Basin
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Basin clear sky chart from 2005-11-05 to 2017-10-06.

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
61%
16192
19%
47105
16%
16154
64%
16154
32%
29071

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
58%
895
14%
2506
12%
895
60%
895
26%
1699
January16th to endNight
58%
822
16%
2550
14%
822
60%
822
26%
1645
February1st to 15thNight
55%
698
11%
2288
11%
698
58%
698
23%
1507
February16th to endNight
52%
632
11%
2004
11%
632
54%
632
22%
1349
March1st to 15thNight
60%
720
15%
2025
11%
720
63%
720
32%
1276
March16th to endNight
54%
750
14%
2094
11%
750
56%
750
27%
1414
April1st to 15thNight
49%
539
14%
1798
12%
539
52%
539
22%
1174
April16th to endNight
51%
540
16%
1689
14%
540
53%
540
24%
1103
May1st to 15thNight
55%
540
17%
1499
14%
540
56%
540
29%
980
May16th to endNight
60%
563
13%
1498
11%
561
61%
561
26%
1032
June1st to 15thNight
56%
540
12%
1440
10%
540
57%
540
31%
951
June16th to endNight
69%
540
19%
1440
14%
540
72%
540
53%
767
July1st to 15thNight
75%
510
18%
1360
14%
510
77%
510
50%
733
July16th to endNight
80%
564
28%
1440
26%
531
81%
531
60%
705
August1st to 15thNight
79%
539
26%
1611
25%
536
81%
536
49%
799
August16th to endNight
75%
573
32%
1806
31%
573
76%
573
48%
873
September1st to 15thNight
70%
540
27%
1800
28%
540
75%
540
39%
927
September16th to endNight
70%
664
30%
1979
27%
664
73%
664
45%
1037
October1st to 15thNight
55%
684
21%
2052
16%
684
57%
684
29%
1299
October16th to endNight
67%
704
26%
2112
21%
704
71%
704
37%
1210
November1st to 15thNight
61%
875
23%
2386
18%
875
63%
875
35%
1484
November16th to endNight
59%
900
20%
2520
17%
900
62%
900
33%
1573
December1st to 15thNight
59%
900
12%
2520
9%
900
60%
900
26%
1725
December16th to endNight
60%
960
16%
2688
12%
960
62%
960
27%
1809

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
45%25%5%345%6%9%47%
48114
Night
5%6%35%5%35%5%6%8%45%
47277
Cloud CoverDay
5%5%244345%6%8%48%
48114
Night
6%6%25%5%345%6%8%45%
47279
TransparencyDay
30%7%17%24%14%5%
47790
Night
34%7%17%24%12%4
47105
SeeingDay
8%418%35%26%6%
6080
Night
8%217%48%20%1
16203

