WhistleStop Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the WhistleStop Observatory clear sky chart from 2005-11-05 to 2017-08-12.

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
31%
14641
14%
46520
8%
14609
38%
14609
17%
32377

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
13%
719
7%
2506
3%
716
18%
716
7%
1963
January16th to endNight
18%
768
11%
2528
5%
768
26%
768
9%
1970
February1st to 15thNight
14%
702
9%
2287
3%
701
21%
701
10%
1770
February16th to endNight
19%
631
13%
1999
5%
631
31%
631
13%
1463
March1st to 15thNight
22%
720
14%
2010
6%
720
29%
720
15%
1485
March16th to endNight
29%
768
20%
2094
7%
768
40%
768
22%
1409
April1st to 15thNight
27%
718
16%
1796
4%
718
33%
718
21%
1302
April16th to endNight
32%
618
20%
1698
6%
618
41%
618
23%
1112
May1st to 15thNight
36%
428
16%
1508
11%
428
45%
428
16%
987
May16th to endNight
41%
374
17%
1497
14%
374
53%
374
19%
857
June1st to 15thNight
40%
360
12%
1440
10%
360
51%
360
18%
888
June16th to endNight
49%
360
11%
1440
14%
360
55%
360
19%
860
July1st to 15thNight
58%
340
17%
1360
23%
340
64%
340
29%
695
July16th to endNight
53%
412
16%
1451
17%
387
63%
387
27%
808
August1st to 15thNight
57%
527
16%
1575
12%
524
62%
524
29%
955
August16th to endNight
60%
607
21%
1657
11%
607
65%
607
36%
1001
September1st to 15thNight
51%
660
19%
1650
10%
660
54%
660
31%
1092
September16th to endNight
52%
657
22%
1823
15%
657
56%
657
29%
1168
October1st to 15thNight
36%
660
17%
1980
14%
660
42%
660
20%
1383
October16th to endNight
22%
704
11%
2117
5%
704
30%
704
14%
1585
November1st to 15thNight
34%
700
18%
2376
16%
700
40%
700
17%
1579
November16th to endNight
19%
720
11%
2520
5%
720
27%
720
10%
1878
December1st to 15thNight
13%
720
6%
2520
2%
720
19%
720
8%
2002
December16th to endNight
11%
768
7%
2688
1%
768
17%
768
6%
2165

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
14%7%36%5%35%5%5%7%36%
47451
Night
16%8%36%5%34446%34%
46694
Cloud CoverDay
14%7%36%5%35%5%5%7%36%
47461
Night
16%8%36%5%3445%6%33%
46699
TransparencyDay
46%7%12%21%10%1
47143
Night
48%7%12%19%11%1
46520
SeeingDay
16%13%23%26%14%5%
5268
Night
17%20%23%24%12%1
14645

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 15thDay
18%14%48%6%45%5%46%21%
1432
16th to endDay
12%14%6%10%9%47%5%5%5%19%
1695
February1st to 15thDay
12%12%5%13%9%35%5%6%5%19%
1575
16th to endDay
19%9%5%8%6%34446%25%
1473
March1st to 15thDay
15%10%37%6%35%5%5%6%29%
1936
16th to endDay
16%8%26%433446%40%
2130
April1st to 15thDay
19%6%36%5%24447%33%
2152
16th to endDay
16%5%25%434446%40%
2271
May1st to 15thDay
13%5%36%5%25%5%6%6%38%
2442
16th to endDay
11%5%245%25%5%5%8%43%
2618
June1st to 15thDay
15%5%25%5%345%5%9%36%
2520
16th to endDay
12%425%5%2446%8%43%
2520
July1st to 15thDay
7%214435%5%6%10%48%
2520
16th to endDay
8%324435%6%8%10%41%
2615
August1st to 15thDay
7%4245%25%6%6%8%46%
2288
16th to endDay
10%5%134245%6%9%47%
2180
September1st to 15thDay
14%5%2442345%7%44%
1980
16th to endDay
12%5%2343345%7%45%
1804
October1st to 15thDay
17%7%35%5%44447%35%
1650
16th to endDay
22%9%35%5%3346%5%28%
1755
November1st to 15thDay
13%7%26%6%445%5%7%35%
1479
16th to endDay
19%11%47%7%45%447%25%
1440
December1st to 15thDay
18%15%5%8%6%36%5%47%19%
1440
16th to endDay
21%14%49%6%35%5%45%18%
1536
January1st to 15thNight
21%14%5%8%6%34446%19%
2506
16th to endNight
15%15%5%9%7%35%5%45%22%
2528
February1st to 15thNight
15%14%6%9%6%35%446%22%
2285
16th to endNight
16%8%46%8%45%5%46%28%
1999
March1st to 15thNight
19%9%36%6%34345%32%
1998
16th to endNight
15%5%35%423445%44%
2094
April1st to 15thNight
20%7%3443445%6%35%
1796
16th to endNight
15%5%2442345%6%44%
1698
May1st to 15thNight
15%5%245%45%5%5%6%37%
1508
16th to endNight
12%5%26%424347%46%
1495
June1st to 15thNight
13%5%35%5%2446%8%39%
1440
16th to endNight
12%425%43446%8%42%
1440
July1st to 15thNight
5%31443445%8%53%
1440
16th to endNight
8%5%2442445%9%48%
1550
August1st to 15thNight
8%4245%345%5%9%47%
1586
16th to endNight
7%415%325%446%52%
1657
September1st to 15thNight
14%424423447%45%
1650
16th to endNight
15%424424345%47%
1823
October1st to 15thNight
20%7%25%5%23446%36%
1980
16th to endNight
21%8%46%5%25%45%6%28%
2117
November1st to 15thNight
16%5%35%5%3445%7%36%
2376
16th to endNight
24%9%36%5%345%47%25%
2520
December1st to 15thNight
22%12%46%5%45%45%6%21%
2520
16th to endNight
21%14%6%8%6%35%445%18%
2688

