Cowles
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cowles clear sky chart from 2005-11-05 to 2017-09-22.

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
42%
15754
28%
46956
14%
15708
55%
15708
33%
26033

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
34%
788
30%
2395
14%
784
52%
784
39%
1201
January16th to endNight
30%
768
33%
2496
16%
768
51%
768
33%
1272
February1st to 15thNight
29%
703
26%
2199
12%
700
46%
700
30%
1193
February16th to endNight
19%
632
34%
1894
9%
632
44%
632
29%
1039
March1st to 15thNight
34%
720
29%
2064
11%
720
48%
720
33%
1163
March16th to endNight
30%
704
32%
2048
12%
704
47%
704
36%
1077
April1st to 15thNight
26%
539
28%
1798
10%
539
49%
539
25%
944
April16th to endNight
26%
540
30%
1800
13%
540
51%
540
26%
941
May1st to 15thNight
33%
540
23%
1796
12%
539
50%
539
29%
968
May16th to endNight
41%
563
28%
1529
13%
561
56%
561
34%
877
June1st to 15thNight
51%
540
22%
1440
13%
540
58%
540
43%
817
June16th to endNight
59%
540
22%
1440
15%
540
65%
540
40%
841
July1st to 15thNight
69%
510
14%
1385
12%
510
70%
510
30%
913
July16th to endNight
70%
564
11%
1593
11%
531
70%
531
26%
1017
August1st to 15thNight
72%
539
12%
1725
14%
536
72%
536
23%
1136
August16th to endNight
68%
573
18%
1910
18%
573
71%
573
26%
1185
September1st to 15thNight
64%
540
20%
1800
18%
540
68%
540
26%
1112
September16th to endNight
61%
513
34%
1831
29%
513
72%
513
36%
912
October1st to 15thNight
44%
660
33%
1980
17%
660
58%
660
40%
1020
October16th to endNight
47%
704
36%
2112
16%
704
61%
704
45%
1027
November1st to 15thNight
45%
814
35%
2214
16%
814
58%
814
49%
1103
November16th to endNight
38%
900
36%
2340
14%
900
53%
900
44%
1264
December1st to 15thNight
28%
900
28%
2479
8%
900
45%
900
32%
1434
December16th to endNight
25%
960
28%
2688
10%
960
41%
960
30%
1577

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%3445%6%50%
47924
Night
342442445%7%55%
47144
Cloud CoverDay
5%6%25%5%35%5%5%6%48%
47922
Night
34245%35%5%5%7%53%
47144
TransparencyDay
31%6%10%19%22%9%
47613
Night
25%7%11%21%24%10%
46957
SeeingDay
13%18%29%25%10%2
5619
Night
8%15%28%27%16%2
15756