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
16%26%7%48%9%10%11%30%
1433
16th to endDay
28%37%7%35%45%9%40%
1686
February1st to 15thDay
26%5%9%9%5%5%7%7%9%31%
1575
16th to endDay
36%47%6%5%5%7%5%9%37%
1470
March1st to 15thDay
25%245%236%6%11%47%
1935
16th to endDay
5%7%35%6%45%5%6%10%40%
2129
April1st to 15thDay
5%7%345%35%6%6%10%42%
2150
16th to endDay
7%8%35%5%346%7%9%38%
2268
May1st to 15thDay
6%7%26%5%345%5%7%45%
2451
16th to endDay
9%5%25%5%35%6%7%10%36%
2616
June1st to 15thDay
6%4145%35%5%8%9%45%
2520
16th to endDay
221232446%8%62%
2520
July1st to 15thDay
2212425%47%11%54%
2520
16th to endDay
21133145%6%9%59%
2628
August1st to 15thDay
33133245%7%10%54%
2340
16th to endDay
321332445%6%62%
2396
September1st to 15thDay
6%31232235%7%61%
2160
16th to endDay
5%413223346%61%
1979
October1st to 15thDay
10%5%25%324346%49%
1705
16th to endDay
5%5%16%4345%5%7%50%
1747
November1st to 15thDay
5%325%425%46%8%52%
1470
16th to endDay
36%35%5%47%5%5%9%43%
1440
December1st to 15thDay
37%27%8%47%7%7%11%31%
1440
16th to endDay
28%47%8%37%7%9%10%31%
1536
January1st to 15thNight
36%37%8%46%7%7%10%35%
2506
16th to endNight
49%37%6%35%5%6%10%36%
2550
February1st to 15thNight
48%5%7%7%46%6%5%9%33%
2277
16th to endNight
27%49%8%46%6%6%8%35%
2004
March1st to 15thNight
37%36%6%25%5%7%9%42%
2025
16th to endNight
8%8%35%5%36%5%7%9%37%
2091
April1st to 15thNight
7%9%36%5%35%5%6%8%39%
1798
16th to endNight
8%8%26%6%35%46%8%37%
1689
May1st to 15thNight
8%6%36%5%34337%45%
1498
16th to endNight
11%5%25%6%45%6%6%9%37%
1498
June1st to 15thNight
8%5%26%435%6%6%7%43%
1440
16th to endNight
321241345%7%62%
1440
July1st to 15thNight
21124246%6%11%56%
1440
16th to endNight
11122235%5%7%66%
1538
August1st to 15thNight
21133245%8%10%56%
1620
16th to endNight
3213423348%61%
1806
September1st to 15thNight
6%314424336%59%
1800
16th to endNight
6%313312346%61%
1979
October1st to 15thNight
12%7%25%5%23447%44%
2052
16th to endNight
5%5%25%43445%7%52%
2112
November1st to 15thNight
45%35%5%345%5%6%50%
2386
16th to endNight
47%35%5%35%46%8%45%
2520
December1st to 15thNight
27%47%7%46%6%7%9%36%
2520
16th to endNight
37%47%6%36%6%6%11%34%
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
26%36%7%46%8%10%11%31%
1432
16th to endDay
38%36%7%3446%7%43%
1686
February1st to 15thDay
36%5%9%7%47%6%7%10%32%
1575
16th to endDay
36%47%6%36%6%6%9%39%
1470
March1st to 15thDay
26%244245%7%10%49%
1935
16th to endDay
7%6%25%5%446%6%9%40%
2130
April1st to 15thDay
6%6%345%35%5%5%9%43%
2150
16th to endDay
7%8%35%5%35%5%6%9%39%
2268
May1st to 15thDay
8%6%25%5%25%45%7%46%
2451
16th to endDay
10%6%24435%6%8%10%37%
2616
June1st to 15thDay
7%423435%5%7%9%46%
2520
16th to endDay
221332445%7%63%
2520
July1st to 15thDay
221332447%11%56%
2520
16th to endDay
21123235%6%8%61%
2628
August1st to 15thDay
321332446%10%55%
2340
16th to endDay
321232435%6%63%
2396
September1st to 15thDay
7%21221235%6%61%
2160
16th to endDay
6%313223346%62%
1979
October1st to 15thDay
12%425%423346%49%
1705
16th to endDay
5%5%25%5%2445%7%51%
1747
November1st to 15thDay
5%425%42345%8%53%
1470
16th to endDay
45%25%5%35%5%5%8%44%
1440
December1st to 15thDay
38%38%6%46%6%7%10%34%
1440
16th to endDay
28%46%7%47%6%7%10%33%
1536
January1st to 15thNight
46%47%6%46%6%7%9%35%
2506
16th to endNight
5%9%37%5%35%5%6%10%36%
2550
February1st to 15thNight
5%8%5%7%6%46%5%6%9%34%
2277
16th to endNight
38%5%9%7%46%6%6%8%34%
2004
March1st to 15thNight
46%36%5%35%5%7%9%42%
2025
16th to endNight
9%7%35%5%35%6%6%8%38%
2094
April1st to 15thNight
8%8%26%6%25%5%6%7%40%
1798
16th to endNight
9%7%36%6%25%5%6%7%38%
1689
May1st to 15thNight
9%7%26%5%34335%47%
1498
16th to endNight
12%4246%445%7%9%38%
1497
June1st to 15thNight
9%5%25%5%25%6%5%7%44%
1440
16th to endNight
3212324348%62%
1440
July1st to 15thNight
21123246%6%11%56%
1440
16th to endNight
211222336%7%67%
1538
August1st to 15thNight
3223335%7%10%57%
1620
16th to endNight
4213313347%62%
1806
September1st to 15thNight
6%313423345%59%
1800
16th to endNight
8%312412446%61%
1979
October1st to 15thNight
13%6%34433446%45%
2052
16th to endNight
6%5%2442445%7%52%
