STAR
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the STAR clear sky chart from 2005-11-05 to 2017-04-02.

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
60%
15343
21%
45350
17%
15302
65%
15302
30%
28524

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
46%
719
19%
2327
15%
716
54%
716
24%
1458
January16th to endNight
44%
768
24%
2403
18%
768
53%
768
29%
1461
February1st to 15thNight
47%
701
24%
2104
17%
699
55%
699
34%
1244
February16th to endNight
47%
631
24%
1853
15%
631
56%
631
34%
1091
March1st to 15thNight
50%
720
30%
1980
17%
720
61%
720
40%
1127
March16th to endNight
54%
768
26%
2112
19%
768
59%
768
40%
1222
April1st to 15thNight
58%
666
24%
1833
19%
666
62%
666
42%
1039
April16th to endNight
59%
660
25%
1719
17%
660
64%
660
39%
1015
May1st to 15thNight
74%
495
30%
1485
23%
495
78%
495
45%
796
May16th to endNight
76%
513
23%
1539
19%
513
77%
513
36%
922
June1st to 15thNight
72%
495
12%
1485
12%
495
73%
495
25%
1015
June16th to endNight
73%
495
7%
1485
7%
495
74%
495
24%
1049
July1st to 15thNight
75%
465
5%
1395
5%
465
75%
465
20%
1053
July16th to endNight
76%
522
6%
1469
6%
489
77%
489
20%
1085
August1st to 15thNight
80%
494
5%
1519
6%
491
80%
491
24%
1055
August16th to endNight
67%
642
8%
1867
8%
642
67%
642
19%
1383
September1st to 15thNight
64%
660
7%
1815
7%
660
65%
660
19%
1356
September16th to endNight
63%
657
12%
1815
11%
657
64%
657
19%
1352
October1st to 15thNight
69%
660
25%
1851
25%
660
71%
660
33%
1151
October16th to endNight
56%
704
27%
2112
24%
704
63%
704
33%
1261
November1st to 15thNight
62%
700
31%
2101
25%
700
69%
700
39%
1160
November16th to endNight
52%
720
29%
2245
22%
720
61%
720
33%
1325
December1st to 15thNight
50%
720
25%
2340
21%
720
58%
720
28%
1415
December16th to endNight
55%
768
24%
2496
24%
768
60%
768
29%
1489

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
8%5%26%6%46%6%6%8%36%
45751
Night
7%425%5%35%5%6%9%44%
45557
TransparencyDay
41%9%13%16%13%5%
45452
Night
34%8%16%19%15%5%
45360
SeeingDay
9%313%22%25%26%
5089
Night
9%218%33%26%9%
15350

By Month

Percentage Cloud Cover SE (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
11%5%25%435%46%10%39%
1611
16th to endDay
10%5%344245%5%7%46%
1818
February1st to 15thDay
12%5%244145%5%6%46%
1750
16th to endDay
7%5%25%42445%8%49%
1619
March1st to 15thDay
5%3134246%5%7%52%
1980
16th to endDay
8%424435%5%5%7%48%
2111
April1st to 15thDay
7%325%5%26%5%6%9%44%
1818
16th to endDay
6%5%244345%6%8%47%
1915
May1st to 15thDay
5%324435%6%6%9%48%
2136
16th to endDay
6%437%7%6%7%7%7%9%33%
2223
June1st to 15thDay
11%6%27%7%5%7%7%8%11%24%
2145
16th to endDay
8%6%48%9%5%7%8%9%9%22%
2145
July1st to 15thDay
10%6%39%10%6%9%7%7%8%19%
2145
16th to endDay
8%6%39%10%5%9%8%7%10%20%
2283
August1st to 15thDay
6%6%47%11%6%8%9%8%10%19%
2096
16th to endDay
10%7%39%9%5%8%7%7%9%21%
1971
September1st to 15thDay
9%7%38%8%5%9%8%8%9%21%
1815
16th to endDay
10%8%37%9%47%7%7%9%23%
1815
October1st to 15thDay
6%6%26%8%47%5%6%8%37%
1777
16th to endDay
10%5%25%5%35%347%46%
1760
November1st to 15thDay
6%325%425%45%7%51%
1757
16th to endDay
8%4245%245%6%7%47%
1713
December1st to 15thDay
10%6%25%5%245%6%8%42%
1620
16th to endDay
8%5%24425%6%5%9%45%
1728
January1st to 15thNight
9%6%36%5%35%45%8%41%
2327
16th to endNight
12%5%245%245%5%7%43%
2403
February1st to 15thNight
11%6%2432445%7%49%
2103
16th to endNight
8%435%43335%7%50%
1853
March1st to 15thNight
5%224433448%55%
1980
16th to endNight
6%3134245%5%7%54%
2112
April1st to 15thNight
5%21442346%10%52%
1833
16th to endNight
6%414324448%54%
1719
May1st to 15thNight
421232346%8%57%
1485
16th to endNight
5%324435%46%9%50%
1538
June1st to 15thNight
8%6%345%35%6%8%11%35%
1485
16th to endNight
6%425%6%46%7%8%14%33%
1485
July1st to 15thNight
6%5%35%5%37%7%8%13%31%
1485
16th to endNight
45%35%7%48%7%11%12%29%
1577
August1st to 15thNight
45%15%6%46%9%8%12%35%
1530
16th to endNight
7%436%7%36%7%8%13%30%
1867
September1st to 15thNight
6%6%26%7%5%5%8%8%12%28%
1815
16th to endNight
8%7%38%6%47%6%7%10%29%
1815
October1st to 15thNight
6%425%5%345%6%8%47%
1851
16th to endNight
8%5%25%42335%7%50%
2112
November1st to 15thNight
5%313435%447%54%
2101
16th to endNight
7%425%5%245%5%8%48%
2245
December1st to 15thNight
10%425%5%3435%8%46%
2340
16th to endNight
7%425%436%5%5%8%46%
2496