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 15thDay
132322335%8%63%
1554
16th to endDay
25%26%5%2333459%
1728
February1st to 15thDay
5%35%5%2436%6%55%
1660
16th to endDay
17%35%43445%6%53%
1580
March1st to 15thDay
28%35%6%23346%53%
1893
16th to endDay
36%35%323446%56%
2175
April1st to 15thDay
36%26%43435%7%51%
2150
16th to endDay
48%35%5%23346%52%
2160
May1st to 15thDay
7%7%345%35%445%48%
2152
16th to endDay
5%414425%45%6%55%
2582
June1st to 15thDay
3224425%5%5%6%57%
2520
16th to endDay
3214435%6%7%8%52%
2520
July1st to 15thDay
7%6%37%7%37%7%7%8%31%
2490
16th to endDay
9%6%39%8%5%9%7%7%8%25%
2467
August1st to 15thDay
8%6%47%9%5%7%6%6%7%29%
2225
16th to endDay
9%7%37%7%46%6%7%6%33%
2292
September1st to 15thDay
11%7%37%7%35%5%5%7%35%
2160
16th to endDay
4425%4435%45%56%
1938
October1st to 15thDay
8%424423345%57%
1650
16th to endDay
441332223467%
1760
November1st to 15thDay
13143235%5%7%61%
1633
16th to endDay
4424313345%62%
1620
December1st to 15thDay
242332345%6%58%
1479
16th to endDay
17%35%6%3445%7%49%
1536
January1st to 15thNight
1313322247%65%
2395
16th to endNight
341242446%6%59%
2496
February1st to 15thNight
16%25%32345%7%57%
2189
16th to endNight
16%1443435%7%58%
1894
March1st to 15thNight
15%245%2446%7%55%
2064
16th to endNight
4314323337%62%
2048
April1st to 15thNight
25%2442445%9%55%
1798
16th to endNight
3424434245%60%
1800
May1st to 15thNight
45%24424446%54%
1796
16th to endNight
5%32332446%6%58%
1528
June1st to 15thNight
12145%35%6%6%7%56%
1440
16th to endNight
2223436%6%6%8%52%
1440
July1st to 15thNight
5%325%5%5%7%8%7%10%37%
1467
16th to endNight
5%425%5%37%9%8%10%35%
1701
August1st to 15thNight
5%425%6%39%8%8%9%35%
1734
16th to endNight
6%427%6%36%7%9%11%37%
1910
September1st to 15thNight
8%325%6%35%7%8%8%39%
1800
16th to endNight
4323433445%60%
1831
October1st to 15thNight
5%31332233463%
1980
16th to endNight
331322345%5%65%
2112
November1st to 15thNight
111222335%6%67%
2214
16th to endNight
2312324346%64%
2340
December1st to 15thNight
35%1433446%7%56%
2479
16th to endNight
26%35%425%46%7%52%
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
141342435%8%60%
1554
16th to endDay
15%36%5%34335%56%
1728
February1st to 15thDay
6%36%6%34446%53%
1660
16th to endDay
17%35%6%25%5%46%50%
1580
March1st to 15thDay
18%36%5%34445%52%
1893
16th to endDay
46%35%5%25%347%53%
2173
April1st to 15thDay
37%36%5%2445%7%49%
2150
16th to endDay
310%35%5%14337%50%
2160
May1st to 15thDay
6%8%35%5%25%45%447%
2152
16th to endDay
6%32442445%6%54%
2582
June1st to 15thDay
431442445%6%57%
2520
16th to endDay
3214436%7%6%7%52%
2520
July1st to 15thDay
8%7%27%7%47%7%6%8%30%
2490
16th to endDay
11%6%38%9%49%6%7%8%24%
2467
August1st to 15thDay
10%6%49%9%5%7%6%48%28%
2225
16th to endDay
10%7%39%7%45%6%6%6%33%
2292
September1st to 15thDay
12%7%37%7%445%46%35%
2160
16th to endDay
44245%33446%56%
1938
October1st to 15thDay
7%424423345%57%
1650
16th to endDay
45%1431223467%
1760
November1st to 15thDay
14144245%5%6%60%
1633
16th to endDay
25%34224345%61%
1620
December1st to 15thDay
15%244245%5%7%56%
1479
16th to endDay
17%36%7%36%5%47%46%
1536
January1st to 15thNight
1423324447%62%
2395
16th to endNight
24235%25%5%5%7%56%
2496
February1st to 15thNight
6%25%6%235%47%53%
2189
16th to endNight
16%246%245%46%54%
1894
March1st to 15thNight
15%25%435%45%6%53%
2064
16th to endNight
3415%323336%62%
2048
April1st to 15thNight
25%25%5%25%45%8%52%
1798
16th to endNight
3425%5%24335%58%
1800
May1st to 15thNight
4424435%5%5%6%53%
1796
16th to endNight
5%323325%45%6%57%
1528
June1st to 15thNight
22145%45%5%5%7%55%
1440
16th to endNight
2214436%5%6%9%52%
1440
July1st to 15thNight
5%325%6%47%8%8%10%36%
1467
16th to endNight
5%426%5%38%8%8%10%34%
1701
August1st to 15thNight
5%426%6%5%8%7%9%8%34%
1734
16th to endNight
6%427%6%36%7%9%11%36%
1910
September1st to 15thNight
8%4246%46%7%7%8%38%
1800
16th to endNight
43135%33445%60%
1831
October1st to 15thNight
6%31442233462%
1980
16th to endNight
221332345%464%
2112
November1st to 15thNight
113425%346%66%
2214
16th to endNight
2413425%435%62%
2340