2112
November1st to 15thNight
5%6%25%4245%5%6%50%
2386
16th to endNight
5%7%35%5%345%5%7%45%
2520
December1st to 15thNight
37%47%7%45%6%6%9%36%
2520
16th to endNight
47%46%6%46%5%6%10%36%
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
38%9%27%20%4
1433
16th to endDay
38%7%23%23%5%1
1686
February1st to 15thDay
42%8%21%22%4
1575
16th to endDay
38%8%24%19%7%1
1470
March1st to 15thDay
27%6%24%32%7%
1935
16th to endDay
37%8%17%29%5%1
2130
April1st to 15thDay
35%7%19%26%10%
2150
16th to endDay
39%8%17%24%8%
2268
May1st to 15thDay
36%7%14%27%11%1
2451
16th to endDay
38%10%19%21%9%
2620
June1st to 15thDay
30%9%19%25%13%2
2520
16th to endDay
18%412%27%23%13%
2520
July1st to 15thDay
21%5%10%24%24%13%
2371
16th to endDay
17%5%9%18%27%21%
2463
August1st to 15thDay
21%5%10%26%21%14%
2325
16th to endDay
19%48%19%30%16%
2396
September1st to 15thDay
22%5%9%24%26%11%
2160
16th to endDay
22%413%30%23%4
1979
October1st to 15thDay
35%6%18%25%11%2
1705
16th to endDay
31%5%18%30%11%2
1747
November1st to 15thDay
28%7%17%26%17%2
1470
16th to endDay
34%7%24%24%8%
1440
December1st to 15thDay
41%10%24%19%4
1440
16th to endDay
39%9%24%19%5%1
1536
January1st to 15thNight
39%10%23%21%4
2506
16th to endNight
39%7%22%23%6%1
2550
February1st to 15thNight
42%8%21%20%6%
2288
16th to endNight
44%7%20%18%7%1
2004
March1st to 15thNight
34%8%22%25%8%1
2025
16th to endNight
40%8%18%24%7%
2094
April1st to 15thNight
41%8%17%22%8%1
1798
16th to endNight
41%7%16%24%9%
1689
May1st to 15thNight
38%7%13%24%14%1
1499
16th to endNight
40%10%18%20%10%1
1498
June1st to 15thNight
36%7%17%22%13%2
1440
16th to endNight
19%5%12%26%25%10%
1440
July1st to 15thNight
19%7%13%25%23%9%
1360
16th to endNight
14%5%9%20%28%21%
1440
August1st to 15thNight
18%6%12%25%23%13%
1611
16th to endNight
21%38%22%26%17%
1806
September1st to 15thNight
26%49%20%25%13%
1800
16th to endNight
23%411%26%26%6%
1979
October1st to 15thNight
39%5%13%26%12%3
2052
16th to endNight
30%5%16%31%13%3
2112
November1st to 15thNight
32%6%15%26%14%2
2386
16th to endNight
35%6%21%26%9%1
2520
December1st to 15thNight
41%9%21%23%4
2520
16th to endNight
40%8%20%24%41
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
6%2437%28%20%
90
16th to endDay
8%9%45%26%10%
96
February1st to 15thDay
7%413%54%20%
90
16th to endDay
3222%54%14%2
81
March1st to 15thDay
46%29%41%15%2
225
16th to endDay
13%8%29%30%15%1
288
April1st to 15thDay
9%11%34%30%12%1
269
16th to endDay
14%13%28%26%15%2
270
May1st to 15thDay
12%7%18%31%24%5%
408
16th to endDay
12%6%16%29%28%6%
517
June1st to 15thDay
8%319%33%28%6%
495
16th to endDay
4220%39%29%3
495
July1st to 15thDay
4214%32%38%7%
475
16th to endDay
3112%42%34%5%
487
August1st to 15thDay
5%221%34%28%7%
315
16th to endDay
5%412%28%33%16%
332
September1st to 15thDay
9%417%30%28%9%
315
16th to endDay
12%418%26%27%11%
263
October1st to 15thDay
10%216%42%27%
107
16th to endDay
5%220%38%20%12%
96
November1st to 15thDay
6%334%30%25%
90
16th to endDay
13%413%52%16%
90
December1st to 15thDay
4215%55%22%
90
16th to endDay
9%6%59%16%8%
96
January1st to 15thNight
7%218%53%15%2
900
16th to endNight
10%215%56%14%1
822
February1st to 15thNight
8%216%52%19%1
704
16th to endNight
7%121%52%15%2
632
March1st to 15thNight
7%219%50%18%1
720
16th to endNight
13%222%45%14%1
750
April1st to 15thNight
11%425%46%12%
539
16th to endNight
13%422%43%13%2
540
May1st to 15thNight
12%417%43%21%1
540
16th to endNight
13%314%41%26%1
563
June1st to 15thNight
12%320%42%20%1
540
16th to endNight
4119%47%25%
540
July1st to 15thNight
4115%46%31%1
510
16th to endNight
2111%52%30%1
564
August1st to 15thNight
410%50%32%2
539
16th to endNight
5%112%41%37%2
573
September1st to 15thNight
8%212%45%27%4
540
16th to endNight
9%111%40%31%4
664
October1st to 15thNight
15%217%41%20%2
684
16th to endNight
8%115%51%20%2
704
November1st to 15thNight
8%319%48%18%1
875
16th to endNight
7%320%52%15%1
900
December1st to 15thNight
7%118%55%16%1
900
16th to endNight
7%215%57%14%2
960
Page generated at 2017-10-07 12:07:47. Version ac04

Copyright 2017 A. Danko


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