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
18%13%5%8%7%45%6%36%19%
1432
16th to endDay
12%14%6%10%8%47%6%46%17%
1695
February1st to 15thDay
12%12%6%12%8%35%5%6%6%19%
1575
16th to endDay
18%9%49%6%45%5%5%6%24%
1473
March1st to 15thDay
16%10%37%6%35%5%5%5%29%
1947
16th to endDay
15%7%26%434446%40%
2129
April1st to 15thDay
19%6%35%5%335%48%34%
2152
16th to endDay
16%5%35%5%3445%6%40%
2271
May1st to 15thDay
13%6%36%5%25%6%6%7%37%
2442
16th to endDay
11%5%245%25%5%5%7%43%
2618
June1st to 15thDay
15%5%35%5%345%5%9%36%
2520
16th to endDay
11%4245%245%5%9%43%
2520
July1st to 15thDay
6%324435%5%6%9%48%
2520
16th to endDay
8%323436%6%7%10%42%
2615
August1st to 15thDay
7%424425%7%6%8%46%
2288
16th to endDay
10%5%234245%5%9%48%
2180
September1st to 15thDay
13%5%2443345%6%45%
1980
16th to endDay
12%5%2443346%7%45%
1804
October1st to 15thDay
16%8%25%5%34446%36%
1650
16th to endDay
22%8%35%6%335%5%5%28%
1755
November1st to 15thDay
13%7%46%5%345%6%7%35%
1479
16th to endDay
18%11%5%7%6%35%5%47%24%
1440
December1st to 15thDay
18%14%6%8%7%36%5%6%6%17%
1440
16th to endDay
21%14%5%7%7%36%45%5%17%
1536
January1st to 15thNight
21%14%5%8%7%3445%5%18%
2506
16th to endNight
15%15%6%9%7%35%5%5%6%20%
2528
February1st to 15thNight
14%16%5%9%6%45%447%21%
2278
16th to endNight
16%9%47%8%45%46%6%27%
1999
March1st to 15thNight
19%10%36%6%34445%31%
2010
16th to endNight
15%6%36%423435%43%
2094
April1st to 15thNight
19%8%345%2445%6%35%
1796
16th to endNight
15%6%2442446%5%44%
1698
May1st to 15thNight
15%5%25%5%446%5%6%37%
1508
16th to endNight
11%5%25%4335%37%46%
1495
June1st to 15thNight
13%6%27%425%5%5%8%38%
1440
16th to endNight
11%5%25%5%345%6%7%42%
1440
July1st to 15thNight
5%313425%5%5%8%53%
1440
16th to endNight
8%4244245%5%8%49%
1550
August1st to 15thNight
8%415%43447%8%46%
1586
16th to endNight
8%314425%45%7%51%
1657
September1st to 15thNight
14%42442345%7%45%
1650
16th to endNight
15%5%25%423446%46%
1823
October1st to 15thNight
19%7%35%6%34446%35%
1980
16th to endNight
22%7%46%5%35%45%6%27%
2117
November1st to 15thNight
16%6%25%5%35%46%7%35%
2376
16th to endNight
23%10%46%6%25%45%7%24%
2520
December1st to 15thNight
22%13%47%5%45%46%6%19%
2520
16th to endNight
21%13%6%9%7%35%446%17%
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
60%6%11%14%5%
1433
16th to endDay
65%6%9%11%5%
1695
February1st to 15thDay
61%8%10%15%3
1577
16th to endDay
57%6%10%14%9%
1473
March1st to 15thDay
54%6%14%17%7%1
1947
16th to endDay
45%5%8%21%15%2
2130
April1st to 15thDay
50%49%19%13%1
2152
16th to endDay
44%5%9%22%16%2
2271
May1st to 15thDay
44%8%10%20%14%1
2442
16th to endDay
39%7%13%23%13%1
2622
June1st to 15thDay
45%6%13%23%10%
2520
16th to endDay
37%8%16%22%14%
2520
July1st to 15thDay
30%8%12%32%15%
2371
16th to endDay
35%11%15%26%10%
2451
August1st to 15thDay
34%10%16%23%13%1
2275
16th to endDay
32%8%14%31%11%1
2180
September1st to 15thDay
37%7%14%28%11%
1980
16th to endDay
37%7%12%30%10%
1804
October1st to 15thDay
47%7%11%22%10%
1650
16th to endDay
56%7%14%14%6%
1755
November1st to 15thDay
46%6%16%18%10%1
1479
16th to endDay
57%8%13%13%6%