December1st to 15thNight
25%135%36%5%47%53%
2479
16th to endNight
26%346%27%5%6%7%49%
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
17%7%9%26%32%6%
1555
16th to endDay
27%39%20%26%12%
1728
February1st to 15thDay
28%7%9%22%19%12%
1661
16th to endDay
28%6%6%21%26%10%
1580
March1st to 15thDay
29%6%6%20%27%9%
1893
16th to endDay
27%5%7%22%29%7%
2175
April1st to 15thDay
31%5%6%22%24%9%
2150
16th to endDay
31%46%20%26%9%
2160
May1st to 15thDay
34%5%9%16%27%7%
2152
16th to endDay
28%5%9%17%25%14%
2586
June1st to 15thDay
23%6%11%15%21%22%
2520
16th to endDay
23%6%12%17%21%18%
2520
July1st to 15thDay
44%9%14%11%11%7%
2343
16th to endDay
48%9%18%17%4
2312
August1st to 15thDay
46%8%14%16%9%3
2210
16th to endDay
44%8%13%18%12%3
2292
September1st to 15thDay
44%7%13%18%12%4
2160
16th to endDay
26%5%7%20%29%10%
1938
October1st to 15thDay
27%46%19%29%13%
1650
16th to endDay
20%36%18%37%13%
1760
November1st to 15thDay
19%410%20%30%14%
1633
16th to endDay
23%25%22%31%15%
1620
December1st to 15thDay
22%6%14%25%24%6%
1479
16th to endDay
32%6%11%22%22%5%
1536
January1st to 15thNight
19%6%8%27%28%9%
2395
16th to endNight
22%5%8%24%27%10%
2496
February1st to 15thNight
26%6%12%22%21%10%
2199
16th to endNight
25%6%5%22%29%10%
1894
March1st to 15thNight
25%7%9%24%24%8%
2064
16th to endNight
22%5%6%22%32%9%
2048
April1st to 15thNight
27%410%24%24%8%
1798
16th to endNight
25%6%6%22%29%10%
1800
May1st to 15thNight
27%6%10%18%27%8%
1797
16th to endNight
24%7%11%19%23%13%
1529
June1st to 15thNight
22%7%14%14%18%23%
1440
16th to endNight
26%7%11%15%20%18%
1440
July1st to 15thNight
34%11%19%14%9%10%
1385
16th to endNight
33%13%27%17%6%
1593
August1st to 15thNight
35%14%17%16%10%4
1725
16th to endNight
33%11%18%20%11%3
1910
September1st to 15thNight
34%10%19%17%11%6%
1800
16th to endNight
22%7%9%20%28%11%
1831
October1st to 15thNight
23%37%22%28%13%
1980
16th to endNight
17%5%7%19%36%13%
2112
November1st to 15thNight
16%48%23%33%12%
2214
16th to endNight
20%45%21%30%16%
2340
December1st to 15thNight
24%8%8%27%23%7%
2479
16th to endNight
28%7%10%23%23%6%
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
15%34%21%18%8%1
90
16th to endDay
14%37%23%15%5%3
96
February1st to 15thDay
10%27%32%27%2
90
16th to endDay
28%24%27%13%41
81
March1st to 15thDay
20%25%39%10%4
183
16th to endDay
22%35%29%10%1
288
April1st to 15thDay
21%37%32%6%1
269
16th to endDay
23%42%24%7%
270
May1st to 15thDay
15%35%35%10%2
289
16th to endDay
9%21%37%22%7%1
517
June1st to 15thDay
8%15%28%28%16%1
495
16th to endDay
46%25%37%21%4
495
July1st to 15thDay
9%223%32%25%5%
447
16th to endDay
15%15%41%20%6%
327
August1st to 15thDay
10%327%42%13%1
315
16th to endDay
13%429%36%13%1
332
September1st to 15thDay
12%435%36%9%1
315
16th to endDay
8%14%38%29%7%1
168
October1st to 15thDay
10%23%30%22%10%4
90
16th to endDay
14%25%26%20%9%4
96
November1st to 15thDay
16%13%34%24%10%1
90
16th to endDay
22%34%22%15%5%
90
December1st to 15thDay
17%34%31%14%2
90
16th to endDay
28%34%22%14%
96
January1st to 15thNight
7%17%35%23%11%3
789
16th to endNight
9%21%35%20%10%2
768
February1st to 15thNight
7%21%38%24%7%
704
16th to endNight
12%28%37%19%2
632
March1st to 15thNight
10%21%30%28%8%1
720
16th to endNight
9%20%36%25%7%
704
April1st to 15thNight
13%26%31%23%4
539
16th to endNight
9%26%33%22%7%
540
May1st to 15thNight
7%18%36%25%12%
540
16th to endNight
8%17%28%31%14%
563
June1st to 15thNight
37%31%38%18%
540
16th to endNight
4325%38%26%1
540
July1st to 15thNight
5%10%38%39%5%
510
16th to endNight
7%6%25%48%10%
564
August1st to 15thNight
7%7%33%41%10%
539
16th to endNight
5%110%33%39%8%
573
September1st to 15thNight
8%116%37%31%4
540
16th to endNight
6%5%20%39%24%2
513
October1st to 15thNight
7%13%29%28%17%3
660
16th to endNight
7%12%29%25%20%5%
704
November1st to 15thNight
7%13%29%30%16%2
814
16th to endNight
9%16%32%25%12%3
900
December1st to 15thNight
11%24%34%21%7%1
900
16th to endNight
11%23%35%22%6%
960
Page generated at 2017-09-23 04:43:42. Version ac04

Copyright 2017 A. Danko


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