58%
2376
September1st to 10thDay
10%5%245%3445%7%53%
2160
11th to 20thDay
12%42343445%7%52%
2106
21st to endDay
15%5%23323446%52%
1980
October1st to 10thDay
17%7%35%43335%6%44%
1841
11th to 20thDay
10%9%46%5%3445%6%45%
1720
21st to endDay
12%8%47%5%345%6%6%41%
1782
November1st to 10thDay
12%11%46%5%35%5%6%5%38%
1673
11th to 20thDay
6%9%48%6%36%6%6%8%38%
1536
21st to endDay
9%10%5%8%7%46%5%5%7%35%
1485
December1st to 10thDay
8%14%5%9%6%45%46%7%30%
1330
11th to 20thDay
5%11%6%10%6%45%47%8%33%
1683
21st to endDay
5%8%48%7%45%5%6%6%42%
2706
January1st to 10thNight
5%8%48%8%48%6%7%8%34%
2647
11th to 20thNight
5%10%6%9%8%45%6%6%7%33%
2549
21st to endNight
5%12%6%11%9%5%7%7%6%7%27%
2772
February1st to 10thNight
7%10%6%9%8%46%5%7%7%30%
2475
11th to 20thNight
7%11%6%9%7%46%5%6%6%33%
2222
21st to endNight
6%8%5%8%7%46%5%6%8%39%
1776
March1st to 10thNight
6%10%5%12%7%446%5%8%33%
2160
11th to 20thNight
8%8%47%7%45%5%6%8%40%
2116
21st to endNight
8%9%48%7%45%6%7%7%34%
1980
April1st to 10thNight
8%7%46%7%446%5%7%43%
1800
11th to 20thNight
13%11%47%6%35%6%6%7%34%
1756
21st to endNight
13%7%35%5%35%5%5%7%42%
1590
May1st to 10thNight
10%6%27%6%345%6%7%44%
1440
11th to 20thNight
12%5%346%35%5%5%8%43%
1417
21st to endNight
16%5%145%35%6%5%8%41%
1552
June1st to 10thNight
11%3244345%6%8%49%
1364
11th to 20thNight
10%6%15%5%35%6%6%8%45%
1260
21st to endNight
6%3334345%7%10%52%
1349
July1st to 10thNight
5%31341345%9%62%
1440
11th to 20thNight
5%3144345%5%7%59%
1440
21st to endNight
32123245%6%8%66%
1576
August1st to 10thNight
5%3145%3346%7%59%
1432
11th to 20thNight
6%324324448%60%
1468
21st to endNight
6%214423447%63%
1980
September1st to 10thNight
8%423324447%60%
1800
11th to 20thNight
9%41432345%7%58%
1854
21st to endNight
12%6%24324448%51%
1980
October1st to 10thNight
12%6%345%3445%7%49%
2115
11th to 20thNight
11%6%26%5%3446%6%45%
2222
21st to endNight
11%7%26%5%36%46%9%42%
2574
November1st to 10thNight
11%10%46%445%5%5%6%40%
2405
11th to 20thNight
7%8%48%7%45%5%5%8%39%
2641
21st to endNight
6%7%48%8%45%5%5%8%40%
2691
December1st to 10thNight
6%8%5%10%8%46%7%5%7%34%
2850
11th to 20thNight
6%10%5%9%8%46%6%6%7%33%
3514
21st to endNight
6%7%47%6%45%5%7%7%42%
4955

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
49%7%17%21%6%
1519
11th to 20thDay
53%5%18%16%6%1
1456
21st to endDay
56%6%15%17%6%
1584
February1st to 10thDay
56%411%20%9%
1502
11th to 20thDay
55%5%15%16%8%1
1633
21st to endDay
50%5%15%20%10%1
1480
March1st to 10thDay
57%5%14%18%5%
1800
11th to 20thDay
45%5%17%26%7%
1842
21st to endDay
49%5%19%20%7%
2376
April1st to 10thDay
42%5%16%23%13%1
2160
11th to 20thDay
53%412%21%10%
2154
21st to endDay
47%314%25%10%1
2366
May1st to 10thDay
43%411%29%12%2
2520
11th to 20thDay
41%412%29%14%
2482
21st to endDay
49%5%15%22%9%
2724
June1st to 10thDay
37%419%28%10%1
2592
11th to 20thDay
38%5%16%28%11%1
2700
21st to endDay
31%6%19%30%13%1
2604
July1st to 10thDay
25%415%36%16%4
2451
11th to 20thDay
23%414%33%21%6%
2445
21st to endDay
22%412%33%26%2
2590
August1st to 10thDay
27%414%30%20%5%
2491
11th to 20thDay
27%312%36%19%3
2432
21st to endDay
26%310%35%24%2
2376
September1st to 10thDay
32%312%31%18%4
2160
11th to 20thDay
33%414%31%16%2
2106
21st to endDay
35%412%32%17%1
1980
October1st to 10thDay
43%314%24%15%2
1841
11th to 20thDay
42%313%29%11%1
1720
21st to endDay
43%5%19%24%9%1
1782
November1st to 10thDay
47%5%11%24%11%2
1673
11th to 20thDay
44%320%23%9%1
1536
21st to endDay
49%422%18%7%
1485
December1st to 10thDay
53%416%21%6%
1330
11th to 20thDay
47%5%26%18%4
1680
21st to endDay
42%211%30%13%1
2706
January1st to 10thNight
46%7%18%20%9%1
2647
11th to 20thNight
49%6%18%21%5%1
2549
21st to endNight
55%7%15%16%7%1
2772
February1st to 10thNight
53%5%14%20%8%
2475
11th to 20thNight
50%5%15%20%9%1
2223
21st to endNight
44%414%25%12%1
1776
March1st to 10thNight
50%417%22%7%
2160
11th to 20thNight
44%313%27%13%
2116
21st to endNight