1440
December1st to 15thDay
62%9%14%11%2
1440
16th to endDay
64%8%11%12%3
1536
January1st to 15thNight
63%6%11%13%4
2506
16th to endNight
61%6%10%14%6%
2528
February1st to 15thNight
60%6%12%14%4
2287
16th to endNight
53%6%12%19%7%
1999
March1st to 15thNight
53%5%11%20%8%
2010
16th to endNight
43%5%9%22%17%2
2094
April1st to 15thNight
47%6%9%18%15%1
1796
16th to endNight
39%6%11%19%20%2
1698
May1st to 15thNight
44%7%10%19%15%3
1508
16th to endNight
36%6%12%24%17%2
1497
June1st to 15thNight
42%6%12%22%15%1
1440
16th to endNight
38%7%14%21%16%2
1440
July1st to 15thNight
27%7%11%28%24%
1360
16th to endNight
32%6%12%28%17%2
1451
August1st to 15thNight
31%8%17%27%14%
1575
16th to endNight
30%6%13%32%16%1
1657
September1st to 15thNight
36%7%15%26%13%
1650
16th to endNight
38%49%30%15%
1823
October1st to 15thNight
48%5%10%21%13%
1980
16th to endNight
54%7%13%14%8%
2117
November1st to 15thNight
44%7%14%19%13%1
2376
16th to endNight
55%8%14%14%5%
2520
December1st to 15thNight
60%9%13%11%3
2520
16th to endNight
64%8%11%10%4
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
20%16%41%15%33
60
16th to endDay
27%14%30%23%4
91
February1st to 15thDay
20%26%44%7%1
90
16th to endDay
30%18%28%18%3
132
March1st to 15thDay
20%21%27%23%5%
270
16th to endDay
22%17%28%23%5%1
288
April1st to 15thDay
19%18%30%22%7%1
270
16th to endDay
17%15%31%19%10%4
270
May1st to 15thDay
22%14%20%25%10%6%
288
16th to endDay
14%12%24%29%14%4
331
June1st to 15thDay
17%14%20%29%15%2
315
16th to endDay
15%12%22%29%13%6%
315
July1st to 15thDay
9%9%13%32%27%8%
305
16th to endDay
10%11%19%33%17%7%
326
August1st to 15thDay
8%7%13%30%27%11%
297
16th to endDay
11%8%13%27%26%11%
268
September1st to 15thDay
12%7%18%23%25%12%
255
16th to endDay
14%8%20%29%18%9%
254
October1st to 15thDay
16%12%25%25%15%5%
255
16th to endDay
22%17%29%23%6%
271
November1st to 15thDay
15%14%29%27%11%2
133
16th to endDay
25%11%20%30%13%
60
December1st to 15thDay
20%26%21%20%11%
60
16th to endDay
28%34%31%6%
64
January1st to 15thNight
21%29%26%15%5%
720
16th to endNight
17%26%28%21%4
768
February1st to 15thNight
20%25%33%18%2
704
16th to endNight
19%23%29%22%4
631
March1st to 15thNight
20%25%25%24%3
720
16th to endNight
17%21%26%24%8%1
768
April1st to 15thNight
18%21%27%24%7%
718
16th to endNight
13%22%26%22%12%1
618
May1st to 15thNight
18%19%17%27%14%1
428
16th to endNight
11%18%21%28%18%
375
June1st to 15thNight
12%18%23%30%13%2
360
16th to endNight
10%17%15%36%19%
360
July1st to 15thNight
5%12%17%38%22%3
340
16th to endNight
6%17%15%36%21%1
412
August1st to 15thNight
9%11%12%33%29%4
527
16th to endNight
7%10%14%32%31%3
607
September1st to 15thNight
13%11%15%25%28%4
660
16th to endNight
13%12%17%24%25%6%
657
October1st to 15thNight
20%14%23%23%16%1
660
16th to endNight
23%17%29%24%5%
704
November1st to 15thNight
15%21%23%26%11%1
700
16th to endNight
22%26%24%18%7%
720
December1st to 15thNight
25%27%28%14%4
720
16th to endNight
24%25%30%15%4
768
Page generated at 2017-08-13 14:16:19. Version ac04

Copyright 2017 A. Danko


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