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
39%7%9%19%18%5%
1612
16th to endDay
35%49%17%24%8%
1818
February1st to 15thDay
36%5%7%15%21%14%
1752
16th to endDay
32%6%11%15%23%10%
1619
March1st to 15thDay
28%6%10%23%21%9%
1980
16th to endDay
32%5%11%20%20%9%
2112
April1st to 15thDay
33%8%12%18%19%7%
1818
16th to endDay
33%6%10%18%23%7%
1915
May1st to 15thDay
29%8%12%23%18%8%
2136
16th to endDay
42%10%12%18%14%1
2227
June1st to 15thDay
49%11%17%17%3
2145
16th to endDay
52%13%19%12%2
2145
July1st to 15thDay
57%15%19%6%1
2006
16th to endDay
53%15%19%9%1
2128
August1st to 15thDay
53%16%22%7%
2083
16th to endDay
54%15%19%9%
1971
September1st to 15thDay
53%16%17%10%2
1815
16th to endDay
52%11%14%12%6%1
1815
October1st to 15thDay
41%8%9%16%15%8%
1777
16th to endDay
37%5%9%14%20%12%
1760
November1st to 15thDay
30%5%8%22%23%10%
1757
16th to endDay
35%6%8%18%17%13%
1713
December1st to 15thDay
39%7%9%20%16%6%
1620
16th to endDay
34%6%12%19%20%7%
1728
January1st to 15thNight
40%5%11%20%16%5%
2327
16th to endNight
38%310%18%19%8%
2403
February1st to 15thNight
33%48%17%22%13%
2112
16th to endNight
32%5%14%17%22%7%
1853
March1st to 15thNight
27%5%10%27%20%7%
1980
16th to endNight
27%5%12%24%21%8%
2112
April1st to 15thNight
26%5%17%23%19%6%
1833
16th to endNight
28%5%12%22%23%7%
1719
May1st to 15thNight
22%6%15%28%20%6%
1486
16th to endNight
28%9%18%24%17%1
1540
June1st to 15thNight
39%11%22%19%6%
1485
16th to endNight
39%14%25%15%4
1485
July1st to 15thNight
41%17%30%7%1
1395
16th to endNight
39%19%28%12%
1469
August1st to 15thNight
34%17%36%10%
1519
16th to endNight
42%17%24%13%1
1867
September1st to 15thNight
44%16%23%13%2
1815
16th to endNight
46%13%17%14%5%2
1815
October1st to 15thNight
33%8%12%19%19%7%
1851
16th to endNight
34%5%9%15%24%10%
2112
November1st to 15thNight
28%411%21%23%9%
2101
16th to endNight
33%5%9%20%17%12%
2245
December1st to 15thNight
36%6%11%23%16%5%
2340
16th to endNight
33%7%12%20%17%7%
2496

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
13%15%31%23%15%1
90
16th to endDay
14%16%30%25%11%2
96
February1st to 15thDay
14%13%24%24%16%6%
90
16th to endDay
11%6%38%29%14%
120
March1st to 15thDay
46%32%35%18%2
270
16th to endDay
11%5%20%31%26%4
288
April1st to 15thDay
7%5%20%30%28%8%
214
16th to endDay
9%5%15%24%32%13%
210
May1st to 15thDay
7%10%25%34%20%
228
16th to endDay
7%9%17%29%35%
267
June1st to 15thDay
11%1312%27%43%
255
16th to endDay
6%37%27%55%
255
July1st to 15thDay
8%6%18%67%
245
16th to endDay
10%2311%21%52%
270
August1st to 15thDay
8%311%19%56%
253
16th to endDay
8%15%11%22%51%
268
September1st to 15thDay
12%1316%23%41%
255
16th to endDay
10%215%33%37%
254
October1st to 15thDay
6%110%30%37%12%
255
16th to endDay
11%119%30%27%9%
272
November1st to 15thDay
9%116%33%29%9%
265
16th to endDay
6%230%33%21%4
183
December1st to 15thDay
8%5%36%33%15%
90
16th to endDay
5%8%12%47%22%3
96
January1st to 15thNight
11%7%25%38%16%
720
16th to endNight
14%6%28%35%13%
768
February1st to 15thNight
14%430%35%13%
704
16th to endNight
9%5%30%43%10%
631
March1st to 15thNight
5%434%42%12%
720
16th to endNight
7%428%41%17%
768
April1st to 15thNight
6%325%44%19%
666
16th to endNight
8%322%44%19%
660
May1st to 15thNight
5%15%44%30%3
496
16th to endNight
6%19%41%35%6%
515
June1st to 15thNight
8%1424%45%15%
495
16th to endNight
5%315%42%32%
495
July1st to 15thNight
8%18%46%33%
465
16th to endNight
5%29%43%39%
522
August1st to 15thNight
5%17%47%38%
494
16th to endNight
10%5%15%38%29%
642
September1st to 15thNight
9%7%22%39%20%
660
16th to endNight
11%7%28%37%13%
657
October1st to 15thNight
7%110%42%33%5%
660
16th to endNight
12%120%33%29%1
704
November1st to 15thNight
6%222%42%22%3
700
16th to endNight
8%327%42%17%1
720
December1st to 15thNight
11%426%36%21%
720
16th to endNight
8%323%42%20%1
768
Page generated at 2017-04-03 01:35:07. Version ac04

Copyright 2017 A. Danko


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