48%417%22%8%1
1980
April1st to 10thNight
42%5%13%24%15%2
1800
11th to 20thNight
50%311%23%12%2
1756
21st to endNight
42%412%27%14%1
1590
May1st to 10thNight
38%412%24%18%3
1440
11th to 20thNight
41%413%26%15%1
1418
21st to endNight
41%6%15%29%9%2
1552
June1st to 10thNight
34%416%29%15%2
1364
11th to 20thNight
35%7%18%25%13%2
1260
21st to endNight
27%5%19%33%14%2
1349
July1st to 10thNight
21%417%37%18%4
1400
11th to 20thNight
25%414%30%20%6%
1392
21st to endNight
18%213%35%26%5%
1488
August1st to 10thNight
26%315%31%19%6%
1424
11th to 20thNight
24%314%35%21%4
1468
21st to endNight
22%310%34%26%5%
1980
September1st to 10thNight
26%310%33%23%5%
1800
11th to 20thNight
28%414%29%22%4
1854
21st to endNight
35%310%31%19%2
1980
October1st to 10thNight
37%414%27%15%3
2115
11th to 20thNight
39%313%29%15%1
2222
21st to endNight
40%5%16%27%11%2
2574
November1st to 10thNight
45%412%23%14%3
2405
11th to 20thNight
45%418%25%7%2
2641
21st to endNight
44%420%24%8%
2691
December1st to 10thNight
50%5%18%21%6%
2850
11th to 20thNight
49%419%22%6%
3516
21st to endNight
42%311%29%13%2
4955

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
9%6%38%29%12%5%
270
11th to 20thDay
10%10%35%30%15%1
246
21st to endDay
12%13%32%33%10%
264
February1st to 10thDay
8%8%43%35%5%
240
11th to 20thDay
14%6%43%30%7%1
240
21st to endDay
9%9%36%33%12%1
198
March1st to 10thDay
18%9%35%25%10%2
240
11th to 20thDay
14%12%32%27%13%3
278
21st to endDay
13%9%36%26%14%2
462
April1st to 10thDay
11%17%38%27%7%
420
11th to 20thDay
23%13%34%21%8%1
432
21st to endDay
18%12%28%27%13%2
600
May1st to 10thDay
15%7%30%29%17%3
603
11th to 20thDay
18%8%26%25%19%5%
625
21st to endDay
18%7%26%23%19%6%
679
June1st to 10thDay
13%9%27%29%18%4
630
11th to 20thDay
11%11%35%28%12%2
630
21st to endDay
9%7%24%36%19%5%
630
July1st to 10thDay
6%5%26%38%22%3
621
11th to 20thDay
6%6%27%40%17%3
618
21st to endDay
45%28%36%23%5%
684
August1st to 10thDay
7%5%24%35%23%6%
626
11th to 20thDay
8%5%24%35%23%5%
610
21st to endDay
8%6%26%32%20%7%
495
September1st to 10thDay
11%9%31%34%13%2
450
11th to 20thDay
16%8%24%27%23%1
396
21st to endDay
21%8%28%27%12%3
270
October1st to 10thDay
24%6%28%24%14%4
270
11th to 20thDay
14%10%35%23%14%4
270
21st to endDay
17%11%42%23%6%1
297
November1st to 10thDay
23%9%29%24%14%1
270
11th to 20thDay
9%13%33%35%10%1
270
21st to endDay
14%8%40%28%10%
270
December1st to 10thDay
11%9%40%27%11%2
270
11th to 20thDay
13%11%43%26%5%2
270
21st to endDay
9%6%40%29%13%3
297
January1st to 10thNight
8%8%38%34%10%1
950
11th to 20thNight
10%10%33%37%9%1
911
21st to endNight
10%12%37%31%9%1
990
February1st to 10thNight
12%8%35%33%10%1
855
11th to 20thNight
12%6%40%29%12%2
704
21st to endNight
9%5%26%40%16%3
592
March1st to 10thNight
9%11%35%34%10%1
720
11th to 20thNight
11%6%39%33%11%1
716
21st to endNight
13%9%30%36%13%
594
April1st to 10thNight
14%9%35%35%7%
540
11th to 20thNight
19%9%35%30%7%
533
21st to endNight
16%7%33%33%11%
540
May1st to 10thNight
13%429%38%15%1
540
11th to 20thNight
16%5%25%36%17%1
531
21st to endNight
18%5%24%36%15%1
584
June1st to 10thNight
12%6%31%36%14%1
540
11th to 20thNight
12%5%37%36%11%
540
21st to endNight
7%5%24%46%17%1
540
July1st to 10thNight
7%226%46%17%
525
11th to 20thNight
5%430%47%13%
522
21st to endNight
4328%49%16%1
591
August1st to 10thNight
6%426%48%15%1
537
11th to 20thNight
8%324%44%20%1
534
21st to endNight
6%426%44%19%1
594
September1st to 10thNight
9%429%41%14%2
540
11th to 20thNight
9%429%43%14%1
540
21st to endNight
14%6%29%38%11%1
540
October1st to 10thNight
16%8%28%35%13%
675
11th to 20thNight
15%9%26%36%14%
782
21st to endNight
14%8%35%32%11%1
990
November1st to 10thNight
15%9%33%36%7%1
925
11th to 20thNight
12%8%39%30%9%1
950
21st to endNight
9%9%37%36%7%1
950
December1st to 10thNight
9%7%36%36%10%2
950
11th to 20thNight
11%11%33%35%9%1
950
21st to endNight
15%8%33%32%12%
1045
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